Indian Patents. 277139:AN IMPROVED PROCESS FOR THE PRODUCTION OF SODIUM CHROMATE FROM STAINLESS STEEL DUST

Title of Invention	AN IMPROVED PROCESS FOR THE PRODUCTION OF SODIUM CHROMATE FROM STAINLESS STEEL DUST
Abstract	The present invention relates to an improved process for the production of sodium chromate from stainless steel dust. This invention is particularly relates to an improved soda ash roasting process by the use of chromium containing waste. This process will be useful for production of high purity sodium chromate used for chemical and leather industry.

Full Text	FIELD OF THE INVENTION The present invention relates to an improved process for the production of sodium chromate from stainless steel dust. This invention is particularly relates to an improved soda ash roasting process by the use of chromium containing waste. This process will be useful for production of high purity sodium chromate used for chemical and leather industry. BACKGROUND OF THE INVENTION Conventionally chromium is recovered from chromite ore by soda-ash roasting method. Reference may be made of V.D.Tathavadkar, M.P.Antony and A.Jha, The soda-ash roasting of chromite minerals: Kinetics consideration, Metallurgical Transactions B, August 2001, Vol. 32B, pp 593-602. After roasting with sodium carbonate, the calcine is leached in hot water. The sodium chromate comes into the solution. It is purified and crystallized to get pure sodium chromate crystal. In an another process sodium dichromate has been prepared using soda roasting of chromite ore. Reference may be made of R.Weber, B.Rosenow, H.D.Block and N.Lonnoff, Process for preparation of sodium dichromate, U.S.patent no. 5273735, 1993, wherein in a furnace, chromite ore was roasted with soda. After roasting the calcine was leached in hot water to get pure sodium dichromate. In a process to recover chromium from chromite bearing ore, reference may be made of Guy W. Lussiez, Chromite recovery from chromium bearing ore, U.S.patent no. 4012484, 1977, wherein ore was leached with strong sulphuric acid at 100°C in presence of manganese dioxide. The leach liquor containing chromium was recovered. In the soda-ash roasting, chromium containing waste is roasted with sodium carbonate at 800°C for 90 minutes. After roasting, the roasted product is leached in hot water to dissolve sodium carbonate formed. The leach liquor is then evaporated to make a concentrated solution of sodium chromate and finally crystallization is done to get pure sodium chromate. Instead of sodium carbonate, sodium hydroxide was also used by the inventors. Reference may be made of D.Chandra, G.X.Wang, M.C.Fuestenar and R.E.Siemens, Alkaline roasting of low grade chromite. Trans. Instn. Min. Metall (Sect C: Mineral process. Extra. Metall), 105, May-August, 1996, pp C105-C112. In this process a low temperature (450-650°C) process was developed by using sodium hydroxide to produce sodium chromate. LACUNAS OF THE PRIOR ART i) The formation of sodium chromate with chromite ore produces low recovery, ii) Chromite ore, a hard material, can not be used in the form of lumps in roasting and require crushing and grinding, iii) Several other impurities like silica, MgO, Al2O3 and TiO2 are associated with the chromite ore which effects the purity of sodium chromate produced, iv) In leaching process with sulphuric acid large amount of acid is generated which causes handling problem, v) All the above processes are high capital cost and high energy intensive. The novelty of the present invention lies in the use of chromium containing waste which is used as raw material to produce high purity sodium chromate. The purity of sodium chromate is ranging between 99 to 99.9%. OBJECTIVES OF THE INVENTION The main object of the present invention is to provide an improved process for the production of sodium chromate from stainless steel dust. The raw material is a chromium containing waste which is hazardous in nature and difficult to throw in the environment and thereby process is reducing the environment problem. Another object of the present invention is to convert sodium chromate from this waste and solve the environmental problem to produce a value added product. Still another object of the present invention is to produce high purity sodium chromate containing negligibly small contamination like Fe, Si, Mg, Al and Ti. SUMMARY OF THE INVENTION In the process of present invention stainless steel dust is taken into a crucible and mixed with sodium carbonate in the ratio of stainless steel dust to sodium carbonate is 2:1. The mixture is roasted in a furnace of controlled temperature at 800°C for 90 minutes of duration. After roasting the calcine is leached with hot water for 60 minutes at 1:5 pulp densities. The diluted solution of sodium chromate is evaporated on hot plate to the concentrated level of crystallization. By cooling the concentrated solution a yellow crystalline mass is obtained which is sodium chromate. DETAILED DESCRIPTION OF THE INVENTION Accordingly, the present invention provides an improved process for the production of sodium chromate from stainless steel dust generated by steel processing plant which comprises: (a) providing stainless steel dust, screening it to the size in the range of 100-150 mesh and removing moisture by a known method; (b) mixing stainless steel dust as obtained in step (a) with sodium carbonate in the molar ratio ranging between 2:1 to 3:1 to get a mixture; (c) roasting the mixture as obtained in step (b) at a temperature in the range of 700 to 900°C in a furnace for 60 to 120 minutes to obtain a roasted product containing sodium chromate; (d) cooling the roasted product as obtained in step (c) to a temperature in the range of 25°Cto30°C; (e) leaching the roasted product as obtained in step (d) in hot water maintaining the solid liquid ratio in the range of 1:5 to 1: 10 for a period in the range of 60 to 90 minutes to obtain a leached liquor ; (f) filtering and crystallizing the leached liquor as obtained in step (e) by known process to obtain pure sodium chromate. In an embodiment of the present invention, the stainless steel dust used is comprising of: Cr : 5-20%, Fe : 80-90%, Ni: 0.5-5.0%, Mn: 0-5.0%. In an embodiment of the present invention, the leaching of roasted product is done in hot water at a temperature in the range of 50 °C to I00°C. In yet another embodiment of the present invention, the purity of sodium chromate is ranging between 99 to 99.9%. In still another embodiment of the present in invention the recovery of the sodium chromate is ranging between 90 to 99%. In the process of present invention stainless steel dust is taken into a crucible and mixed with sodium carbonate in the ratio of stainless steel dust to sodium carbonate is 2:1. The mixture is roasted in a furnace of controlled temperature at 800°C- 900 °C for 60- 90 minutes of duration. At this temperature only chromium oxide present in the stainless steel dust reacts with the sodium carbonate. After roasting the calcine is leached with hot water for 60-90 minutes at 1:5 to 1:10 pulp densities. The diluted solution of sodium chromate is evaporated on hot plate to the concentrated level of crystallization. By cooling the concentrated solution a yellow crystalline mass is obtained which is sodium chromate. Therefore, the present invention makes use of chromium wastes, where sodium chromate is formed by roasting with sodium carbonate according the following reactions, (Formula Removed) The sodium chromate thus formed is soluble in hot water and after crystallization pure sodium chromate is obtained. During the operation of this process iron containing residue is obtained which is free from hazardous chromium and used for landfill. The following examples are given by way of illustration and should not be construed to limit the scope of the invention. Example - 1 In an alumina crucible 10 g of stainless steel dust containing 7.72% Cr, 3.55 % Mn, 1.71% Ni, 1.36% silica and rest of iron with particle size of 100 mesh was taken. It was mixed with 5 g of AnalaR grade sodium carbonate. It was heated in a muffle furnace at 800°C. At this temperature the mixture was kept for 90 minutes. After cooling to room temperature (25 °C) the roasted product was taken out from furnace. It was then leached in hot water (90°C) for 60 minutes. After leaching solid liquid separation was done. The leached liquor obtained was evaporated on hot plate. The solution was then kept for crystallization and pure sodium chromate was obtained. A fraction of sodium chromate was dissolved in hydrochloric acid and analyzed with the help of atomic absorption spectrometer to check the purity and found that it is 99% pure with respect to chromium content. The overall recovery of chromium by this process was found 95%. Example - 2 In this experiment 20 g of same stainless steel dust containing 7.72% Cr, 3.55 % Mn, 1.71% Ni, 1.36% silica and rest of iron with particle size of 150 mesh was taken in a crucible. It was mixed with 7 g commercial grade sodium carbonate. The mixture was heated in a furnace at 850°C for 60 minutes. After cooling to room temperature (28 °C) the roasted product was taken out from furnace. The roasted product obtained was leached with hot water at a temperature 90°C for 60 minutes and a yellow solution of sodium chromate was obtained. After evaporation and crystallization sodium chromate crystal of 99.2% purity was obtained. The maximum recovery of chromium by this method was found 95.5%. Example - 3 10 g of stainless steel dust containing chromium was mixed with 4 g of sodium carbonate. It was heated in a furnace at 800°C. At this temperature the mixture was kept for 120 minutes. After roasting, the furnace was cooled to room temperature (30 °C) and solid calcine was obtained. It was ground and leached with hot water at a temperature 90 °C for 90 minutes. Sodium chromate formed was dissolved in water and leach liquor was obtained. After crystallization the pure yellow sodium chromate was obtained. It was then analyzed for purity and found that it is 99.5% pure. The overall recovery of chromium was found 96%. THE MAIN ADVANTAGES OF THE PRESENT INVENTION: 1. In this process hazardous waste containing chromium is used as raw material for the production of value added product, sodium chromate. 2. All industrial wastes containing chromium can be used by this process. 3. The energy consumption in this process is less than the conventional process of sodium chromate formation. 4. High purity of sodium chromate is obtained with negligibly small contamination with iron and nickel. 5. The process is simple to operate and hence lower plant cost. We claim: 1. An improved process for the production of sodium chromate from stainless steel dust generated by steel processing plant which comprises: (a) providing stainless steel dust, screening it to the size in the range of 100-150 mesh and removing moisture by a known method; (b) mixing stainless steel dust as obtained in step (a) with sodium carbonate in the molar ratio ranging between 2:1 to 3:1 to get a mixture; (c) roasting the mixture as obtained in step (b) at a temperature in the range of 700 to 900°C in a furnace for 60 to 120 minutes to obtain a roasted product containing sodium chromate; (d) cooling the roasted product as obtained in step (c) to a temperature in the range of 25 °C to 30 °C; (e) leaching the roasted product as obtained in step (d) in hot water maintaining the solid liquid ratio in the range of 1:5 to 1: 10 for a period in the range of 60 to 90 minutes to obtain a leached liquor ; (f) filtering and crystallizing the leached liquor as obtained in step (e) by known process to obtain pure sodium chromate. 2. An improved process as claimed in claim 1, wherein the stainless steel dust used is comprising of: Cr : 5-20%, Fe : 80-90%, Ni: 0.5-5.0%, Mn: 0-5.0%. 3. An improved process as claimed in claim 1, wherein the leaching of roasted product is done in hot water at a temperature in the range of 50 to 100°C. 4. An improved process as claimed in claim 1, wherein the purity of sodium chromate is ranging between 99 to 99.9%. 5. An improved process as claimed in claim 1, wherein the recovery of the sodium chromate is ranging between 90 to 99%. 6. An improved process for the production of sodium chromate from stainless steel dust generated by steel processing plant is substantially described with reference to the examples.

Full Text

FIELD OF THE INVENTION
The present invention relates to an improved process for the production of sodium chromate from stainless steel dust. This invention is particularly relates to an improved soda ash roasting process by the use of chromium containing waste. This process will be useful for production of high purity sodium chromate used for chemical and leather industry.
BACKGROUND OF THE INVENTION
Conventionally chromium is recovered from chromite ore by soda-ash roasting method. Reference may be made of V.D.Tathavadkar, M.P.Antony and A.Jha, The soda-ash roasting of chromite minerals: Kinetics consideration, Metallurgical Transactions B, August 2001, Vol. 32B, pp 593-602. After roasting with sodium carbonate, the calcine is leached in hot water. The sodium chromate comes into the solution. It is purified and crystallized to get pure sodium chromate crystal.
In an another process sodium dichromate has been prepared using soda roasting of chromite ore. Reference may be made of R.Weber, B.Rosenow, H.D.Block and N.Lonnoff, Process for preparation of sodium dichromate, U.S.patent no. 5273735, 1993, wherein in a furnace, chromite ore was roasted with soda. After roasting the calcine was leached in hot water to get pure sodium dichromate.
In a process to recover chromium from chromite bearing ore, reference may be made of Guy W. Lussiez, Chromite recovery from chromium bearing ore, U.S.patent no. 4012484, 1977, wherein ore was leached with strong sulphuric acid at 100°C in presence of manganese dioxide. The leach liquor containing chromium was recovered.
In the soda-ash roasting, chromium containing waste is roasted with sodium carbonate at 800°C for 90 minutes. After roasting, the roasted product is leached in hot water to dissolve sodium carbonate formed. The leach liquor is then evaporated to make a concentrated solution of sodium chromate and finally crystallization is done to get pure sodium chromate.
Instead of sodium carbonate, sodium hydroxide was also used by the inventors. Reference may be made of D.Chandra, G.X.Wang, M.C.Fuestenar and R.E.Siemens, Alkaline roasting of low grade chromite. Trans. Instn. Min. Metall (Sect C: Mineral process. Extra. Metall), 105, May-August, 1996, pp C105-C112. In this process a low temperature (450-650°C) process was developed by using sodium hydroxide to produce sodium chromate.
LACUNAS OF THE PRIOR ART
i) The formation of sodium chromate with chromite ore produces low recovery,
ii) Chromite ore, a hard material, can not be used in the form of lumps in roasting
and require crushing and grinding, iii) Several other impurities like silica, MgO, Al2O3 and TiO2 are associated with
the chromite ore which effects the purity of sodium chromate produced, iv) In leaching process with sulphuric acid large amount of acid is generated
which causes handling problem, v) All the above processes are high capital cost and high energy intensive.
The novelty of the present invention lies in the use of chromium containing waste which is used as raw material to produce high purity sodium chromate. The purity of sodium chromate is ranging between 99 to 99.9%.
OBJECTIVES OF THE INVENTION
The main object of the present invention is to provide an improved process for the production of sodium chromate from stainless steel dust. The raw material is a chromium containing waste which is hazardous in nature and difficult to throw in the environment and thereby process is reducing the environment problem.
Another object of the present invention is to convert sodium chromate from this waste and solve the environmental problem to produce a value added product.
Still another object of the present invention is to produce high purity sodium chromate containing negligibly small contamination like Fe, Si, Mg, Al and Ti.
SUMMARY OF THE INVENTION
In the process of present invention stainless steel dust is taken into a crucible and mixed with sodium carbonate in the ratio of stainless steel dust to sodium carbonate is 2:1. The mixture is roasted in a furnace of controlled temperature at 800°C for 90 minutes of duration. After roasting the calcine is leached with hot water for 60 minutes at 1:5 pulp densities. The diluted solution of sodium chromate is evaporated on hot plate to the concentrated level of crystallization. By cooling the concentrated solution a yellow crystalline mass is obtained which is sodium chromate.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention provides an improved process for the production of sodium chromate from stainless steel dust generated by steel processing plant which comprises:
(a) providing stainless steel dust, screening it to the size in the range of 100-150 mesh and removing moisture by a known method;
(b) mixing stainless steel dust as obtained in step (a) with sodium carbonate in the molar ratio ranging between 2:1 to 3:1 to get a mixture;
(c) roasting the mixture as obtained in step (b) at a temperature in the range of 700 to 900°C in a furnace for 60 to 120 minutes to obtain a roasted product containing sodium chromate;
(d) cooling the roasted product as obtained in step (c) to a temperature in the range of 25°Cto30°C;
(e) leaching the roasted product as obtained in step (d) in hot water maintaining the solid liquid ratio in the range of 1:5 to 1: 10 for a period in the range of 60 to 90 minutes to obtain a leached liquor ;
(f) filtering and crystallizing the leached liquor as obtained in step (e) by known process to obtain pure sodium chromate.
In an embodiment of the present invention, the stainless steel dust used is comprising of: Cr : 5-20%, Fe : 80-90%, Ni: 0.5-5.0%, Mn: 0-5.0%.
In an embodiment of the present invention, the leaching of roasted product is done in hot water at a temperature in the range of 50 °C to I00°C.
In yet another embodiment of the present invention, the purity of sodium chromate is ranging between 99 to 99.9%.
In still another embodiment of the present in invention the recovery of the sodium chromate is ranging between 90 to 99%.
In the process of present invention stainless steel dust is taken into a crucible and mixed with sodium carbonate in the ratio of stainless steel dust to sodium carbonate is 2:1. The mixture is roasted in a furnace of controlled temperature at 800°C- 900 °C for 60- 90 minutes of duration. At this temperature only chromium oxide present in the stainless steel dust reacts with the sodium carbonate. After roasting the calcine is leached with hot water for 60-90 minutes at 1:5 to 1:10 pulp densities. The diluted solution of sodium chromate is evaporated on hot plate to the concentrated level of crystallization. By cooling the concentrated solution a yellow crystalline mass is obtained which is sodium chromate.
Therefore, the present invention makes use of chromium wastes, where sodium chromate is formed by roasting with sodium carbonate according the following reactions,
(Formula Removed)
The sodium chromate thus formed is soluble in hot water and after crystallization pure sodium chromate is obtained.
During the operation of this process iron containing residue is obtained which is free from hazardous chromium and used for landfill.
The following examples are given by way of illustration and should not be construed to limit the scope of the invention.
Example - 1
In an alumina crucible 10 g of stainless steel dust containing 7.72% Cr, 3.55 % Mn, 1.71% Ni, 1.36% silica and rest of iron with particle size of 100 mesh was taken. It was mixed with 5 g of AnalaR grade sodium carbonate. It was heated in a muffle furnace at 800°C. At this temperature the mixture was kept for 90 minutes. After cooling to room temperature (25 °C) the roasted product was taken out from furnace. It was then leached in hot water (90°C) for 60 minutes. After leaching solid liquid separation was done. The leached liquor obtained was evaporated on hot plate. The solution was then kept for crystallization and pure sodium chromate was obtained. A fraction of sodium chromate was dissolved in hydrochloric acid and analyzed with the help of atomic absorption spectrometer to check the purity and found that it is 99% pure with respect to chromium content. The overall recovery of chromium by this process was found 95%.
Example - 2
In this experiment 20 g of same stainless steel dust containing 7.72% Cr, 3.55 % Mn, 1.71% Ni, 1.36% silica and rest of iron with particle size of 150 mesh was taken in a crucible. It was mixed with 7 g commercial grade sodium carbonate. The mixture was heated in a furnace at 850°C for 60 minutes. After cooling to room temperature (28 °C) the roasted product was taken out from furnace. The roasted product obtained was leached with hot water at a temperature 90°C for 60 minutes and a yellow solution of sodium chromate was obtained. After evaporation and crystallization sodium chromate crystal of 99.2% purity was obtained. The maximum recovery of chromium by this method was found 95.5%.
Example - 3
10 g of stainless steel dust containing chromium was mixed with 4 g of sodium carbonate. It was heated in a furnace at 800°C. At this temperature the mixture was kept for 120 minutes. After roasting, the furnace was cooled to room temperature (30 °C) and solid calcine was obtained. It was ground and leached with hot water at a temperature 90 °C for 90 minutes. Sodium chromate formed was dissolved in water and leach liquor was obtained. After crystallization the pure yellow sodium chromate was obtained. It was then analyzed for purity and found that it is 99.5% pure. The overall recovery of chromium was found 96%.
THE MAIN ADVANTAGES OF THE PRESENT INVENTION:
1. In this process hazardous waste containing chromium is used as raw material for the production of value added product, sodium chromate.
2. All industrial wastes containing chromium can be used by this process.
3. The energy consumption in this process is less than the conventional process of sodium chromate formation.
4. High purity of sodium chromate is obtained with negligibly small contamination with iron and nickel.
5. The process is simple to operate and hence lower plant cost.

We claim:
1. An improved process for the production of sodium chromate from stainless steel
dust generated by steel processing plant which comprises:
(a) providing stainless steel dust, screening it to the size in the range of 100-150 mesh and removing moisture by a known method;
(b) mixing stainless steel dust as obtained in step (a) with sodium carbonate in the molar ratio ranging between 2:1 to 3:1 to get a mixture;
(c) roasting the mixture as obtained in step (b) at a temperature in the range of 700 to 900°C in a furnace for 60 to 120 minutes to obtain a roasted product containing sodium chromate;
(d) cooling the roasted product as obtained in step (c) to a temperature in the range of 25 °C to 30 °C;
(e) leaching the roasted product as obtained in step (d) in hot water maintaining the solid liquid ratio in the range of 1:5 to 1: 10 for a period in the range of 60 to 90 minutes to obtain a leached liquor ;
(f) filtering and crystallizing the leached liquor as obtained in step (e) by known process to obtain pure sodium chromate.
2. An improved process as claimed in claim 1, wherein the stainless steel dust used is
comprising of:
Cr : 5-20%, Fe : 80-90%, Ni: 0.5-5.0%, Mn: 0-5.0%.
3. An improved process as claimed in claim 1, wherein the leaching of roasted product is done in hot water at a temperature in the range of 50 to 100°C.
4. An improved process as claimed in claim 1, wherein the purity of sodium chromate is ranging between 99 to 99.9%.
5. An improved process as claimed in claim 1, wherein the recovery of the sodium chromate is ranging between 90 to 99%.
6. An improved process for the production of sodium chromate from stainless steel dust generated by steel processing plant is substantially described with reference to the examples.

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Patent Number

277139

Indian Patent Application Number

673/DEL/2010

PG Journal Number

47/2016

Publication Date

11-Nov-2016

Grant Date

11-Nov-2016

Date of Filing

22-Mar-2010

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	SANJAY PRASAD	NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR, -831007 JHARKHAND, INDIA

PCT International Classification Number

C01G37/14;

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA