OUTLINE FOR CALCIUM CARBONATE PROCESS
1.
INTRODUCTION
Calcium carbonate is an important inorganic chemical for a variety of chemical industries. It is basically an Extender which enhances the properties of end product by virtue of its characteristics and offers cost advantages also. The crystal structure, shape and density are measure characteristics used for various applications thus giving a choice of variety of grades that can be manufactured. The main raw material for producing Precipitated Calcium Carbonate (P.C.C.) is Limestone of high purity (>98%).
2. APPLICATIONS
Various applications of P.C.C. are given below: i.
In natural and synthetic rubber for tyre/tube industry and in Hawai chappal - to improve strength as well as whiteness.
ii.
In Plastics - for PVC pipes, cables, profiles - as a filler to improve impact strength and other physical & electrical properties.
iii.
In Dentrifice industry - as cleaning and polishing agent (tooth paste).
iv.
In Paints - for improving opacity and whiteness.
v.
In Paper Industry - for brightness, opacity and ink receptivity.
vi.
In Pharmaceuticals Pharmaceutical s - as calcium source, as an antacid and as a bulking agent.
vii.
In foods & beverages - as calcium supplement and filteration aid.
viii.
In Sealant & Maskets - as rheology modifier.
ix.
In Cosmetics - to control absorption characteristics & increase fluffiness.
x.
In Printing inks - to control strength & body of the ink.
The specifications of each grade are altered/controlled during reaction to achieve required particle size, density, shape of particles, viscosity of powder, oil absorption and flow point, etc.
3. PROPERTIES
Structure
Calcitic/Aragonitic
CaCO3 %
98
Density (gpl)
400 - 600
Average Particle Size( µ)
2 - 10
Viscosity (cps)
3000 - 60000
Moisture (%)
<1
Surface area (m2/g)
10 - 20
Grits (%)
<0.1
Brightness %
99
Whiteness %
98
4. PROCESS DESCRIPTION
The process for the manufacture of CaCO3 consists of following stages: i.
Calcination of Limestone
ii.
Slaking/Hydration of Quick Lime
iii.
Carbonation of Lime Slurry (Calcium Hydroxide)
iv.
Sieving/water leaching (decantation)
v.
Drying
The process utilizes evolved CO2 from Lime Kilns for carbonation. Limestone is presently
from
Himalayan
ranges
mainly
from
Himachal
Pradesh
Mines
(Sataun/Renuka).
CALCINATION
Calcination is done at Vertical Shaft Kilns. Limestone of 4"/6" size is fed to these Kilns. Producer gas is used as fuel for Calcination which is obtained from Gas producers. The Gas producers are charged with mixed fuel (Pearl Coke and Charcoal). The flue gas from each kiln is drawn through a series of Water Scrubber, Soda Scrubber/Roots Blower and sent for carbonation. The composition of Producer Gas CO
:
25-27%
CO2 :
4-6%
O2
:
1%
H2
:
10%
N2
:
Balance
Typical analysis of Kiln gas CO2 :
25-28%
O2
:
3%
N2
:
Balance
SLAKING (HYDRATION OF QUICK LIME)
Lime drawn from kilns is charged into lime slakers in a batch process. Water is taken into slakepot and batch of lime is charged in (2) lots. After completion of reaction, slurry is diluted with water and pumped into process block through a vibratory 150 mesh sieve. Concentration and temperature of Lime Slurry are adjusted as per set conditions for various grades of Calcium Carbonate.
CARBONATION OF LIME SLURRY
Carbonation on reaction is carried out in Cylindrical/Rectangular wooden/Mild steel barrels provided with Agitators. The Kiln gas is drawn by a Roots Blower (exhauster) through a water scrubber/soda ash scrubber. This is done to wash and cool the gas to our room temperature. Washed gas is then fed to carbonation barrels. Carbonation is carried out in batch process by taking measured quantity of Lime Slurry and reacting it upon Kiln Gas in the wooden barrel. Particle size and shape of various grades are controlled by the following factors.
(1)
Lime Slurry Temperature/Concentration
(2)
Agitator Speed
(3)
Gas flow
(4)
Gassing Time
(5)
Leave (quantity of gassed slurry left out in barrel for seeding purpose in some grades. In other grades barrel is thoroughly washed and cooled after completion of every charge).
Primarily, two crystal forms of Calcium Carbonate are produced (i) the needle like Aragonitic form based upon arthorhombic structure (ii) calcitic barrel shaped based upon hexagonal rhombic structure.
SIEVING/DECANTATION (FILTERATION)
Chalk slurry is passed through 325 mesh screen and sent for settling. In case of Precipitated grades, the chalk (CaCO3) slurry is stored in settlement tanks for a particular time, after that top layer is drained out by decantation and thick slurry is pumped to Film Dried feed tank after giving a thorough agitation. For coated grades, their slurry is coated with a solution of Sodium Stearate at a particular condition to make 1.5/3.0% coated grades. Coated slurry is filtered through High Pressure Filter to produce thick cake having 35% moisture.
DRYING
Precipitated grades are dried in steam heated drum dryers (Film dryers). Chalk slurry is fed from one side through wet trough and from other side of drier, the dry material is removed by Doctor's knife. Finally, the product is conveyed to Silos after passing through vibratory screen and roller magnets for final packaging in HDPE/PP bags.
In case of Coated grades, the Calcium Carbonate cake discharged from the Press Filter is fed into a back mixer where some dry powder is mixed with to make it free flowing and suitable for feeding into Thermoventury Dryer. Hot air is generated in air heater by burning superior Kerosene Oil. This hot air is fed to drying plant (Thermoventury Dryer) where pulverizing and drying of coated Calcium Carbonate taken place. Outlet of drying unit is led to cyclone, where larger particles are separated and taken for packing into 50 kg bags. Finer particles along with moist air goes into a Reverse PulseJet type bag filter for collecting the product (Finer) and letting out clean air with combustion gases. Finer particles from R.P. Jet Silos are separately packed into 50 kg bags. The crystalline form of particles, size and degree of aggregation are regulated by varying the process conditions giving many different grades designed to meet the various needs of customers, viz. Plastics, Rubbers, Surface Coating, Paper, Pharmaceuticals, Cosmetics, Dentrifice, Food & Beverages, Sealants, Printing Inks, etc.
5. QUANTITATIVE REQUIREMENTS
Basis
1 ton of Calcium Carbonate
Limestone
1.2 MT
Coke/Charcoal
250 kgs
Power
220 KWH
Water
5 KL
Diesel for drying
150 lts
Economic Plant Capacity
60 tons per day
6. ECONOMICS OF PRODUCTION
Land
10000 sq metres
Building
3000 sq metres covered area
Plant & Machinery
7 crores
Power
1000 kVA
The breakeven points comes to 60% of plant capacity utilization and internal rate of return works out to 27%. 7. MEASURE PROBLEMS
a. Selection of highly efficient equipments b. Nearness to source of limestone c. Nearness to source of water d. Abrasion of pipelines and gas lines.
8. MARKET SCOPE
Worldwide precipitated calcium carbonate stands at 6 million tons/year. Present capacity in India is 3 lac tons i.e. only 5% of total capacity. With the growth in Industrial sectors mentioned in application, there exists good scope of adding up the capacity. Also, special grades have good market for Export to far east and middle east countries. The demand of calcium carbonate has good relation with the growth of population which can be considered while estimating the requirements.
9. MAJOR PRODUCERS IN INDIA
a. M/s Citurgia Biochemical Ltd., Rishikesh b. M/s Lime Chemical, Roha c. M/s Gulshan Chemicals, Muzaffanagar d. M/s Kavery Chemicals, Sahoranfia e. M/s Dewan Chemicals, Meerut f. M/s Sai Chemicals, Surat g. M/s Indochem, Bhiwani h. M/s Chemical & Solven, Jodhpur 10. MAJOR EQUIPMENTS
a. Lime kilns b. Slakers c. Reactors d. Flash dryers e. Refrigeration plant f. Boiler g. Vacuum filters h. Rotary sieves i.
Settlement tanks
11. NEW PRODUCT INFORMATION
a. Paper filling grade - for writing papers
b. Paper coating grade - For special fax paper c. Adhesives d. Oil base paints e. Microbiological free grades Above grades are under production in Europe and need to be developed for meeting local demand and exports to middle east/far east.
12. CONCLUSIONS
A state of art plant for Precipitated Calcium Carbonate with ISO Certification, Quality Circles, having good Automation and well equipped laboratory, supported by a Pilot Plant for development is likely to have a good potential in Indian market giving boost to different industries and meet our present import demand of Calcium Carbonate.
