FIELD OF THE INVENTION
This invention relates to a process for the production of low silica concentrates, from
low grade dolomite ore deposits or fines. The low silica concentrates can be used in iron
and steel making processes. The invention further relates to agglomeration of these
dolomite fines by using briquetting subsequent beneficiation by using dry air
classification technique followed by flotation.
BACKGROUND OF THE INVENTION
Dolomite is a carbonate mineral with composition of calcium magnesium carbonate. The
proportion of CaCO3 and MgCO3 varies as per mining zones and geological environment.
Dolomite is used in many industries such as additive for thermoplastic, powder coating,
ceramics, reinforced polyester glass fiber, glass industry etc. Further, dolomite is used
as hardening agent for rubber industry, source for magnesium oxide, flux in the steel
industry as an impurity remover, decorative and dimension/ornamental stone. It is also
used for de-dusting in coal mines and conditioning soil to "sweeten" acidic soils and
make them more alkaline or neutral. Out of above mentioned applications, use of
dolomite in iron and steel making area plays a significant role on the utilization front of
dolomite.
Commonly lump or the coarse size of the dolomite with silica content less than 4% is
used as a flux in the iron and steel making industry. For generating or achieving these
sizes, mining and product sizing viz. crushing and screening are carried out. Due course
of these processes, huge quantities of fines (< 3-10 mm) are produced and this
contains higher content of siliceous gangue minerals which is detrimental to the user
industries.

Patent US4372843A discusses about a method of beneficiating phosphate ores
containing Dolomite. The process involves testing the ore for excessive levels of
carbonate impurities, the use of conventional flotation steps to remove silica, grinding
the ore to a particle size in which at least 90% of the ore particles are less than 0. 355
mm and thereafter reagentizing the ground phosphate containing ore with water, a
carbonate collector, a phosphate depressant and a pH regulator to obtain a flotation
feed having a solids level of about 20-30% and a pH of about 5. 6-6.0.
Patent publications US4636303A, US4648966A, US4600505A, from Tennessee Valley
Authority, discusses about beneficiation of phosphate ores. However, none of the
above specified references discuss about use and recovery of the fines generated
during the dolomite crushing process. These fines are wastes for the industry and also
creates environmental concerns. In the literature, there is no such comprehensive
process to utilise these fines from waste dump ore low grade deposit.
OBJECTS OF THE INVENTION
An object of the present invention is to propose a process for the production of low
silica dolomite concentrate from low grade dolomite fines.
Another object of the invention is to develop an agglomeration process of low silica
dolomite concentrate from low grade dolomite fines.
Another object of this invention is to reduce the silica content in the dolomite fines.
Further object of this invention is to propose a dry air classification and flotation process
for minimizing the silica content from the low grade dolomite ores or fines.

SUMMARY OF THE INVENTION
Accordingly, there is provided a process for utilisation of high silica dolomite fines to
produce low silica concentrate and an agglomeration process, comprising the steps:
selective classification of dolomite fines by using dry air classifier to produce the coarser
product of size greater than 0.8 mm with low silica content, subjecting the finer fraction
to a grinding mill to produce the particles of size less than 0.2 mm, further subjecting
the grounded products to flotation process, and mixing the coarser product of the
classifier and flotation concentrate.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Process flow diagram for the process of production of low silica dolomite
concentrate from fines.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure 1, the inventive beneficiation and agglomeration process of
minimizing the silica content from low grade dolomite ore or fines consists the steps of
dry classification, flotation and briquetting. The dolomite fines have a particle size of
less than 10 mm and the silica concentration in the dolomite fines varies in the range of
8 to 15 wt. %.
The generated dolomite fines are fed to an air classifier (2) for classifying the fines,
from which coarser product can be collected as coarser product (3) whereas another
product is collected as fines (4). The fines are fed to a grinding mill (5) and then
subjected for the separation of silica in a flotation (6) process. From the floatation

process, the low silica concentrate (7) is collected as product and tailing (8) with high
silica is either discarded or stored for future utilization. Further, the collected products
from classifier (3) and flotation concentrate (7) are mixed and the final product (10) can
be used as a flux in the iron and steel making industry with or without briquetting (9).
As depicted in Figure 1, a method is provided to produce low silica dolomite fines from
low grade dolomite ore or fines. The process handles the coarser particles of maximum
particle size having 10 mm. Further, 80% (by weight) of the fines have a particle
diameter in the range of 0.3-6 mm. The low grade dolomite ore or fines contain
dolomite, calcite, quartz, feldspar as the primary minerals along with minimum amount
of pyrite and others. The dolomite fines or low grade ore contains silica in the range of
8-15% with CaO (20-28%), MgO (15-21%) and loss on ignition (LOI) of 35 to 43%.
The low grade dolomite ore or fines of above mentioned composition are fed in to the
air classifier (2) to classify the feed into two fractions. The classification of the fines is
targeted to classify the material in the particle size range of 0.8-3 mm (cut size or D50).
The dry classifier as per the current invention is a gravitational type dry classifier or an
inertial type of centrifugal dry classifier or combination of both. The classifier
parameters are adjusted to an optimum level for minimum amount of ultra-fines (below
0.1 mm) to the coarser product. The ultrafine content in the coarser product (3) (i.e.
misplacement of ultra-fines) is less than 12%. The silica content in the coarser product
(3) is in the range of 3 to 7% with a yield of 20-45% whereas silica content of the finer
fraction is in range of 10 to 20%.
The fines (4) generated from classifier are subjected to grinding (5) for reducing the
particle size to less than 0.212 mm with 80% of mass with particle size in between 0.2
to 0.1 mm. The grinding can be carried out either in the wet or dry. Further, the
product from the grinding mill is fed to a flotation (6) process to separate the silicate

bearing gangue minerals. The flotation process can be carried out as direct or reverse
method. The conditioning of the feed material was carried out at a higher pulp density
i.e. the solid concentration in the range of 35-45% solids (by wt.) in water media for 3-
5 minutes. In case of direct flotation, the oleic acid or sodium olate can be used as
collector to float the dolomite from the low grade dolomite ore or fines. The collector
dosage can be varied from 0.3 to 0.7 kg/t of ore. Similarly depressing agent to depress
the silicates in the slurry was maintained by adding sodium silicate at optimised level in
the range of 0.2 to 0.5 kg/t of ore. By adding these reagents slurry was conditioned for
another 3-5 minutes. After conditioning, pulp density of the slurry can be maintained in
the range of 10 to 25% solids by wt. and thereafter, air was allowed in to tank and
froth can be collected. The froth product can be collected and sink can be drained as
another product. In case of direct floatation process, pH of the slurry is maintained in
the range of 8 to 9.
Similarly reverse flotation can be carried out by floating selectively only silicate bearing
gangue minerals from dolomite by maintaining pH at optimum level in between 4-7.
Different amine based collectors such as dodecyl amine can be used as a collector. The
optimum dosage of collector is in the range of 0.2-0.5 kg/t of ore. From the flotation, a
concentrate (7) with silica content in the range of 1 to 2.5% can be achieved by
discarding the tailing (8) product with silica level in between 25 to 40%. The silica level
of the final product is in the range of 2.5 to 4% and is varied in between 50 to 70%.
The coarse product (3) of air classifier and concentrate (7) product of flotation is mixed
and composite product with silica content in between 3 to 5% is achieved. The
composite final product can be mixed with sinter or pelletisation feed as a flux. Also, the
composite product can be agglomerated by mixing with binders such as: lime or
molasses. The briquettes were prepared with different particle size range, binder
concentration, compaction load, and at different firing temperature. It was found that a

good quality briquette of CCS> 350 kgf can be produced with particle size of < 3-10
mm, binder combination of molasses of 3-7% by wt. and lime of 2-3% by wt. along
with 3 ton of compaction pressure and firing temperature of 650-750 ° C. Similar
experiments on the dolomite fines were carried out as mentioned in the flow sheet.
As mentioned in Fig.l, the feed material is subjected to gravitational and inertial
classifier. The test results along with their process conditions are given in below Table3.
Test 1 and 3 are carried out in a gravitational air classifier whereas test 2 was in
gravitational-inertial type air classifier. Both the classifiers are of 6 inch diameter of
laboratory unit.

The fines generated from the classifiers are subjected to ball mill and grounded for 3
minutes. D80 size (particle size where 80% by wt. passes) after grinding for the test 1, 2
and 3 were205 μm, 200 μm and 180 μm respectively. The grounded products from the
air classifier are subjected to flotation to separate the silicate bearing gangue minerals.
Flotation test No. 1 and 2 are carried out as direct method where dolomite is floated by
depressing silicate bearing minerals. Test No. 3 is carried out as reverse method to float

silica gangue minerals by discarding dolomite in the sink of the cell. The test results
along with their process conditions are given in Table 2. Experiments were carried out
by adding the required amount of water and dolomite fines into the flotation cell to
adjust the mentioned pulp density. As per the prerequisite, collector and depressing
agent (in case of direct flotation) was added by measuring the pH. In case of reverse
flotation, pH was adjusted by adding the acid (either HCI or H2S04) into the slurry. After
conditioning for the mentioned time, air is allowed to flow (4 lpm/25 cm3 volume).
During that time, froth was collected and make-up water was added to fill the volume.
After collection of the products, it was allowed to dry and analysed for yield and grade.

Further, the coarse product of air classifier and flotation concentrate are mixed and
subjected for briquetting for better utilization of the dolomite fines. The briquette has
cold compression strength in the range of 350 to 450 kgf. The conditions and the
obtained results for briquetting are given in Table 3.
Table 3: Results and experimental conditions of briquetting process

Note: The wt. % mentioned against the molasses and lime is with respect to the feed.
The current method of the invention reduces silica level from the dolomite fines for
effective utilization. Further, the invention opens up possibility of utilization of low grade
siliceous dolomite fines in the iron and steel industry. The invention of course is useful
from environment perspective as it ensures effective utilization of high silica dolomite
fines produced from the dolomite processing plant. This results in avoiding the
environmental hazard such as storage of these plant waste. The process combines
further dry and wet processes which decreases the water consumption for the
processing.

WE CLAIM :
1. A process of reducing silica content in dolomite fines, the process comprising:
- classifying the dolomite fines to achieve a cut size (D50) in the range of 0.8
to 3 mm;
- grinding the finer product to reduce particle size in the range of 0.2 to 0.1
mm;
- flotating the grounded product by maintaining solid concentration in the
range of 30-45 wt.% in a water media and adding a collector; and
- mixing concentrate collected from the flotation step and coarser fraction from
the classification step.

2. The process as claimed in claim 1, wherein the dolomite fines have a particle size of
less than 10 mm.
3. The process as claimed in claim 1, wherein silica concentration in the dolomite fines
varies in the range of 8 to 15 wt. %.
4. The process as claimed in claim 1, wherein the dolomite fines are classified in a dry
air classifier.
5. The process as claimed in claim 1 and claim 4, wherein the dry air classifier is a
gravitational type dry classifier.
6. The process as claimed in claim 1 and claim 4, wherein the dry air classifier is an
inertial type of centrifugal dry classifier.
7. The process as claimed in claim 1, wherein the grounded product is floated using a
direct floatation process.

8. The process as claimed in claim 7, wherein the collector concentration varies in the
range of 0.3 to 0.7 kg/t of ore.
9. The process as claimed in claim 8, wherein the collector is sodium olate or oleic
acid.
10. The process as claimed in claim 1 to claim 9, further comprising the step of adding a
depressant.
11. The process as claimed in claim 10, wherein the depressant is sodium silicate.
12. The process as claimed in claim 1 to claim 11, wherein pH is maintained in the
range of 8 to 9.
13. The process as claimed in claim 1, wherein the grounded product is floated using a
reverse floatation process.
14.The process as claimed in claim 13, wherein the collector is dodecyl amine.
15. The process as claimed in claim 14, wherein dodecyl amine is used in the
concentration range of 0.2 to 0.5 kg/t.
16. The process as claimed in claim 13, wherein pH is maintained in the range of 4 to 7.
17.The process as claimed in any of the preceding claims, wherein concentration of
silica in the concentrate after the floatation process is in the range of 1 to 2.5 wt.%.
18. The process as claimed in any of the preceding claims, wherein concentration of
silica in the final product after mixing the concentrate collected from the flotation
process and coarser fraction from the classification step is in the range of 2.5 to 4
wt.%.

19. The process as claimed in claim 1 further comprising the step of briquetting,
wherein the dolomite fines are mixed with molasses in the concentration range of 3-
7 wt. % . and lime in the concentration range of 2-3 wt. %. along with 3 to 3.5 ton
of compaction pressure and firing temperature in the range of 650 to 750 °C.
20. The process as claimed in claim 19, wherein the briquette has cold compression
strength of at least 350 kgf.
21. The process as claimed in claim 19, wherein the briquette has cold compression
strength in the range of 350 to 450 kgf.

ABSTRACT

The present invention is related to a process for the production of low silica concentrate
from the low grade dolomite ore or fines. Further, invention also provides an
agglomeration process for the utilisation of such fines. The process of the current
invention comprises of selective dry classification, flotation, and briquetting of dolomite
fines by minimizing the silica level to <4% from low grade fines containing 8-15% silica.
The low silica containing product obtained from the current process can be
agglomerated by mixing with binders such as: lime or molasses. The briquettes were
prepared with different particle size range, binder concentration, compaction load, and
at different firing temperature. A good quality briquette of CCS> greater than 350 kgf
can be produced with particle size of 3-10 mm, binder combination of molasses of 3-7%
(by wt.) and lime of 2-3% (by wt.) along with compaction pressure in the range of 3 to
3.5 ton and firing temperature of 650-750 °C.