PROCESS AND APPARATUS FOR MANUFACTURE OF HYDROXIDE
SLURRY
TECHNICAL FTELD
[0001] The present invention relates to relates broadly to a process and apparatus for
manufacture of high solids hydroxide slurries from caustic calcined carbonate
powders that may be produced from calcination of magnesite, dolomite and
limestone and mixtures thereof, whereby the slurries have low resistance to shear
thinning to facilitate reconstitution after months of storage with mild agitation.
BACKGROUND
[0002] Hydroxide slurries are aqueous suspensions of solid hydrated oxides of primarily
magnesium, as Mg(OH) , calcium Ca(OH)2 and mixtures thereof, in water. They are
widely used in many industrial processes. An example of which is the treatment of water
to raise the pH and eliminate odours, particularly for sewerage treatment, and to
precipitate heavy metals. These slurries are increasingly replacing sodium hydroxide
because of their inherent properties.
[0003] For slurries with a high pH or about 12.0, a hydrated lime slurry Ca(OH) or
dolime slurry Ca(OH)2.Mg(OH)2 is used, whereas for slurries with a pH of about 10.4
when diluted in water, a magnesium hydroxide slurry Mg(OH) or semidolime
Mg(OH) .CaC03 slurry is used. For sewerage treatment, magnesium hydroxide slurry or
semidolime slurry is preferred because any excess magnesium hydroxide entering
digesters does not kill the bacteria, whereas overdosing of sodium hydroxide or hydrated
lime slurry can destroy the bacteria and close down the digestion process. In heavy metal
removal, the pH of the magnesium hydroxide or semidolime slurry is in the desirable
range where amphoteric hydroxides of toxic metals precipitate, whereas at the pH of
sodium hydroxide or hydrated lime, the initial precipitates re-dissolve.
[0004] Hydrated lime slurries are produced using lime, CaO, from a kiln, and the
hydrators for that processes are a known art. Generally, the lime is sufficiently reactive
that the shiny s formed quickly, typically less than sixty nutes from granules of the
order of 1 mm. The process may include ri a ing the lime, and. generally the process
requires the addition of dispersants to provide the required stability and to assist the
slurr production process . ny o these dispersants are destroyed at high fe mpe t ir e
so that a lime hydraior is ge erall cooled ,
[0005] Magnesium hydro de slurries arc produced fr om either precipitated magnesium
hydroxide, or from hydrating magnesium oxide which has bee generally produced from
the calcination of the i eral agi site .
[0006] Preferred properties of hydroxide slurries may include
. Th y should contain particles that ca react quickly whe the slurry is,
example, mixed w th water. Thus slurry y he composed of small
particles with a median siz of 5 an a low surface area about 4 : gm (as
measured by gas absorption methods of the drie powder), or median of 20
p n and a hig surface area about 20 n gr , A wide, or multiple peaked, particle
si e distribution, with a sharp a is preferable to promote the slurry stability.
Suc a particle size distribirrion may a of less tha 1 0 microns,
. The chemistry of th surfaces of th particles i th slurry are strongly dependen
on th surface area of the oxide particles that are h dra ed to form the slurry, and
me range of applications of the slurry depend oo these surface chemical
properties. Thus a slurry produced- from oxide-particles that have a surface area of
order 150 'Vgm or high have markedly different exterior chemical surface
properties than a slurry produced front conventional oxide fe a surface area of
20 rr /gro. Without being by theory, oxide particles having a high
area v larger amounts of energetic chemical delects, suc as superoxides and
peroxides, at the crystal grain boundaries, a d these species largely survive
hydration to confer on th slurry particle surfaces a different degree of surface
chemistry that cor elates w h the r&ce are of the initial oxide used to form th
slurry.
The percentage of solids by weight is at s 50%, and preferably 55- 5% The
higher solids c n sat the less water is required t be shipped. However,
the higher imit s.generall y he result of the requirements that th slurry has a lew
resistance to t ning, and thus a low apparent viscosit a low shear rates. f the
solids; a o too g i : gel tends t form which has high resistance to
thinning and. h gh apparent viscosity, an is not readily usable in the
applications described abo ve.
The viscosity at arnbient temperature is: 50-900 centipoise ( ps ,
preferably 5 -300 ops at a shear rate of 200 rp or less with such a lo resistance
to h r n g, th slurry S regarded^ as preferably when the apparent viscosity
is less tha 00 P The achievement of such thin, h gh solids, slurries generally
requires a viscosit modifier to facilitate the breakdow of gels with i inrun
agitation. The d sirah y of thin s rri es Is that they are easil handled, an are
more amenable to the application of subsequent processing steps that deli ver
desirable properties such a sprayed coatings that have a high strength because the
low viscos ty delivers an ease of application a surface coverage with a low
water that would otherwise cause cracking and/or low strength when dried
due to the excessively hi h p r eaht ty from water evaporation during dr ing.
In many applications, this desirahle property of th n slurries i augmented fey the
surface chemistry that arises fr the us of high surface area oxide particles sed
to produce the slurry.
The of the slurry such that it can be used up to many months after
a ufacture The characterisation of slurry stability s often somewhat arbitrary
and may nvolve meeting a n n er of criteria. For example;--
i one criterion ay be that of pourab iity/ wabiliry so that mor than 80% by
weight, preferably greater than 90% by weight, of a d sample in, say, a .1. n
container pours off after 7 days of undisturbed gravity settling<
i another criteri o :may be that after ays of un distur bed g t d gravit
settling, water , called syne sis vessel of container of metre
depth i s .30 millimeters; or 30 days the sy e sis is ess tha 50
millimeters.
(ill) another criterion ma be that the slurr has less than % sediment ("heel")
after 30 days.
iv) another criterion may b that th syneresis less than 5% after days, or
preferably 3%.
[0007] t is recognised that these this, high solids slurries of variabl e chemical reactivity
do not: have a long intiinsic lifetime, and some degree of sedimentation oc urs For ma
the :re-s s end i y of the slurry s much more critical than long-term
slurry homogeneity because agitation can: h provided at the point of storage, and i
required suc agitation daring storage ma b r ii e t.
[0008] The choice of viscosity modifiers and stabilisers to produce stable, thin, high
solids slurries ar generally associated wit the surface charges on the particles, a the
ionic strength of the water. The viscosity modifiers and stabilisers are generally not
speci i t the method of manufacture of the slurry.
[0009] The prior art for the production of hydrated lime slurries from kil lime is wel
understood f relevance to the present invention, US Patent No. 3 573 02 discloses a
two stage process, and uses steam pressure to overcome differences in the hydration rates
of lime and magnesia that otherwise cause significant issues in making slurries of mixed
alkaline earth hydroxides. The use of pressure vessels adds to the i an cost of
a slurry plant. The kiln lime is generall burne lime, in which the granules ar inter d
to give a moderate surface area. This is acceptable because the hydration process of li e
causes the granules to break u from the stresses induced as the particles expand during
the reaction to accommodate the water. There is a need for a process that can produce
hydroxide slurries from u -smte ed materials without the need for the of high pressure
processing. Convention iirne hydrator plants, operating a ambient pressure, require
inputs of i with low magnesium oxide content,
[0010] The prior art for the production of magnesium hydrox ide slurries that eet the
Industrial requirements listed above, is characterised by the initial solids materials used to
make the slurry. It is noted that these processes do not generally use the approach of
using; gh pressure steam, as disclosed for hydrated l e with magnesia, to accelerate the
magnesia hydration reactions. The magnesia particles generally do not exhibit significant
fracturing from the: hydration processes. These classes of materials for magnesium oxide
slorries are;-
a Precipitated Magnesium Hydroxide (PMH), M is general produced b the
precipitation of magnesium hydroxide from brines by the addition o f h rated
lime. The prior art, described below, bc sse on th use of (i viscosity
modifiers that thin the slurry an (si) stabilisers that facilitate the: stability of the
slurry, ror by agitating d fl o c at ng the washed precipitate n water, The
spec fi viscosity modifiers are selected to deal with the presence of sign fi can
amounts of residuai h oride io u i the washed precipitate, PMH does not
requi r grinding or hydration to .make th slurry because the particle of th
PM is similar to that of the desirable slurry namely 25 microns or less.
Specifically, Patent No. 54 0395 describes the production of slurry by
grinding dried magn es m hydroxide to a specified particle size and then mixing
with water under agitation. US Patent No. 5,762,901 and No. 5,514,357 describe
the siabiiisation of slurries n which the slurry contains chloride ions in 030-
0,42%, by weight on an Mg basis, These describe the use f a caiioriic polymer,
and, if required a thickening agent t o stabilise th slurry, s that the s fanned
by physical d fl oc ulaiio is stable, so that it au be irausported and stored
without st bst nt agglo r atio of the magnesium hydroxide solids. The
patent US 5,877,247 describes th stabilisation of slurries formed fr solid
magnesium hydroxide usin a combination of one or more polymeric dispersanis
and one or ore water -soJ e alkali metal salts. The patent E 0097 A4
sc s s the production of stabilized magnesium hydroxide slurry using we
milling f ap hydroxide to give control of the median particle
i controlled partic le size range, ami controlled surface area of the
Mg( solids t e slurry.
The addition of viscosity modifying agents and di p rsa t to to control
viscosity, stability dispersabiliiy, is well established art Such viscosity
modifying agents or disperssuts can include decomposable phosphates (FR
239 4S ) ; ear o ylic acid type poiy ue surfactants ( P -20 810); po ya o
and x k of strong acids such HQ 3S or 4302539); polymeric
anion dispersant and water soluble alkal metal sal t (AO 48785/93);
s dphom th d acryla i e homopolymers or copolymers ( S 4743396};
alkaline salt of l s c inic ester product (DE 3730 ; alkal metal silicate
and hydroxide and/or mineral acid salts (.162(107439), organic or inorganic
dispersants 6 70214); xs than gum and l gnin snlphouates (CA 110
( }:? 37e); earboxy ethyl ulose ( . 4 (6):39729:k); caiio i polymers
(US 443 248}; ferrous hydroxide or ah omn hydroxide (CA 79(8 ):44 ) an
polyaery a (US 42306 ).
Dead Burned M gn sia (DBM). DBM generally in granule of about 25 ram or
less, is generally produced by the calcination of the mineral magnesite. DBM is
sintered, with a very low specific surface area, often belo 0 m gn . When
mi d i water, the hy dration of DBM produce n u hydroxide is ver
slow, ove many days and weeks. The prior ar r slurries formed f om DIM is
focused on activating the hydration process, and dealing with the propensity o
so l magnesium d to coat th small surface area presented, and to
slow down the reaction. The means of activation include wet milling to
regenerate the surface, and pre era ly in hot te to take advantage of the fact
that the hydration reaction is thermally activated, and the use of chemical
additives that are associated with lifting the coating from the surface, Generally,
viscosity modifiers and stabilisers are used to produce a thin, stable high solids
slurry, in the same manner required for (a).
Specifically the US Patent No. 5,4 7,879 A describes the process of production
of a stabi lized, press rre hydra ed magnesium hydroxide starry from ground
DBM. A xture comprising ground DBM and water is pressure hydrated to
provide a pressure hydrated slurry. The: u hydrated slurry is then deagglomerated.-
If desired, chloride ions and catiortie polymer can be added to
fiirther stabilize the slurry. The pressure preferably 2-7 bar and the temperature
s preferably that of wet steam at that pressure Th process is catalysed by th
introduction o chloride ions. This patent teaches the use of magnesium chloride
to catalyse the hydration of the DBM,
As an alternative to pr ssure process, the wet milling of the DBM granules is
described i the prior art. US Patent 5906804 A and European Patent No.
0772570 describe a process for producing stable magnesium hydroxide
siurry
:
which wet grinding calcined magnesia granules having particle: size of
about 25 mm o less, and hydrating the finely divided magnesia i a hydration
wherein said finely divided agnesi is mixed wit water under agitation
a hea s as to produce a agnesi hydroxide s rry having at least 80%
hydration; and passing the slurry through second particle reduction zone so as
t produce slurry particles: wherein 90% of said slurry particles have size less
tha S microns. A viscosity modifying agent is added to e sure a maximum
viscosity of 1000 cP. The final product is described as stable., pumpable,
magnesium hydroxide slurries having a solids content of at least 40%. Th
viscosity modifying agent is selected fro the group consisting of either inorganicacids
having a molecular weight less tha 130 a r or inorganic salts thereof
having an alkali meta a a sole cation; o earboxylie acids having a molecular
weight of less than 20 ana* optionally containing one or more hydroxy! groups
and salts thereof exc ding salts having alkali metal a a sole cation; or p lyhydri
alcohols, and. carbohydrates con ng two or more hydroxyl groups and ha ng
molecular we ht of less than 500 amu; or alkaline earth ox des, hydrox ides and a
con inati thereof. This patent teaches recirculating th parent (uiihydrated
solids) through a until the particle is substantially c .
Generally, viscosity modifiers and stabilisers are used to produce a thin, stable,
gh-s s slurry from DBM, i the same manner required or (a).
Granular Calcined Magnesia (GCCM). GCCM i als produced from the
alc ati of the magnesite, GCCM granules ay be extracted from a kiia at a
earlier stage of process than DBM. The surface area of GCCM is typically n th
range of 25-60 m. GCCM is also sintered, bu to lesser degree than DBM:,
i the case of CM th rate limiting process for slurry formation is the wet
milling of the granules. Th fester hydration is associated with the use of shorter
nulling processes compared to DBM. n common with slurries made f
PM and DBM, hig solids n rie : require viscosity modifiers and stabilisers to
produce a thin, i.ab e, high solids slurry.
c the production Of slurries from. GCCM is described in Japanese
Patent No, 5-2790 1.7 and Japanese Patent No, 3-279018. GCCM which is
Introduced into a hydration tank equipped with a sinter or agitator and which is
simultaneously milled b steel balls or other form of abrading apparatus. ro et
Chemical Abstracts A) 6 (2) 58S4e (1966) refers to the hydration of
rfsagnesite-denved MgO. In this ease, magnesium hydroxide was produced daring
boiling or short wet grinding of the MgO with water in bail mill European
Patent No 0599085 describes process in which GCCM is comminuted the
wet state with a wet-pul veriser and bydrated i the presence of an alkaline
aqueous medium which Included sodiu hydroxide at an elevate temperature of
no less tha 7( The resultant pulverised materia! is classified into fine and
coarse particles using a classifying means which is generally set to restrict the
passage of part les n e ess of 20 micron. Subsequently the coarse particles are
recycled to the wei-pn!veriser. By b tin GCCM to concurrent wetpulverisation
md hydration in the presence heated alka ne aqueous d ,
mag nes ca be simultaneously d a d hydrated te rap heat g to.
produce a act ve magnesium hydro de showing a lo viscosity even at a hig
concentration.
South Korean Patent 93 6 describes formation of active magnesium
hydrate ad fro light bt n d rnagnesite which s subjected to wet crushing
with water, an alkali stabiliser inclusive of sodium hydroxide md dispersing agent
inclusive of po!year o y -using reaction beat and crushing heat.
[001 i ] Generally, viscosity modifiers and stabilisers are use to produce a thin, stable,
high solids from GCCM, in the same ma er requited fo (a) and (b),
[0012] Powdered Caustic Calcined Magnesia (PCCM). PCCM ay be produced by
simply grinding GCCM , or may be directly produced by the flash calcination of ground
niagnesite powders ay be used, or by i stales produced by any of the
a.f ren t tioned processes and flash calcining the dried hydroxide. Most flash ealeiners,
however, generally have the undesirable property that some particles are exposed t very
high -temperatures the hot combustion gas, and calcine and sinter quickly, so that th
product has variable specific surface area, and. variable hydration properties;. The average
properties of flash calcined PCCM are otherwise similar to those of PCCM from grinding
granules. Indirect heating, co ter caiciners, a described by Scents an oi y for
example i 2007/ 496 (incorporated herein by reference) produce uniformly
calcined PCCM with ini al sintering and a high specific surface area, which ca be in
the range of 00- 25 m~/¾rn with the dearee of calcination of 90-98%. Such calcined
PCCM has markedly different surface .chemical properties than PCCM produced by
conventional methods
[0013] The production of slurries fr Q has bee previously described, P-2-
484 refers to a process of producing slurry from PCCM having a solids content of 5-
70% w at above 5 a under agitation, wherein some slurry is periodically removed
and replaced by hot water a d magnesia to obtain a uniform slurry density. 3-2523
-!Orefers
to a process for preparing P gr inding the GCCM to a mea particle size o 5-
1 micro a d then subjecting the ultra-fine powder in an acidic reaction. JP V -212214
refers to a method of nr n ac re a PCCM slurry having -50% wt Mg 2 wherein
magnesia having a mean particle dia re er of less than 100 micron is hydrated » the
presence of alkali metal ions and/or alkaline earth etal ions an also in the presence of
the hydroxide ion, nitrate ion, carbonate ion, chloride ion and/or sulphate ion. D 272288
which describes hydration of MgO resulting f m MgC¾ thermal which is
carried ou by (a) pfe-hydratmg MgO in one or more series r parallel connected
hydration reactors; and (b) grinding i one or ore series or parallel connected hydration
reactors. JP 03-60774 refers to the production of magnesium: hydroxide slurries which
incl ude the step of slakin finely pul verised light burn t magnesia which is obtained by
firing naturally produced mag e ite with water with heating to S- 0& Sodium
hydroxide is added a s a hydration accelerator. It i known ro JP 5-2 8 0 and JP 3 ~
252 , i r example, that magnesia ma be produced b calcination of gne e
followed by particle reduction to obtain ultra-fine particles having mean particle size of
5- 0 micron which is en hydrated to form magnesium hydroxide slurry. The hydration
process ca be carried out in particle reduction zone.
[00 t is also known to use addi tives to accelerate the hydration of MgO to
g(O and/or t modify the crystal shape of the magnesium hydroxide product during
hydration. Su additi ve includ citric acid or magnesium chloride (see CA
i l 24 .215 23t) , short chain carboxylic acids or corresponding salts such a magnesium
acetate JF 3-197 , JP 0 1-131022 and DD 280745), monm chloride (DD 241247);
magnesium chloride, magnesium acetate, magnes m sulp t or magnesium nitrate ( D
2469 ); inorganic or organic acids suc as C 1or acetic acid or their magnesium salts
such as nt gnesi i chloride, or magnesium acetate (CA ( 1 ) ; 90 19, proprioBie
acid (JP 63-277510), a -butyric aci (JP 63-27751 , and sodium hydroxide (JP 03-
60774).
[00 15] -3- 73 15 refers to the production of a magnesium hydroxide slurry 3-
70 t and more preferably 20-50 t solids as an intermediate in the production of
- -
magnesium hydroxide crystals hexagonal plate-like crystals are obtained a
a fi al product of of mag esia These .crystals are utilised a fire re rdan
JP 1-1 3 22, which is s in the p or art preamble of iP-3~ 1973 , states that the
purpose of addition of magnesium salts such as magnesium acetat or organic acids such
as acetic acid, controlling th rate of hydration for controlling the growth of
magnesium hydroxide crystals. The crystal that are obtained by the hydration proces of
this reference are regular n shape thereby avoiding the ior of agglomerates.
[00 ] Generally, viscosity modifiers and stabilisers are used to produce a thin, stable,
high solids slurry from PCCM * i the same manner required for (a), (b) and (e .
[001 ] There is need to produce ow emissions intensity starry products to mitigate the
impact of global a i Th production of from brines is energy mteosive and
uses hydrated lime that is generally produced in ime kilns that have significant C¾.
emissions from both the energ consumed and from the calcination of limestone The
production of DBM and CC V us energy intensive kites, that als have high
emissions fro both the energy consumed as wel as f m the calcination of magnesite,
as doe the production of PCCM from traditional flash ale ers,
[0018] In contrast, the production of .PCCM using oalemers, of the type described b
Sceats a or ey, with indirect heating of magnesite entrained in steam produces, after
steam condensation, ore ( stream that can be liquefied, and sequestered, thereby
significantly reducing the carbon footprint indirect heating, with counter-flow, is energ
efficient, and for th purpose of this specification, produce a PCCM with a ver high
surface area, in the range of 0-200 /g . I addition, this type of calciner also
produces high surface area lime a i CaO.MgC and seniidoiime M CaC .v
More generally, carbonate minerals are a mixture of limestoBe, dolomite and
magnesite, and the calcined material fr m this reactor is a powder mixture of the oxides
d un e ied carbonates. These powdered caustic l in d carbonate powders arc the
feedstock to produce the hydro de s rries descr ed in this invention. The high
Surface area calcined carbonate powders are ver reactive, and there is a nee for
manufacturing process that ca use suc feedstocks for the production of slurries.
In e of this process for surface area materials, the process
described herein: e a rs also he to h formation of slurries caustic calcined
carbonate powders, such as , d usin traditional: flash ca iner s or by
grinding granular caustic calcined carbonate materials, such as GCCM, The prior art
described above o r the production of slurries frorn PCCM ar ot b e use for the very
reactive powders. There i eed for slurry production process for powdered caustic
calci ed carbonates that ca e applied generally to powdered caustic calcined feedstock
produced by any means, and of any composition Th description below is based on
PCCM because magnesi is the ost difficult calcined material t slurry. The
The invention i equally applied to any powdered calcined earth carbonate, including
mixtures.
[0 ] Alternatively, high surface area PCCM can be obtained by drying a m gne n
hydroxide s rry formed by any of the aforementioned processes, and flash calcining this
material at lower temperatures, preferably below 600 C to rapidly dehydroxylate th
hydroxides t refo rm PCCM. Th lower temperature deh roxyl ion compared to
de arbonah n means that the sintering of the PCCM produced by the calcination of
hydroxide particles i significantly reduced, so t ha th surface area is e increased.
The caleiners described by Seeats and ore!y produce a very high surface area PCCM, of
about 250 -gnu from dried hydroxide feed, fro an initial PCCM: having &surface
area of l es than 200 this approach, th surface area of the PCCM is highest
when the slurry has been produced by ydra ng high surface are PCCM. As described
above, the surface particles i slurries produced fro higher surface area .PCCM are
characterized by higher concentrations of reactive species such as peroxide and
superoxide, and in marry applications, these species ar beneficial . Repeating th steps
of dehydration, flash: calcination and hydration allows a nere eu al increase in th
surface ar a of the PCCM produced in each step, and thus the concen trati of reactive
species. The hydration process in each of these hydration steps requires the use of the
production process described i this invention because the rat of heat release
becomes too fast fo conventional slurr production processes.
[0020] Any discussion of the prior art throughout the specification should in no way be
considered as an admission that such prior art is widely known or forms part of common
general knowledge in the field.
SUMMARY
[0021] PROBLEMS TO BE SOLVED
[0022] The present invention may aim to provide a process, system, device and
apparatus for production of hydroxide slurries from caustic calcined carbonate or
hydroxide powders.
[0023] It is an object of the present invention to overcome or ameliorate at least one of
the disadvantages of the prior art, or to provide a useful alternative.
[0024] MEANS FOR SOLVING THE PROBLEM
[0025] A first aspect of the present invention may relate to a process for producing a
hydroxide slurry from caustic calcined carbonate powder, comprising the following
steps: mixing caustic calcined carbonate powder with water in a reactor vessel and
forming a reaction mixture; applying a shearing force to the reaction mixture using a
mixing apparatus; allowing heat of hydration to raise the temperature of the reaction
mixture to near the boiling point, preferably about 95°C, and allowing steam to
evaporate from the reaction mixture as hydration proceeds, to remove excess heat
and control reaction temperature to just below or at the boiling point.
[0026] A second aspect of the present invention may relate to a process for producing a
hydroxide slurry from caustic calcined hydroxide powder, comprising the following
steps: mixing caustic calcined hydroxide powder with water in a reactor vessel and
forming a reaction mixture; applying a shearing force to the reaction mixture using a
mixing apparatus; allowing heat of hydration to raise the temperature of the reaction
mixture to near the boiling point, preferably about 95C, and allowing steam to
evaporate from the reaction mixture as hydration proceeds, to remove excess heat
and control reaction temperature to just below or at boiling point.
[0027] Preferably, the process is adapted for the production of a high solid fraction
hydroxide slurry from the reaction mixture, wherein the slurry has a relatively low
resistance to shear thinning.
[0028] The process is further adapted for the production of high solid fraction
hydroxide slurry from the reaction mixture, wherein the slurry has both a relatively
low resistance to shear thinning and a relatively high concentration of reactive
chemical species such as peroxide and superoxide ions.
[0029] The preferred process may additionally comprises the following steps: metering
an input of a viscosity modifier to enable the mixing apparatus to maintain uniform
mixing under thin slurry conditions promoted by the viscosity modifier; allowing the
reaction to proceed spontaneously, during boiling, until the water has ceased to boil and
the temperature dropped to a first set point; and quenching the slurry to drop the
temperature to a second set point.
[0030] The preferred powder may include ground particles and wherein said ground
particles are less than 100 microns in diameter. More preferably, said ground particles
have a particle size distribution in the range of 0.1 up to 150 microns, and preferably with
a d of less than 100 microns.
[003 1] The preferred powder may have a surface area preferably in excess of 100 m2/ m,
and more preferably in excess of 200 m2/gm.
[0032] The slurry may have a final solids content, after accounting for the water loss
from boiling, is in the range of 40-70%. More preferably, the slurry has a final solids
content, after accounting for the water loss from boiling, in the range of 55-65%.
[0033] Preferably, during the process additional water is added, if required, during the
process to ensure that the final solids fraction meets the solids fraction specification of
the slurry product.
[0034] The preferred temperature of the water during the first step is within a range of
10-25 C. The preferred mixing apparatus may comprise at least one high shear mixer, and
preferably such shear pump being external to the reactor vessel through which the slurry
is circulated by the pump action, and optionally further, a paddle or other similar mixer is
used o agitate the slurry in the reactor vessel.
[0035] The process may be configured to be run by a device that continuously operates
all of the steps of the process in a predefined order. A viscosity modifier, including but
not limited to acetic acid, or magnesium acetate, may be added to maintain the apparent
viscosity in the range of 60-300 cP during the process, where the apparent viscosity is
that of the slurry at a shear rate of preferably 200 cycles per second.
[0036] The first set point is after the time at which the temperature has reached a
maximum, and this maximum is preferably above 90 C. Preferably, the first set point is in
the range of about 85-93 C, and preferably about 93 C, with the provision that the heat
losses in the reactor apparatus are sufficiently low that the length of time to reach the set
point is less than 60 minutes. The preferred process may also include a step of quenching
of the slurry at the end of processing is in the range of about 10-60 C and preferably
about 40 C. This quenching may be conducted by transport of the slurry to a second
vessel that has a temperature and heat capacity such that a desired quenching temperature
is achieved.
[0037] The preferred process is adapted to yield a high solids slurry which, after 1 month
standing without agitation exhibits syneresis of preferably less than 5% of the height of
the storage vessel and preferably 3%, and a toe of preferably less than 1% of the height of
the storage vessel, and which can be remixed and made to flow and pour by mild
agitation.
[0038] The preferred carbonate material is calcined limestone, magnesite or dolomite
[0039] A third aspect of the present invention ay relate to a reaction apparatus for
producing a hydroxide slurry from a reaction mixture of at least caustic calcined
carbonate powder or caustic calcined hydroxide powder and water, wherein the
reaction apparatus comprises; reaction vessel having a first inlet adapted for
receiving caustic calcined carbonate powder an a second inlet adapted for receiving
water and a controller that is adapted to eiectroracally control the process within the
reaction vessel; shearing apparatus positioned within the reaction vessel for shearing
the reaction mixture and wherein the rate of shearing is controlled by the controller; a
viscosity sensor positioned within the reaction vessel adapted to supply viscosity
information about the reaction mixture to the controller; a temperature sensor
positioned within t e reaction vessel adapted to supply viscosity information about
the reaction mixture to the controller; an steam outlet for release of steam from the
reaction vessel, such that in use the reaction is controlled by the controller s that the
heat of hydration may raise the temperature of the reaction mixture, allowing water
to boil off from the reaction mixture a hydra tion proceeds, and removing steam via
the steam outlet t remove excess heat and control reaction temperature at boiling
point
[0040] A fourth aspect of the present invention may relate to a process and apparatus
for production of hydroxide slurries from caustic calcined carbonate powders, or
caustic calcined hydroxide powders, whether such powder is derived for example
from traditional flash ca emer , from the ea! ers decribed by Sceais and orle , or by
grinding of granular calcined carbonate or hydroxide powders.
[0041] n one form, the disclosure provides a process of producing hydroxide slurry
fro caustic calcined carbonate or hydroxide powder, including;
a) mixing caustic calcined carbonate or hydroxide powder with water i
reactor vessel;
b) shearing the reaction mixture; and
c) allowing heat of hydration to raise the temperature of the reaction mixture
to a maximum near the boiling point, and allowing water to boil off from
the reaction mixture as hydration proceeds, to remove excess heat. The
maximum and reaction temperature is bounded by the boiling point of
water in the mixture.
[0042] Optional, and preferred, process steps include one or more of:
d) metering an input of a viscosity modifier to enable the mixing system to
maintain uniform mixing under thin slurry conditions promoted by the
viscosity modifier;
e) allowing the reaction to proceed spontaneously, during boiling, until the water
has ceased to boil and the temperature dropped to a first set point; and
f) quenching the slurry to drop the temperature to a second set point.
[0043] Preferably the reactor vessel is insulated to facilitate accelerated increase in
temperature of the reaction mixture up to boiling point. The vessel also preferably
has a steam outlet allow escape of ready steam from the reaction vessel, such that the
reaction takes place at substantially ambient pressure.
[0044] Further preferred aspects of control of the reaction include one or more of:
[0045] (i) minimising the heat losses such that the hydration heat liberated
spontaneously heats the slurry and accelerates the hydration process such that the
water boils, to provide the constant conditions at the boiling temperature and pressure
to allow the remainder of the hydration to be controlled in a simple self—regulating
manner; and
[0046] (ii) mixing of the water and particles to reduce the formation of bubbles, to
break up the formation of aggregates during the slurry production, and to provide
mixing so that the hydration reaction at the surfaces of all particles can occur at a
fast, uniform rate; and
[0047] (iii) adding a viscosity modifier to maintain thin slurry during production,
with the modifier being added at a rate and to a degree necessary to allow the ixing
to take place without a substantial change in the energy consumption of the mixing
system,
[0048] The slurry is preferably quenched to terminate the hydration of residual
magnesium oxide, and cooled to ambient conditions. The process may require no
stabilisers to achieve the criteria for stable, readily thinned, hig solids magnesium
slurries,
[0049] A further aspect of the present invention may provide for a reaction apparatus
for producing hydroxide slurry from caustic calcined carbonate powder or caustic
calcined hydroxide powder, including: a reaction vessel having a inlet for the
powder and a water inlet; shearing apparatus for shearing the reacti on mixture; and a
steam outlet for release of steam from the reaction vessel, such that in use the
reaction is controlled by allowing heat of hydration to raise the temperature of the
reaction mixture, allowing water to boil off fr o the reaction mixture as hydration
proceeds, and removing steam via the steam outlet to remove excess heat and control
reaction temperature at boiling point.
[0050] n a fifth aspect of the present invention, a hydroxide slurry is provided and
whereby it may comprise: particles of caustic calcined carbonate or hydroxide
powder, and water; wherein the particles within the slurry have particle size
distribution in the range of 0.1 to 0 microns; and an apparent viscosity in the range of
60-200 cP.
[005 ] In a sixth aspect of the invention, a hydroxide slurry is provided and whereby
it may comprise: particles of caustic calcined carbonate or hydroxide powder having
a surface are preferably in excess of 0 m grn, or more preferably in excess of 200
m7gm, and water; wherein the particles withi the slurry have particle size
distribution in the range of 0.1 to 100 microns; and an apparent viscosity in the range of
60-300 cP.
[0052] Preferably, the slurry is made by the processes described in the aforementioned
aspects of the present invention.
[0053] Further forms of the invention will be apparent from the description and
drawings, and from the abstract and claims.
[0054] In the context of the present invention, the words "comprise", "comprising" and
the like are to be construed in their inclusive, as opposed to their exclusive, sense, that is
in the sense of "including, but not limited to".
[0055] The invention is to be interpreted with reference to the at least one of the technical
problems described or affiliated with the background art. The present aims to solve or
ameliorate at least one of the technical problems and this may result in one or more
advantageous effects as defined by this specification and described in detail with
reference to the preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE FIGURES
[0056] Embodiments of the invention will be better understood and readily apparent
to one of ordinary skill in the art from the following written description, by way of
example only, and in conjunction with the drawings, in which:
[0057] Figure 1 depicts a schematic drawing of a process for production of stable,
thin, high solids magnesium oxide slurry from powders of caustic calcined magnesia.
DESCRIPTION OF THE INVENTION
[0058] Preferred embodiments of the invention will now be described with reference to
the accompanying drawings and non-limiting examples.
[0059] The production of a stable, thin, high solids magnesium hydroxide slurry from
caustic magnesia starts from the production of the PCCM. In one embodiment, the
CCM is produced by grinding granules from a conventional kiln to achieve the
desired particle size distribution, in another embodiment it is produced by flash
calcining pre-ground magnesite powders in a flash calciner. These embodiments
produce powders with moderate specific surface area in the range of 20-60 m27gm. In
the preferred embodiment, the PCCM is prepared from flash calcining pre-ground
magnesite powders in an indirectly heated, counter flow reactor to produce a high
surface area material, with a specific surface area in the range of 100-200 m2 an.
Alternatively, the PCCM is prepared by drying a slurry, and flash calcining the
hydroxide powder in an indirectly heated, counterflow reactor to produce a very high
surface are material, with a specific surface area in excess of 200 m7 gm.
[0060] In this first preferred embodiment of the present invention, the powder is
mixed into a container with water in a ratio to give the ultimate solids/water ratio in
the product, when account is taken of the consumption of water to form the
hydroxide, and the water loss from boiling as described below. The solids and water
are agitated during mixing to prevent clumping. The temperature of the water and
powder may be at ambient, or either may have been preheated.
[0061] The basis for the process is that the energy released from hydration of the
MgO to Mg(OH)2 by liquid H20 is used to heat the products of the reaction and the
excess water to 100 C, and the excess heat spontaneously boils a portion of the
excess water. In the ideal case of a reactor at ambient pressure, with no heat loss
and inputs at 25 C, a 60% slurry can be made in which the heat released
spontaneously raises the temperature to 100 C, and the remaining heat from the
subsequent reaction spontaneously boils the water. Thus one tonne of 60% slurry
(containing 600 kg of Mg(OH)2 and 400 kg of water) is produced at 100 C by boiling
off an additional 76 kg of water. This slurry is made by mixing 415 kg of MgO and
661 kg of water at 25 C. From the known thermodynamics of the reactions, the
hydration of the MgO by liquid water releases 387 MJ of heat of which 186 MJ is
used to heat the materials to 100 C, and 201 MJ is used to heat and evaporate the
water. In the design of the reactor, with inputs at 25 C, it follows that about 48% of
the hydration reaction is complete before the boiling of the excess water occurs. Real
reactors have heat losses, and minerals have impurities, so these quantities provided
above are for guidance only. In the prior art, the released heat is removed using heat
exchangers, or for very slow reactors the heat is lost by convection or conduction. In
this invention, the evaporation of the water is used to remove the heat, and the
boiling point of water provides a stable operating condition for rapid processing.
[0062] The kinetics of hydration plays a very important role in the formation of
slurries from DBM and CCM. It is well established that the initial reaction rate (a)
scales proportionally to the surface area of the solid particles, and (b) has an
activation energy of about 60 k.T/mol This means that the hydration reaction rate at
50 C, 75 C, and 100 C is, respectively, 4.3, 18.1 and 57.6 times faster than that at 25
C. However, it is often observed that the rate of reaction slows down significantly
before the reaction is complete, and this is attributed to the low solubility of
Mg(OH)2, such that Mg(OH)2 crystallites coats the pore surfaces. This is particularly
evident from dead burned magnesia. The solubility of Mg(OH) 2 also increases with
temperature, so this effect become less important at higher temperatures. For deadburned
materials the very low porosity is such that it is believed that the Mg(OH) 2
crystals formed during the reaction are separate from the parent particle. Wet milling
of DBM will remove any coating, and expose new surfaces, on the particles. While
the grinding process of DBM is essential, the prior art also describes the use of hot
water to increase the hydration rate. The milling conditions then determine the time
to produce the slurry. In contrast, for very high surface area CCM particles, the
specific surface area may exceed 100 m /gm, and there is little evidence of pore
blocking effects. Without being limited to theory, migration of water to such CCM
particles is probably not a rate limiting step because of the high porosity of the
particles. The most important observation is that the hydration reaction of CCM, in
a well stirred thermally insulated reactor, exhibits thermal run-away. For example,
using a material with a surface area of 190 m /gm, the temperature of the well-stirred
reactor initially rises spontaneously to 50 C over 30 minutes, and this is followed by
a fast process in which the temperature spontaneously rises to 100 C within 10
minutes. The heat released by the initial hydration increases the water temperature,
which increases the reaction rate so that heat released further increases the
temperature. This is thermal run-away. Importantly, the boiling point of water is
reached preferably within thirty minutes, and the temperature stabilizes, such that the
remaining reaction can be completed, say, with an additional 120 minutes of
processing at a fixed temperature through the release of steam. In this invention, the
boiling of the water circumvents the need to control the temperature of the reactor to
avert damage or hazards. Furthermore, the signature that the reaction is substantially
complete is that boiling ceases and the temperature begins to fall, at a rate determined
by reactor heat losses and residual hydration. t would be appreciated by a person
skilled in the art that PCCM produced with a high surface area, in the range of 100-
200 m7gm will be preferred as a source of PCCM, compared to PCCM with a
surface area of 20-60 m7gm because the processing time will be shorter, and less
susceptible to heat losses that might otherwise result in the slurry not reaching the
boiling point of water.
[0063] The slurries produced by a fast reaction at high temperature are characterized
by particles that are bonded aggregates of small crystallites of magnesium hydroxide.
These crystallites support a range of defect centres at the boundaries, which is
believed to contribute to the reactivity. The higher the initial surface area of the
PCCM, the higher the concentration of these defect centres.
[0064] For reasons considered below, to terminate the production, it is preferable that
the reaction is rapidly quenched to below about 60 C when the desirable degree of
reaction has been reached, ie as determined by monitoring the drop of temperature
described above. It has been demonstrated that the properties of such a quenched
material does not change significantly over months. For most applications, the
performance of the slurry is not impacted by a small amount of residual oxide
material, so there is no absolute requirement to achieve complete hydration. When
the set point is achieved, the slurry can be quenched. In a preferred embodiment this
is simply achieved by transferring the slurry batch to a steel vessel with adequate
heat capacity and/or cooling, to quench the product to below about 60 C.
[0065] In summary, the evaporation of water, releasing up to about 7% of the initial
water, provides a simple means whereby the slurry can continue to hydrated to the set
point for completion without the need for external control or heat transfer systems
during the reaction.
[0066] In the description above, a condition for the reactor is that the slurry must be
well stirred to achieve uniform kinetics.
[0067] There are several other requirements, which require more detailed
consideration of the mixing process. Thus the mixing:-
[0068] A) rapidly mixes the water and the particles so that the hydration reaction
occurs quickly.
[0069] B) rapidly mixes the water and particles so that concentration gradients do not
develop, which would otherwise slow down the reaction and reduce the productivity
of the plant. From a quality control perspective, the removal of concentration
gradients gives a uniform product because all particles have the same temperature,
and see the same aqueous environment;
[0070] C) breaks down aggregates of particles that otherwise form lumps which lead
to the collapse of the slurry. There is a strong tendency of particles to agglomerate at
high solids fractions, and the mixing is required to shear aggregates of particles. It is
noted that aggregation leads to concentration gradients, which are to be avoided;
[0071] D) prevents the development of bubbles of steam in the mixture, which
otherwise leads to foaming which also leads to an inhomogenous solids-liquid
environment and concentration gradients; and
[0072] E) comminutes the particles, so that a broader particle distribution is
developed. Comminution occurs when the particles are subject to high shear forces.
It is noted that the hydration process weakens the structure of the particles as the new
molecular configurations are developed. During this process, the initial particles can
fragment if subject to strong shear forces.
[0073] Notwithstanding the concepts described above, experiments show that the
formation of a stable slurry is facilitated by the use of a high shear mixing apparatus
which is capable of inducing each of the mechanisms described above. In more
general terms, the formation of a stable slurry is rendered more difficult to achieve
without the use of such a high shear mixing apparatus. In the preferred embodiment,
the shear mixing pump is external to the reactor and draws the slurry from the base of
the reactor and returns the sheared slurry to the top of the reactor. A smaller pump is
used to agitate the slurry in the reactor. It is observed that the reaction rate can be
moderated, if required, by the settings of the high shear mixing apparatus. It is
stressed that an objective of the current invention is to minimize the use of dispersion
agents and the like, because the prior art describes instances in which these agents
interfere with the applications of the slurries.
[0074] The comminution of the particles during the slurry production process has
been observed during the process by sampling and measuring the change of the
particle size distribution during the course of the reaction. It is believed that the
stability of high solids slurry is enhanced if the particle size distribution is broad.
This broadening has been observed during the slurry formation using the high shear
mixing apparatus, and is likely to positively contribute to the stability of the slurry.
Preferably, the particle size distribution of the raw feed should be a broad
distribution.
[0075] In summary, the mixing of the solids is preferably accomplished using a high
shear mixing apparatus that substantially dissipates concentration gradients,
agglomerates, steam bubbles and induces comminution.
[0076] All high solids magnesium hydroxide slurries exhibit non-Newtonian
viscoelastic properties, as shown by the formation of a gel, to some degree. The
requirement of the gelled slurry is that it exhibits little resistance to thinning, and to
that extent it can be classified as a thin slurry. During production, the slurry must be
agitated sufficiently to break down the gel structure so that the slurry can feed to the
high shear mixing apparatus, described above. Post production, the slurry must
exhibit a low resistance to shear thinning so that gentle agitation thins the slurry, to
enable the slurry to be pumped or poured for application. The means of thinning of
magnesium hydroxide slurries is well described in the prior art, and for high solids
fraction slurries, the approach of using a viscosity modifier or dispersion agent is
common to all the processes previously described. That is, the use of a viscosity
modifier is a factor to be considered independently of materials and method used to
form the slurry. The preferable viscosity modifier is one which is low cost, and
added in small amounts, typically <1%. The prior art shows that soluble salts are
commonly used for this role. It is noted that the solubility of magnesium hydroxide
is low, and at the pH of 10.4, the ionic strength of the water is not very high. A
preferred approach to increase the ionic strength is to use an acid, such as acetic acid,
which reacts essentially completely with the magnesium hydroxide to form
magnesium acetate ions, which act as the viscosity modifier. The pH of such a
slurry is lowered to about 9.5 as a result of the ionic strength, and this pH increases
back to 10.4 when the slurry is diluted.
[0077] The stability of the slurry is, as described in the prior art, an important
characteristic. Measurements during the production of the slurry show that the
stability of the slurry increases during the hydration process. That is, samples of
slurry extracted from the reactor during the early stages of hydration immediately
collapse, while samples taken at later stages take progressively longer to settle, and
towards the end of the reaction, the slurry does not settle on the timescale of months.
These characteristics do not apparently change during cooling of the sample. The
evolution of the slurry stability is a complex process that is linked to the degree of
hydration, the mixing process, especially shear, and the use of viscosity modifiers.
Importantly, there is no adverse effect of boiling water on the slurry characteristics
provided that the water content is managed to account for the loss.
[0078] The embodiment of the process shown in Figure 1 shows a batch reactor for
the production of a magnesium hydroxide slurry from PCCM. The batch process
starts with filling the reactor vessel 100 with preferably cold water 101, and then the
PCCM 102 is metered into the water, preferably over a 10-15 minute timescale. The
reactor is preferably insulated. The slurry 103 is stirred by a paddle 104 and a
portion of the material is sheared by a shear pump 105 after routing through valve
106 at the base of the reactor. The sheared slurry is returned to the reactor near the
top of the slurry surface at 107. As the reaction proceeds, the temperature of the
slurry in the reactor rises due to the exothermic hydration process. As the slurry
begins to gel, a viscosity modifier, such as acetic acid, 108 is metered into the reactor
the apparent viscosity, so that the amount of modifier is just sufficient to maintain the
apparent viscosity at a sufficiently low level that the paddle stirrer 104 and the share
pump 105 can operate within their specifications. In addition, as the reaction
proceeds, the water approaches the boiling point, slightly below 100 C, and steam
109 is ejected from the reactor through the stack 110. The mass loss of steam is
preferably measured. The viscosity, at one or more shear rates is measured, along
with the temperature, and the mass flow of slurry through the shear mixer. When the
reaction is nearly complete, the boiling ceases and the temperature in the reactor
begins to fall. Samples of the slurry may be taken and important properties, such as
the stability, /eta potential and the viscosity are measured to determine that the
reaction has progressed to the point that a thin, stable slurry have been obtained. The
shear pump is turned off, and the slurry 111 is drained from the reactor at the base
through valve 106. The slurry is preferably quenched during transport to a vessel
(not shown), in which the transport pipes and/or the vessel has either the required
heat capacity, or is cooled, so that the slurry rapidly cools to below 60 C. When
quenched in this manner, the thin, stable high solids slurry is formed with the
desirable attributes. The slurry can be left to cool to ambient temperatures.
[0079] Process control is thereby simplified, and costs reduced, by using boiling
point as a bound to control the process temperature, and optionally by using a simple
quenching mechanism to stop the reaction.
[0080] The simplicity of the process allows the establishment of transportable slurry
plants that can be conveniently located relative to the site of production of the
magnesium oxide powder, and the sites of consumption, to reduce the costs of
transporting slurries over long distances. For example the invention may be
embodied in a compact apparatus that can be stationed in processing plants that are
distant from the source of production of the magnesite powder.
[0081] The slurries can be produced from a wide variety of caustic calcined
carbonate and hydroxide materials, and mixtures thereof. Such slurries can have
substantially different chemical properties that depend on the surface area of the
powder.
[0082] Preferably, the slurry forming part of the first preferred embodiment of the
present invention may include a slurry of a predetermined viscosity. Specifically, the
viscosity of the slurry may be sufficient to allow for the slurry to be sprayed onto the
walls of a sewage pipe so as to coat the interior of the said pipe. Preferably, the
viscosity of the slurry is sufficiently high enough to allow the slurry to adhere to the
walls of the pipe without falling off, whilst maintaining a maintaining a viscosity low
enough to allow the slurry to be pumped and applied to the said walls by a pumping
apparatus or spraying machine.
[0083] In this specification, the word "comprising" is to be understood in its "open"
sense, that is, in the sense of "including", and thus not limited to its "closed" sense,
that is the sense of "consisting only of. A corresponding meaning is to be attributed
to the corresponding words "comprise, comprised and comprises where they appear.
[0084] It will be understood that the invention disclosed and defined herein extends
to all alternative combinations of two or more of the individual features mentioned or
evident from the text. All of these different combinations constitute various
alternative aspects of the invention.
-2R-
[0085] While particular embodiments of this invention have been described, it will be
evident to those skilled in the art that the present invention may be embodied in other
specific forms without departing from the essential characteristics thereof. The
present embodiments and examples are therefore to be considered in all respects as
illustrative and not restrictive, the scope of the invention being indicated by the
appended claims rather than the foregoing description, and all changes which come
within the meaning and range of equivalency of the claims are therefore intended to
be embraced therein. It will further be understood that any reference herein to known
prior art does not, unless the contrary indication appears, constitute an admission that
such prior art is commonly known by those skilled in the art to which the invention
relates.
[0086] Although the invention has been described with reference to specific examples, it
will be appreciated by those skilled in the art that the invention may be embodied in
many other forms, in keeping with the broad principles and the spirit of the invention
described herein.
[0087] The present invention and the described preferred embodiments specifically
include at least one feature that is industrial applicable.
THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:
1. A process for producing a hydroxide slurry from caustic calcined carbonate
powder or caustic calcined hydroxide powder, comprising the following steps:
a) mixing the powder with water in a reactor vessel and forming a
reaction mixture;
b) applying a shearing force to the reaction mixture using a mixing
apparatus;
c) allowing heat of hydration to raise the temperature of the reaction
mixture to near the boiling point, and
d) allowing steam to evaporate from the reaction mixture as hydration
proceeds, to remove excess heat and control the maximum reaction
temperature to by near to the boiling point.
2. The process of claim 1, wherein the process is adapted for the production of a
high solid fraction hydroxide slurry from the reaction mixture, wherein the
slurry has a relatively low resistance to shear thinning.
3. The process of claim 2, wherein the process additionally comprises the following
steps:
e) metering an input of a viscosity modifier to enable the mixing apparatus to
maintain uniform mixing under thin slurry conditions promoted by the
viscosity modifier;
f) allowing the reaction to proceed spontaneously, using evaporation to
balance the heat release , until the water temperature has reached a
maximum and the temperature dropped to a first set point; and
g) quenching the slurry to drop the temperature to a second set point.
4. The process of claim 3 in which the maximum temperature is the boiling point of
water in the reaction mixture.
5. The process of claim 3 wherein the term 'near to boiling point' means a first set
point in the range of 85-95°C.
6. The process of claim 3, wherein the powder includes ground particles and wherein
said ground particle distribution has a d9o of less than 100 microns.
7. The process of claim 6, wherein said ground particles have a particle size
distribution in the range of 0.1 to 150 microns.
8. The process of claim 3, wherein the slurry has a final solids content, after
accounting for the water loss from boiling, is in the range of 40-70%.
9. The process of claim 8, wherein the slurry has a final solids content, after
accounting for the water loss from evaporation, in the range of 55-60%.
10. The process of claim 3, in which additional water is added, if required, during the
process to ensure that the final solids fraction meets the solids fraction
specification of the slurry product.
11. The process of claims 1-10 wherein the process is adapted for the production
of a high solids fraction from the reaction mixture, wherein the slurry has
a relatively high concentration of chemically reactive species.
12. The process of claim 11 in which the concentration of reactive species in the
slurry is enhanced using caustic powers that have a surface area between
100-200 m7gm, and is further enhanced using caustic powders that have a
surface area in excess of 200 m2/am.
13. The process of claim 2, wherein the temperature of the water during step (a) is
within a range of 10-25°C.
14. The process of claim 2 in which the mixing apparatus comprises at least one high
shear mixer, and preferably such shear pump being external to the reactor vessel through
which the slurry is circulated by the pump action, and optionally further, a paddle or other
similar mixer is used to agitate the slurry in the reactor vessel.
15. The process of claim 2, wherein a device continuously operates steps (a)-(g) in a
predefined order.
16. The process of claim 1 wherein a viscosity modifier is added to the process to
maintain the apparent viscosity in the range of 60-300 cP during the process, where the
apparent viscosity is that of the slurry at a shear rate of preferably of 200 rpm.
17. The process of claim 2 , whereinthe first set point is in the range of about 85-
95°C, with the provision that heat losses in the reactor apparatus are sufficiently low that
the length of time to reach the set point is less than 60 minutes.
18.
19. The process of claim 2 or claim 11 wherein quenching is conducted by transport
of the slurry to a second vessel that has a temperature and heat capacity such that a
desired quenching temperature is achieved.
20. The process of claim 19, wherein the quenching of the slurry at the end of
processing is in the range of 10-60°C
2 1. The process of claim 19, wherein the quenching of the slurry at the end of
processing is 40°C.
22. The process of any one of the previous claims, wherein the process is adapted to
yield a high solids slurry which, after 1 month standing without agitation exhibits
syneresis of less than 5% of the height of the storage vessel, and a toe of less than 1% of
the height of the storage vessel, and which can be remixed and made to flow and pour by
mild agitation.
23. The process of anyone of claims 1 to 22, wherein the powder is calcined
limestone, magnesite or dolomite.
24. A reaction apparatus for producing a hydroxide slurry from a reaction mixture
of at least caustic calcined carbonate or caustic calcined hydroxide powder, and
water, wherein the reaction apparatus comprises:
a . a reaction vessel having a first inlet adapted for receiving caustic
calcined carbonate powder and a second inlet adapted for receiving
water and a controller that is adapted to electronically control the
process within the reaction vessel;
b. shearing apparatus positioned within the reaction vessel for shearing
the reaction mixture and wherein the rate of shearing is controlled by
the controller;
c . a viscosity sensor positioned within the reaction vessel adapted to
supply viscosity information about the reaction mixture to the
controller;
d . a temperature sensor positioned within the reaction vessel adapted to
supply viscosity information about the reaction mixture to the
controller; and
e . a steam outlet for release of steam from the reaction vessel, such that
in use the reaction is controlled by the controller so that the heat of
hydration may raise the temperature of the reaction mixture, allowing
water to boil off from the reaction mixture as hydration proceeds, and
removing steam via the steam outlet to remove excess heat and control
reaction temperature at boiling point.
25. A hydroxide slurry comprising: particles of caustic calcined carbonate
powder, and water; wherein the particles within the slurry have particle
size distribution in the range of 0.1 to 150 microns; and an apparent viscosity
in the range of 60-300 cP.
AMENDED CLAIMS
received by the International Bureau on 13 March 201 5 (13.03.201 5)
1. A process for producing a hydroxide slurry from caustic calcined carbonate
powder or caustic calcined hydroxide powder, comprising the following steps:
h) mixing the powder with water in a reactor vessel and forming a reaction
mixture;
i) applying a shearing force to the reaction mixture using a mixing apparatus;
j ) allowing heat of hydration to raise the temperature of the reaction mixture
to near the boiling point, and
) allowing steam to evaporate from the reaction mixture as hydration
proceeds, to remove excess heat and control the maximum reaction
temperature to near the boiling point.
2 . The process of claim 1, wherein the process is adapted for the production of a
high solid fraction hydroxide slurry from the reaction mixture, wherein the slurry has a
relatively low resistance to shear thinning.
3 . The process of claim 2, wherein the process additionally comprises the following
steps:
1) metering an input of a viscosity modifier to enable the mixing apparatus to
maintain uniform mixing under thin slurry conditions promoted by the
viscosity modifier;
m) allowing the reaction to proceed spontaneously, using evaporation to balance
the heat release , until the water temperature has reached a maximum and the
temperature dropped to a first set point; and
n) quenching the slurry to drop the temperature to a second set point.
4. The process of claim 3, wherein the maximum temperature is near the boiling point of
the reaction mixture.
5. The process of claim 4, wherein the term 'near the boiling point' means a first set
point in the range of 85-95°C.
6. The process of claim 3, wherein the powder includes ground particles and wherein
said ground particle distribution has a d of less than 100 microns.
7. The process of claim 6, wherein said ground particles have a particle size distribution
in the range of 0.1 to 150 microns.
8. The process of claim 3, wherein the slurry has a final solids content, after accounting
for the water loss from boiling, is in the range of 40-70%.
9. The process of claim 8, wherein the slurry has a final solids content, after accounting
for the water loss from evaporation, in the range of 55-60%.
10. The process of claim 3, in which additional water is added, if required, during the
process to ensure that the final solids fraction meets the solids fraction specification of the
slurry product.
11. The process of any one of the preceding claims, wherein the process is adapted
for the production of a high solids fraction from the reaction mixture, wherein the slurry
has a relatively high concentration of chemically reactive species.
12. The process of claim 11, wherein the concentration of reactive species in the
slurry is enhanced using caustic powers that have a surface area between 100-200 m /gm,
and is further enhanced using caustic powders that have a surface area in excess of 200
m /gm.
13 . The process of claim 2, wherein the temperature of the water during step (a) is within
a range of 10-25°C.
14. The process of claim 2, wherein the mixing apparatus comprises at least one high
shear mixer, and preferably such shear pump being external to the reactor vessel through
which the slurry is circulated by the pump action, and optionally further, a paddle or other
similar mixer is used to agitate the slurry in the reactor vessel.
15. The process of claim 3, wherein a device continuously operates steps (a)-(g) in a
predefined order.
16. The process of claim 1, wherein a viscosity modifier is added to the process to
maintain the apparent viscosity in the range of 60-300 cP during the process, where the
apparent viscosity is that of the slurry at a shear rate of preferably of 200 rpm.
17. The process of claim 3 , wherein the first set point is in the range of about 85-95°C,
with the provision that heat losses in the reactor apparatus are sufficiently low that the length
of time to reach the set point is less than 60 minutes.
18. The process of claim 3 or claim 11 wherein quenching is conducted by transport of
the slurry to a second vessel that has a temperature and heat capacity such that a desired
quenching temperature is achieved.
19. The process of claim 18, wherein the quenching of the slurry at the end of processing
is in the range of 10-60°C
20. The process of claim 18, wherein the quenching of the slurry at the end of processing
is 40°C.
21. The process of any one of the preceding claims, wherein the process is adapted to
yield a high solids slurry which, after 1 month standing without agitation exhibits syneresis of
less than 5% of the height of the storage vessel, and a toe of less than 1% of the height of the
storage vessel, and which can be remixed and made to flow and pour by mild agitation.
22. The process of any one of any one of the preceding claims, wherein the powder is
calcined limestone, magnesite or dolomite.
23. A reaction apparatus for producing a hydroxide slurry from a reaction mixture of
at least caustic calcined carbonate or caustic calcined hydroxide powder, and water,
wherein the reaction apparatus comprises:
f . a reaction vessel having a first inlet adapted for receiving caustic calcined
carbonate powder and a second inlet adapted for receiving water and a
controller that is adapted to electronically control the process within the
reaction vessel;
g . shearing apparatus positioned within the reaction vessel for shearing the
reaction mixture and wherein the rate of shearing is controlled by the
controller;
h . a viscosity sensor positioned within the reaction vessel adapted to supply
viscosity information about the reaction mixture to the controller;
i . a temperature sensor positioned within the reaction vessel adapted to
supply temperature information about the reaction mixture to the
controller; and
j . a steam outlet for release of steam from the reaction vessel, such that in
use the reaction is controlled by the controller so that the heat of hydration
may raise the temperature of the reaction mixture, allowing water to boil
off from the reaction mixture as hydration proceeds, and removing steam
via the steam outlet to remove excess heat and control reaction
temperature at boiling point.
24. A hydroxide slurry manufactured by the process of claim I , the hydroxide slurry
comprising: particles of caustic calcined carbonate powder, and water; wherein the
particles within the slurry have particle size distribution in the range of 0.1 to 150 microns;
and an apparent viscosity in the range of 60-300 cP.