FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION: SYNTHETIC GYPSUM
2. APPLICANT(S):
NAME: ACC Limited
NATIONALITY: Indian Company registered under The Companies
Act, 1913
ADDKESS: Cement House, 121 Maharshi Karve Road,
Mumbai 400 020, Maharashtra, India.
PREAMBLE TO THE DESCRIPTION: The following specification particularly describes the invention and the manner in which it is to be performed.
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FIELD OF INVENTION:
This invention relates to a method of manufacture of synthetic gypsum from lime rich material and spent/waste sulphuric acid.
BACKGROUND OF INVENTION:
Gypsum, which is essentially CaS04.2H20, is an important and essential component of cement and constitutes 4-5% of cement. It is added to regulate the quick setting reaction of cement, when it reacts with tri-calcium aluminate, in the presence of water, to form fine crystals (ettringite). The ettringite is responsible for the slow two stage setting of portland cement. In addition Calcium sulphate in the form of gypsum functions as an accelerator by increasing the rate of strength development of cement, and as a modifier of volume change characteristics of cement.
Over the last couple of years, there has been an increased demand for gypsum with increased cement production in the country. However with depletion of mineral gypsum reserves, the availability of marine/mineral gypsum is relatively limited. In addition, with increased transportation costs, the landed price of the mineral / marine gypsum have escalated considerably. Attempts have been made to use alternatives for gypsum such as phosphogypsum, borogypsum, titanogypsum, citrogypsum, fluorogypsum and other chemical gypsum to partially substitute mineral / marine gypsum as an additive in cement manufacture, but the level of impurities present in these gypsum affect the properties of cement, especially with respect to setting of cement for which purpose, gypsum is added. Some of them like titanogypsum which is of high quality is fine and wet, hence difficult to handle. Further, because of their limited availability, the usage is restricted.
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The other type of gypsum, which is commonly used is FGD gypsum, produced by flue gas desulphurisation (FGD) equipment at coal-fired power stations. This type of gypsum is very fine in nature and can have moisture ranging from 10% to 15%, which is not ideal for cement manufacture.
The present invention describes for the first time, aprocess for the manufacture of _ synthetic gypsum from spent/waste sulphuric acid and lime rich materials so as to make it a suitable substitute for mineral or marine gypsum for use in industrial and commercial applications. Spent / waste sulphuric acid is a hazardous waste product, and unless treated, is difficult to dispose off. The invention enables use of this hazardous waste sulphuric acid, in an industrially usable form. Synthetic gypsum made by this process is equivalent to mineral / marine gypsum in its performance in cement, in terms of setting characteristics and compressive strength development of cement. Ordinary Portland Cement (OPC), Portland Slag Cement (PSC) and Portland Pozzolana Cement (PPC) manufactured with synthetic gypsum manufactured by the said process, along with clinker, is functionally equivalent to that of OPC, PSC & PPC manufactured with clinker and mineral/ marine gypsum employed by most cement manufacturers.
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SUMMARY:
In its main aspect, the invention refers to a method of manufacturing synthetic gypsum from lime rich material and spent / waste sulphuric acid. By a process of nodulisation / pellitisation of lime rich material with a spray of spent / waste sulphuric acid, pellets are made and and conditioned at ambient temperature.
In another aspect of this invention, the lime rich material used is the lime rich waste containing at least 35 % CaCCh, from industry , in the form of pulp, sludge or effluent or lime rich material from one or more among limestone/ shale/ marble/ kilnfeed and for kiln dust. The concentration of sulphuric acid in spent / waste sulphuric acid ranges from 30% to 80%.
In yet another aspect of this invention, the nodulisation /pelletisation of the lime rich material is done in a pan noduliser / pellitiser. The nodule / pellet size ranges from 5-10 mm to 200 to 300 mm depending upon the nature of the lime rich material.
In a further aspect of this invention, the nodules / pellets are kept for conditioning for 10-12 hours. The nodules / pellets may be conditioned on a conveyor belt as they are taken to the site of storage or production or by aeration.
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DESCRIPTION:
The following is a brief description of working of this invention. In its main embodiment, synthetic gypsum is prepared from lime rich material and spent/ waste sulphuric acid by a process of nodulisation / pelletisation of lime rich material with a spray of spent / waste sulphuric acid. The lime rich material can also be the lime rich waste from industries like paper, steel, ceramic, sugar, carbide, fertiliser and fluoride, or can be even kiln dust. Lime rich material like limestone, shale, marble fines, kilnfeed or kiln ESP dust could also be used, and is not limited to the afore referred sources. The waste lime rich material may be in the form of pulp, sludge or effluent. The spent / waste sulphuric acid can be from industry, where sulphuric acid is formed as a by-product or waste product. In order to obtain a purity of at least 65% synthetic gypsum, and to be used industrially, where mineral or marine gypsum is used in cement manufacture in particular, the lime rich material should have a lime content of at least 35 %, (usually in the form of calcium carbonate or calcium hydroxide), and sulphuric acid should have a concentration of 30 to 80%. Nodules / Pellets are made and conditioned at ambient temperature.
In another embodiment of this invention, the nodulisation /pelletisation of the lime rich material is done in a pan noduliser / pellitiser. The nodules / pellet size ranges from 5-10 mm to 200 to 300 mm depending upon the nature of the lime rich material.
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In a further embodiment of this invention, the nodules / pellets are kept for conditioning for 10-12 hours. The nodules / pellets may be conditioned on a conveyor belt as they are taken to the site of storage or production, or conditioned by aeration. Conditioning would depend on concentration of acid, moisture content of the nodules / pellets and atmospheric conditions.
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During the process, the calcium carbonate from lime rich material reacts with spent
waste sulphuric acid to form calcium sulphate dihydrate (CaS04.2H20), hemi hydrate,
anhydrite forms, co-products and intermediates. The unreacted calcium carbonate in
the synthetic gypsum is in the range of 0.5% to 8%.
CaC03 + H2S04 ► CaS04.2H20 + 2C02
Depending on the type of lime rich material or substrate used, it is likely to have varied presence of various oxides, more particularly Si02, Al 203, Fe 203, and MgO in addition to CaO present as Ca(OH)2 or CaC03, as seen in Table 1. The CaC03, if present in the form of calcite in the lime rich material, is often present as unreacted calcium carbonate in the Synthetic gypsum . CaC03, is otherwise specifically added as a perfomance improver in cement manufacture, since it assists in the hydration of cement.
TABLE 1

% Oxides Lime richWaste formdifferentindustries Limestone Shale Marble fines Kilnfeed Kiln ESP dust
Si02 2.0-8.0 8.0-18.0 10.0-23.0 1-5 10-13 13-24
A1203 0.3-5.0 0.5-2.5 3.0-10.0 0.1-2 2.5-3.5 5-10
Fe203 0.1-4.0 0. 5-3.0 3.0-6.0 0.1-1 2.0-2.5 1.5-3
CaO 45.0-54.0 40.0-48.0 30.0-41.0 48-53 40.0-44.0 37-43.0
MgO 0.1-1.5 1.0-2.0 0.5-5.0 0.2-1.5 0.5-2.0 0.5-2.0
Loss onignitionat 1000°C 35.0-51.0 35.5-38.5 25-35 38-45 34.0-38.0 20.0-35.0
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The quality of synthetic gypsum from lime rich waste , like pulp from the paper industry and spent / waste sulphuric acid from industry, is in the range of over 90% purity as seen in example 1.
EXAMPLE 1.
QUALITY OF SYNTHETIC GYPSUM FROM LIME RICH WASTE AND SPENT / WASTE SULPHURIC ACID

Lime rich Spent/waste Synthetic Mineral/ IS-1290-
% Oxides waste from sulphuric gypsum marine 1973
paper industry acid gypsum requirement for gypsum
CaO 79.5 % — ~ ~ —
on loss
free basis
Sulphates 0.2 70% 46.0% 30.0-46.0% --
expressed
as % S03
%Pu irisynthetic] gypsum 96.0 70-95 70-75
The quality of synthetic gypsum from lime rich material like the kiln feed and spent waste sulphuric acid from industry, is also in the range of over 90 % purity as seen in example 2
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EXAMPLE 2.
QUALITY OF SYNTHETIC GYPSUM FROM KILNFEED AND SPENT / WASTE SULPHURIC ACID

% Oxides kilnfeed Spent/waste Synthetic Mineral / IS-1290-1973
sulphuric acid gypsum marine gypsum requirement for gypsum
CaO 75.4 % ~ ~ — —
On loss free
basis
Sulphates 0.01 65% . 44.8% 30.0-46,0% —
expressed
aas %S03
% Purity of synthetic gypsum 95.8 70-95 70-75
Depending on the source/location of spent/waste sulphuric acid industry, the synthetic gypsum manufactured by the above process is a cheap substitute for mineral /marine gypsum used in any industrial and commercial application.
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CLAIMS:
1. A method of manufacturing synthetic gypsum from lime rich material and spent waste sulphuric acid by nodulisation / pellitisation of lime rich material with a spray of spent / waste sulphuric acid to form pellets, and conditioning the nodules / pellets at ambient temperature.
2. A method as claimed in claim 1, where the lime rich material is one or more among lime rich waste , containing at least 35 % lime ( CaO), from industry in the form of pulp, sludge or effluent.
3. A method as claimed in claim 1, where in the lime rich material is one or more among limestone/ shale/ marble/ kilnfeed and for kiln dust
4. A method as claimed in claim 2 or 3, where in the concentration of sulphuric acid in spent / waste sulphuric acid ranges from 30% to 80%.
5. A method as claimed in claim 4, where in nodulisation /pelletisation is done in a pan noduliser / pellitiser
6. A method as claimed in claim 5, where in the nodules / pellet size ranges from 5-10 mm to 200 to 300 mm
7. A method as claimed in claim in 4 or 6 where in the pellets are kept for conditioning for 10-12 hours
8. A method as claimed in claim 7 where in the conditioning of the nodules / pellets is done on a conveyor belt or by aeration
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Dated this l4th day of November 2005 at Mumbai
For ACC Limited
Usha A. Chandrasekhar Patent Attorney
To,
The Controller of Patents,
The Patent Office,
Mumbai.
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ABSTRACT:
The invention describes a method of manufacturing synthetic gypsum from lime rich material and spent / waste sulphuric acid, by nodulisation / pellitisation of lime rich material with a spray of spent / waste sulphuric acid to form nodules / pellets, and conditioning the pellets at ambient temperature.
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