Claims:We Claim:

1. A process of recovering calcium as precipitated calcium carbonate (PCC) and iron as iron oxidefrom calcium containing feedstock including waste such as generated in steel making processes comprising:
extracting the calcium from the calcium containing feedstock following treating the same with 0.1-1.5M HCl solution such as to recover dissolved calcium in an acidic calcium containing solution by filtration;
neutralizing the thus obtained acidic calcium containing solution with a base thereby generating a basic calcium containing solution to precipitate out the iron content which is converted to iron oxide by roasting;
carbonizing the said basic calcium containing solution with carbon dioxide whereby carbon dioxide reacts to form bicarbonate which in turn reacts with some of dissolved calcium thereby forming calcium carbonate precipitate and finally recovering therefrom the said calcium carbonate precipitate by filtration.

2. The process as claimed in claim 1 wherein said calcium containing feedstock is selected from waste comprises steel converter slag preferably EOF slag, LF Slag, EAF slag, said HCL comprises industrial waste HCL and said base may, in particular, be an alkali, or hydroxide base, preferably, the base is ammonium hydroxide (NH OH).
3. The process as claimed in anyone of claims 1 or 2 wherein said calcium containing feedstock include steel converter slag preferably EOF slag of size range <1mm
4. The process as claimed in anyone of claims 1 to 3 wherein the EOF slag included has the composition range comprising by wt%: Si02: 20-35; CaO: 35-50; MgO: 5-10; MnO: 2-10; Al203 : 2-8; FeO: 25-40.
5. The process as claimed in anyone of claims 1 to 4 wherein the liquid to solid ratio maintained in the dissolution step is 6-7ml/g of steel slag
6. The process as claimed in anyone of claims 1 to 5 wherein the base used in neutralization step is ammonium hydroxide which is provided to increase the pH from 4 to 10 and to remove the impurities iron oxide.
7. The process as claimed in anyone of claims 1 to 6 wherein the Carbon di oxide (CO2) used with the flow rate of 0.3 – 4 liters per minutein the carbonation process which is obtained from flue gas generated in iron and steel making process.
8. The process as claimed in anyone of claims 1 or 7 wherein the product PCC is obtained in size < 2 micron which contributes 90%.
9. The process as claimed in anyone of claims 1 to 8 wherein the product iron oxide recovery percentage is 4-8 %.
10. The process as claimed in anyone of claims 1 to 9 wherein the precipitated iron is subjected to roasting in the temperature range of 400 -500 oC to obtain iron oxide.

Dated this the 12th day of April, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
A PROCESS OF RECOVERING CALCIUM AND IRON FROM ENERGY OPTIMIZING FURNACE SLAG.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company.

Address : Salem Works, Pottaneri P.O., Mecheri, Mettur Taluk, Salem District- 636453, Tamil Nadu, India;

Having the Regd. Office at:
JSW CENTRE, BANDRA KURLA COMPLEX, BANDRA(EAST), MUMBAI-400051, MAHARASHTRA, INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE








The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a process of recovering calcium as precipitated calcium carbonate(CaCO3) and iron as iron oxide from the industrial by-products with the usage of waste hydro chloric acid(HCL). More particularly, the invention relates to the usage of Energy Optimizing furnace slag and greenhouse gas (CO2) to precipitate CaCO3.This invention also relates to a precipitation of iron hydroxide by the addition of ammonium hydroxide and obtained the iron oxide by roasting process.The products developed are considered to be eco-friendly, techno-economically attractive and takes care of the waste generated in the process of steel making process.

BACKGROUND OF THE INVENTION
JSW Steel Salem Works is a one million tonne per annum integrated special steel plant producing wire rods, round bars and RCS (round-corner-square) blooms. Steel is refined in an energy optimising furnace (EOF) and secondary refining is carried out in a ladle heating furnace. The plant generates about 600 tonnes of steel slag including LHF (Ladle Heating Furnace) slag every day. The steel slag is crushed and the metallic-magnetic portion (amounting to about 150 tonnes) is recovered and partly recycled within the plant and partly sold to external customers. The remainder of the 450 tonnes of non-magnetic slag is screened and graded and about 300 tonnes in the size range of 4mm – 40mm is utilised by cement plants. This leaves approximately 150 tonnes per day of steel slag in the size range 0-4mm for which an outlet needed to be found to prevent it otherwise accumulating on site.

Also the increasing atmospheric concentration of CO attributed to fossil fuel combustion is a serious problem, which contributes significantly to global warming. It is estimated that CO emission by the year 2100 will be approximately four times greater than that in 2000. Therefore, finding a practical method of reducing CO2 emissions is paramount.

Evidently, Iron and steel industries produce many millions of tons annually of Solid waste (which has very little economic value) and greenhouse gases that pose a serious environmental impact. Accordingly, there remains a need in the art for a method of reducing such waste that is both economically and environmentally viable. As such, the invention described herein address therequirement for sequestration of environmentally harmful carbon dioxide and the usage of low value steel slag and industrial waste HCL and ammonium hydroxide to precipitate the value added calcium carbonate and the iron oxide.

The calcium carbonate produced in the mineral carbonation process could then possibly be used to meet the high standards of synthetic calcium carbonates, it could probably be sold at a higher price. Precipitated calcium carbonate (PCC) is used as a filler and coating pigment in plastics, rubbers, paints, and papers. It is over 96% pure calcium carbonate, manufactured conventionally from limestone by a carbonation process. Using steelmaking slags to replace the limestone used in PCC manufacturing would simultaneously save virgin material resources and refine a low-value by product slag into a value added product.

Synthetic calcium carbonate, or precipitated calcium carbonate (PCC), is today produced by three different processes: a lime-soda process, a calcium chloride process, and a calcination/carbonation process. In the lime-soda process, calcium hydroxide is reacted with sodium carbonate to produce a sodium hydroxide solution, from which the calcium carbonate is precipitated. In the calcium chloride process, calcium hydroxide is reacted with ammonium chloride, forming ammonia gas and a calcium chloride Solution. After purification, this solution is reacted with sodium carbonate to form a calcium carbonate precipitate and a sodium chloride solution. In the third, and most commonly used, production process, calcium oxide is hydrated with water, producing a calcium hydroxide slurry. The slurry is reacted with a CO-rich flue gas, from which the calcium carbonate is precipitated.The common PCC production processes require calcium oxide or -hydroxide as raw material, which is typically produced by calcining (i.e. burning) limestone, and causes significant CO2 emissions. The Virgin limestone used also needs to have low levels of impurities in order to affect the quality of the PCC.

PRIOR ART
1.Reference is made to WO 2012/095815 Al, wherein the invention relates to a process of producing precipitated calcium carbonate. The process includes, in a calcium dissolution step, contacting a solid particulate feedstock (BOF Slag), which contains calcium as a calcium compound in a calcium-containing matrix, with hydrochloric acid. The calcium is thereby dissolved from the matrix and is recovered as an acidic calcium-containing solution. In a neutralisation step, a base (Ammonium hydroxide) is admixed with the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution. In a carbonation step, the basic calcium-containing solution is contacted with carbon dioxide. The carbon dioxide reacts to form bicarbonate which, in turn, reacts with at least some of the dissolved calcium, thereby forming a calcium carbonate precipitate and a resulting calcium-depleted solution. In a calcium carbonate recovery step, the calcium carbonate precipitate is recovered from the calcium-depleted solution as a calcium carbonate product. However, the drawback of this prior art is it doesn’t discuss about the usage of EOF slag which have different chemical composition than BOF slag. Moreover, there is no mention about the recovery of iron oxide and also the usage of industrial waste hydrochloric acid in the present invention is economically effective than the prior art.

2. Reference is made to CN104828850Awhich recites a method of preparing high-purity light calcium carbonate with steel slagThe method is characterized by comprising following steps: (1) grinding the steel slag to enable free calcium oxide to expose on the surface of the steel slag; (2) with a wet method, extracting the calcium oxide from the steel slag with acetic acid; (3) performing filtration and clarification, feeding CO2 gas for carbonization; and (4) performing dehydration, washing, drying, cooling, smashing and screening to obtain the high-purity light calcium carbonate.However, the drawback of this prior art is it doesn’t discuss about the usage of EOF slag which have different chemical composition than BOF slag. Moreover, it uses the acetic acid which is not economical and no mention about the recovery of iron oxide.

3. Reference is made toEP2632855A1 wherein the invention relates to a process for the preparation of precipitated calcium carbonate comprising the steps of a) providing and calcining calcium carbonate comprising material; b) slaking the reaction product obtained from step a) with an aqueous ammonium chloride solution; c) separating insoluble components from the calcium chloride solution obtained from step b); d) carbonating the calcium chloride solution obtained from step c); e) separating the precipitatedcalcium carbonate obtained from step d); the precipitated calcium carbonate obtained by this process, as well as uses thereof. However, the drawback of this prior art is that it uses the naturally occurring resources such as sand and thus causes depletion of natural resources. Even though it discussed about the properties of final product, the addition of flocculants will be additional cost to this invention and the purity of product will be affected by this flocculants. Moreover, the solvent used is ammonium chloride which is another economically unattractive when it moves to commercial scale.

4. Reference is made to US 8,603,428 B2 which discloses a method for producing calcium carbonate from alkaline industrial waste or by-products, such as an iron- and steelmaking slag, while raising the concentration of valuable metals, such as Vanadium, in the waste or by-product, which can subsequently be extracted therefrom. However, the drawback of this prior art is that it uses ammonium acetate (CHCOONH) as a major solvent which is not economically viable when it moves to commercial production. Moreover, this invention does not discuss the recovery of final product PCC and the iron oxide. And also this process includes the extraction of vanadium which makes the process further complicate.

5. In Chinese patent application No. 1757597, a process for preparing porous superfine calcium carbonate is described. This isachieved by preparing an aqueous solution of calcium chloride, as well as, separately, an aqueous solution of ammonium hydrogen carbonate and carbon dioxide, andreacting these solutions in a colliding reaction, being a rather complex reaction in practise, while controlling flow and temperature, resulting in the formation of porous superfine precipitated calcium carbonate having a high specific surface area. However, the process according to CN 1757597 starts with a high quality starting material, wherein high porosity and high fineness particularly are achieved by the specific type of colliding reaction which is a complex process. Nomention is made in this document about the recovery of iron oxide.

6. In WO 2002/085788 is disclosed a process for mineral carbonation wherein carbon dioxide is reacted with a bivalent alkaline earth metal silicate, which silicate isimmersed in an aqueous electrolyte solution. It is mentioned that the residual compounds obtained after carbonisation, i.e. the mixture of carbonate and silica formed, can be used as filler in construction materials. Natural minerals suitable for carbonation can be found in abundance and should theoretically provide enough storagefacility to sequestrate all the carbon dioxide produced worldwide. When a carbon dioxide sequestration process is located near a mineral production site, the transportcost is low, since the mineral carbonate formed could be stored in used mining pits. However, exploitable mineral resources are generally located far from the placewhere the carbon dioxide is produced and where it would preferentially be sequestrated. This can lead to high transportation cost for both the reactant and the formedmineral, affecting the industrial applicability of the process.

7. Reference is made to Arshe Said, published in Applied Energy 112 (2013) 765–771, wherein a process of producing precipitated calcium carbonate (PCC) from steelmaking slag is disclosed. Calcium was extracted selectively from the slag with aqueous solution of ammonium salt (NH4NO3, CH3COONH4 or NH4Cl) in an extraction reactor. After removal of the residual slag, the calcium-rich solution reacted with CO2 in a carbonation reactor producing PCC. Based on the experimental results, the slag’s grain size has a clear effect on the calcium extraction efficiency; the smaller the steel converter slag’s grain size, the larger the surface area, and the better the mass transfer rate which in turn results in a higher extraction efficiency. Grinding to smaller sizes is therefore one strategy towards improved efficiencies and chemical conversion rates. Solid to liquid ratio is another important parameter for improving extraction efficiency. However, the drawback of this prior art is, the inventor suggests to maintain low solid to liquid ratio 5g/l and this option will be operationally expensive because of larger reactor volumes. Moreover,it uses ammonium salt as a major solvent which is not economically viable when it moves to commercial production.

8. Reference is made to Sanni Eloneva, publishedin Ind. Eng. Chem. Res. 2008, 47, 7104–7111, studied the possibility of utilizing steel converter slag, a byproduct of steel manufacturing, for production of pure calcium carbonate. The dissolution of calcium from steel converter slag by using acetic acid as a solvent and the precipitation of pure calcium carbonate from the resulting solution were experimentally investigated. It was found that, while strong solutions of acetic acid dissolve most of the calcium from the slag, weak acetic acid solutions dissolve calcium selectively. Precipitation of any substantial amount of calcium carbonate was found to require the addition of sodium hydroxide. Calcium conversion from the solution intothe precipitate was as high as 86%, and the purity of the precipitate was over 99%. The calcium carbonate particles produced at low temperatures were found to have rhombohedral particle shapes, a very high brightness (98.7%), and a small particle size (mean value of 0.6 μm).However, the drawback of this prior art is that it uses acetic acid as a major solvent which is not economically viable when it moves to commercial production. Moreover, it doesn’t discuss about the impurities extracted during the carbonation process.

Thus, in short it may be summarized that the prior art processes have the following limitations:
a) The production cost of Precipitated Calcium Carbonate (PCC) is relatively high when it uses costly acetic acid, ammonium chloride, ammonium acetate
b) Theusage of flocculants to get the high purity level of PCC
c)The production of PCC by using virgin resources and causes depletion of natural resources.
d)No mention to recover iron as value added iron oxide.

The present invention is thus directed to provide PCC which would be produced by utilizing steel plant wastes instead of consuming naturally occurring material, and uses the industrial waste hydrochloric acid as a major solvent. By using ammonium hydroxide the precipitation of iron hydroxide is obtained and then by roasting iron oxide is produced. For carbonation CO2 generated in iron and steel making process can be utilized.

OBJECTS OF THE INVENTION

The basic object of the present invention is thus directed to provide a process to recovering calcium as precipitated calcium carbonate(PCC) and iron oxide from abundantly available waste materials such as EOF slag and industrial waste HCL which eliminates the drawback of known process.

A further object of the present invention is to provide a process of recovering calcium as PCC using CO2 gas from in plant flue gas.

A still further object of the present invention is to provide a process of recovering calcium as PCC which is high purity and desired particle shape which is equivalent to the conventional PCC produced from natural resources.

Yet another object of the present invention is to provide a process of producing PCC, which improves the techno economic properties of the product.

A further object of the present invention is to provide a process of recovering calcium as PCC and iron as iron oxide, whereinammonium hydroxide is used to precipitate the iron.

Yet another object of the present invention is directed to a process wherein the precipitated iron is subjected to roasting to obtain iron oxide from the EOF slag.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a process of recovering calcium as precipitated calcium carbonate (PCC) and iron as iron oxide from calcium containing feedstock including waste such as generated in steel making processes comprising:
extracting the calcium from the calcium containing feedstock following treating the same with 0.1-1.5M HCl solution such as to recover dissolved calcium in an acidic calcium containing solution by filtration;
neutralizing the thus obtained acidic calcium containing solution with a base thereby generating a basic calcium containing solution to precipitate out the iron content which is converted to iron oxide by roasting;
carbonizing the said basic calcium containing solution with carbon dioxide whereby carbon dioxide reacts to form bicarbonate which in turn reacts with some of dissolved calcium thereby forming calcium carbonate precipitate and finally recovering therefrom the said calcium carbonate precipitate by filtration.

A further aspect of the present invention is directed to said process wherein said calcium containing feedstock is selected from waste comprises steel converter slag preferably EOF slag, LF Slag, EAF slag, said HCL comprises industrial waste HCL and said base may in particular, be an alkali, or hydroxide base, preferably, the base is ammonium hydroxide (NH OH).
A further aspect of the present invention is directed to said process wherein said calcium containing feedstock include steel converter slag preferably Energy Optimizing furnace slag (EOF) having size <1mm.

A still further aspect of the present invention is directed to said process wherein the EOF slag included has the composition range comprising by wt%: Si02: 20-35; CaO: 35-50; MgO: 5-10; MnO: 2-10; Al203 : 2-8; FeO: 25-40.

A still further aspect of the present invention is directed to said process wherein the liquid to solid ratio maintained as 6-7 ml/g of steel slag in the dissolution step.
A still further aspect of the present invention is directed to said process wherein the base used in neutralization step is ammonium hydroxide which is provided to increase the pH from 4 to 10 and to remove the impurities iron oxide.
A still further aspect of the present invention is directed to said process wherein precipitated calcium carbonate having particle shape of common calcite and vaterite structure with 90% of size range between 2-4 microns.
A still further aspect of the present invention is directed to said process wherein steel slag include other metallic compound iron(lll)oxide (Fe2O). These iron oxide gets precipitated in the neutralization step as iron hydroxide while adding the base ammonium hydroxide and it is removed through filtration. Then this iron hydroxide is subjected to roasting (400 0C) to obtain value added iron oxide.
A still further aspect of the present invention is directed to said process whereinin the carbonation step industrial flue gas carbon di oxide generated in iron and steel making processis used with the flow rate of 0.3 – 4 liters per minute.
A still further aspect of the present invention is directed to said process wherein the carbonation step carried out with the time duration of about 20 minutes to 40 minutes.

Another aspect of the present invention is directed to said process wherein the precipitated iron is subjected to roasting in the temperature range of 400 -500 oC to obtain iron oxide.
Yet another aspect of the present invention is directed to said process wherein the product iron oxide recovery percentage is 4-8 %.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawing and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: illustrates a detailed Flowchart of the process steps involved in the production of precipitation of calcium carbonate and iron oxide with steel slag.
Fig.2: shows laboratory setup of experiments.
Fig.3: shows calcium extraction in 1.5MHCl.
Fig.4: shows filtrate analyses in 1.5MHCl.
Fig.5: shows Spherical vaterite particles 2500x.
Fig.6: shows XRD analyses of PCC.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The present invention relates to provide precipitated calcium carbonate with the usage of steel slag, industrial waste hydro chloric acid and the carbon dioxide from flue gas.The invention comprises of four steps comprises i) Dissolution step ii) Neutralization step + Recovery of iron oxide iii) carbonation step and iv) Recovery step
EOF slag is generated in the process of primary steel making. The generation of EOF slag is huge and is expected to further rise. Till now, there is no commercial process for utilization of EOF slag. The present invention provides a process which can utilize EOF slag for the preparation of PCC and iron oxide which is having the potential for commercialization. The products developed by this process are ecofriendly, techno-economically attractive and will take care of waste generated in steel making processes.Following Table: 1 shows the XRF analysis of typical EOF Slag indicating its constituents in wt%:
Table 1:
CaO MgO Fe2O3 SiO2 Al2O3 MnO P2O5 TiO2 Na2O SO3 Cr2O3 V2O5
38.1 8.4 25 16.0 2.5 6 2.1 0.5 0.2 0.3 0.4 0.1

The waste HCl is generated in the chemical industry which having very low cost and value and it’s an added advantage in this invention.
Carbon di oxide is obtained inside the plant itself which is generated from flue gas from iron and steel making process.
Hence the present invention comprises the following process.
i)Dissolution step: contacting a solid particulate feedstock, which contains calcium as a calcium compound in a calcium-containing matrix, with hydrochloric acid (HCI), thereby dissolving the calcium from the matrix, and recovering the dissolved calcium in an acidic calcium-containing solution
ii) Neutralisation step:ammonium hydroxide baseis added to the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution. Here the iron gets precipitated and subjected to roasting (400 oC) to obtain iron oxide
iii) Carbonation step: contacting the basic calcium-containing solution with carbon dioxide, such that the carbon dioxide reacts to form bicarbonate which, in turn, reacts with dissolved calcium, thereby forming a calcium carbonate precipitate and a resulting calcium-depleted solution;
iv)Recovery step, recovering the calcium carbonate precipitate from the calcium-depleted solution.
Accompanying Figure 1 illustrates a detailed Flowchart of the process steps involved in the production of precipitation of calcium carbonate and iron oxide with steel slag.

EXAMPLE:
The following example are provided by way of illustration of the process according to present invention and therefore should not be construed to limit the scope of the present invention.
Extraction of calcium from steel converter slag was investigated by dissolving steel converter slag in HCL solutions of different concentrations. The steel slag used in the experiments having size <1mm.
The experimental setup (Figure 2) contained a 1000 mL glass reactor, in which the different solution was stirred by a magnetic stirrer (650 +-50 rpm). The glass reactor was heated by using a hot plate along with stirrer. 150 grams of the steel converter slag, was added to the reactor containing HCL 1.5M HCl at 30 °C, 40 °C, 50 °C were tried. To observe the progression of the dissolution, residue samples were taken from the solution 5,10, 20, 30, 45,60, 90, and 120 min after the slag was added. To analyze the composition of the residual (i.e., undissolved) slag, this residue was dried in the oven at ∼120 °C and sent to XRF. The solution samples were filtered (Whatman 3, 1 μm membrane) and analyzed for Ca, Fe by titration method.
The maximum dissolution of calcium from the steel slag was observed in 1.5M HCl solution, nearly CaO % in slag reduced to 26.69% from 38.1 % and the rate of dissolution achieved is 0.026. (Figure 3).
The filtration solution was analysed to find out the amount of Ca and Fe dissolved in the solution. The filtration carried out by the following steps.
Determination of Calcium by Standardized EDTA solution:
Step 1: 50ml from filtrate solution
Step 2: Indicator – Patent rador in solution
Step 3: Add 10ml NaOH in solution to adjust pH -7
Step 4: Add 10ml 8.5Ml NH3-NH4Cl
Step 5: Titrate solution with the EDTA solution until the color changes from Wine red, through purple, to a pure rich blue color. At the end point, the last Traces of purple in the solution will have just disappeared. Bythe use of end point value, the Ca dissolved can be calculated.
Nearly 16 gm /l, Ca got dissolved in 1.5M HCl solution, but at the same time Fe also got dissolved (8 gm/l) in that solution. (Figure 4).
The Fe dissolved is separated in the neutralization step by adding ammonium hydroxide in the solution and it is separated by filtration and obtained a clear solution. The separated residue is subjected to roasting (400 oC) and obtained iron oxide.

Precipitation Experiment
The solution obtained from the dissolution method were used for the calcium carbonate precipitation experiment. Nine batches of solution were taken from 1.5 M HCL dissolution experiment with temperature 30°C, 40°C, 50°C and time of 60,90 and 120 mins. The experimental setup contained a glass reactor (1000ml) with one end connected to Carbon dioxide cylinder through a flow rate controller, allowing CO2bubbled through the solution.
Normally when the liquid is acidic no reaction will occur. Since the obtained solution from 1.5M HCl dissolution experiment had the pH value – 4, itis treated with NH4OH in order to increase the pH to 9. Due to the presence of Fe in the solution, precipitation occurs when NH4OH poured in the solution, but it is an advantage to get high purity calcium carbonate. Around 100 ml of NH4OH (1:1) were used for 1000 ml of dissolution experiment solution to increase the pH to 9 as well as to complete removal of Fe. This step is to ensure to get the pure calcium carbonate without contamination. Then the solution is heated to 40oC in order to catalyst the reaction.
Intermediate addition of NH4OH helps to obtain more calcium carbonate. Because once the CO2 bubbles were passed the pH level will reduce due to increase in HCl in the solution (equation 2). Two or three times NH4OH (20 ml) can be added intermediately to increase the pH which in turn results in more calcium carbonate formation.
Precipitation of Iron oxide:
FeCl3 + NH4OH ↔ Fe(OH)3 +NH4Cl (1)
The formation of CaCO3 shown in below equation,
CaCl2+ H2O + CO2↔ CaCO3 + 2HCl (2)
CO2 (g) ↔ CO2 (aq) (3)
CO2 (aq) + H2O (l) ↔ H2CO3 (aq) (4)
Ca2+ (aq) + H2CO3– (aq) → CaCO3 (s)↓ + 2H+ (5)
Results of Precipitation experiment:
Calcium dissolution efficiency and efficiency of carbonation can be calculated as the following
Calcium dissolution efficiency (%) = (Ca in extraction filtrate/Ca in feed sample) x100 (6)
Calcium dissolution efficiency (%) = (16/38)X 100  42%

Efficiency of carbonation
Ca + CO2 ↔ CaCO3 (7)

By stoichiometric 14 gm of Ca can produce 35 gm of CaCO3.This is well achieved in this experimental value of 38 gm of CaCO3 with purity level of 99%. Hence efficiency of carbonation is 100%.

In this experiment the filtrate solution is heated to 35 – 40oC and the final crystal structure we obtained is spherical vaterite (Figure 5). The size of the particles ranges from 2-4 microns.
To find out the purity the obtained PCC is analysed in XRD and it is greater than 96% (Figure 6).

It is thus possible by way of the present invention to provide Precipitated calcium carbonate(PCC)made of steel plant waste materials, waste hydrochloric acid and carbon di oxide from flue gas and a method for making PCC with purity level greater than 96% and the common shape of calcite and vaterite required for pulp industry. Hence by this invention solid waste generated in the steel industry would be taken care and in the point of emerging greenhouse gas, the CO2 utilized in this process is a unique technology to reduce the same.