Title of Invention

"AN IMPROVED PROCESS FOR THE PRODUCTION OF ELECTROLYTIC ZINC POWDER"

Abstract An improved process for the production of zinc powder from zinc dross waste generated by steel galvanising plant which comprises: (i) crushing zinc dross to size in the range of 2-3" and keeping in a porous bag for use as anode, (ii) dipping anode and cathode in a electrolytic bath of sodium zincate of pH in the range of 8-12. (iii) passing current to the said electrolytic bath in the range between 5 to 15 Amps and voltage in the range of 3 to 5V for a period ranging between 2-6 hours to deposit zinc powder in the cathode, (iv) removing zinc powder from cathode, washing and drying thereafter by known method to obtain pure zinc powder, characterised in that the crushed zinc dross kept in porous bag used as anode..
Full Text The present invention relates to an improved process for the production of electrolytic zinc powder from zinc wastes. This invention is particularly relates to an improved electrolytic process for the production of zinc powder from zinc dross generated by steel galvanising plant. This invention will be usefiil for producing electrolytic grade zinc powder used for paint/pigment, chemical and non-ferrous industry.
BACKGROUND OF THE INVENTION
Conventionally zinc is produced by roasting - leaching - electrowinning method. The zinc concentrate is roasted in a fluidised bed reactor. Reference may be made of K.H. Heino, R.T. McAndrew, N.E.Ghata, Fluid bed roasting of zinc concentrate at Canadian electrolytic zinc limited, Valleyfield, American Institute of Mining and Metallurgical Engineers Symposium on Mining and Metallurgy of Lead and Zinc, Vol. 2, P. 157, 1970, wherein the roasted product is leached in sulphuric acid. The leach liquor after purification is electrowon in the electrolytic cell. In the electrolytic cell a series of rectangular cells, usually constructed of PVC sheets supported by a steel frame is used. In the cell the cathodes are made of aluminium sheet and anodes are made of cast or rolled lead sheet. The temperature of the electrolytic cell is generally maintained in the range of 30-40'C.
In an another process zinc powder is produced by zinc-air elecfrochemical cell. Reference may be made of US patent no. 4,038,458 dated 26 July 1977, wherein a cell comprises on several unitary cell blocks each containing two substantially identical modules are arranged one above the other. Zinc powder is produced in suspension.
In a process to produce zinc powder from galvanised steel scrap, reference may be made of US patent no. 5,106,467 dated 21 april 1992, wherein leaching solution depleted of its oxidant is tranferred to an electrolytic cell where zinc is removed and plated as metallic powder onto a cathode. The leaching solution is recycled by removing zinc.
For electrowinning the zinc from zinc sulphate solution elecfricity is passed through rectifier. The operation is done at the current density which may vary from 400-500 A/m^. Cell voltage, determined by current density and the conductivity of the solution, varied from 3.3 to 3.5 V per cell, while current efficiency varies from 88 to

92 percent. The consumption of energy during electrolysis lies generally with in the range 3.1-3.3 kWh/kg of zinc cathode produced.
The present demand of zinc is about 0.25 million tonnes with about a 40% gap in the demand of indigenous production. In order to fulfil the demand and to exploit the zinc waste such as zinc dross (80-95% Zn, 1-5% Fe), zinc ash (70-80% Zn, 3-10% Fe), a process would be needed to produce zinc of purity 99%. In a known distillation condensation method zinc dross is treated to produce zinc powder. Reference may be made to P.T. Craddock, I.C. Fraestone, L.K.Gurjar, K.T.H. Hegde and V.H. Sonawane, Mining Magazine, p. 44-56, Jan. 1985, wherein the zinc dross is heated at elevated temperature in vacuum chamber and the zinc vapour is condensed under inert atmosphere. (Reference may be made of I.Nirdos, R.K.Kalia and S.V.Muthuswami, Hydrometallurgy, 1988, 20, 203-217), wherein impure zinc is cut into pieces and dissolved either in sulphuric acid or hydrochloric acid. After purification the solution is electrolysed as per the method mention above.
In all the hitherto known process for the production of zinc mention above have the following drawbacks, i) Roasting leaching - electrowinning process is suitable for high-grade zinc
concentrate, ii) Zinc dross, a hard material, can not be used in the form of lumps in roasting -
leaching process, iii) Lead is used as an anode, which causes extra cost, iv) In distillation condensation process where zinc dross can be used to produce
zinc powder tends to oxidation with finer size and also contains impurities
which volatilises along with zinc, v) All the above processes are high capital cost and high-energy intensive, vi) Large amount of effluents are generated in the above processes such as SO2,
solid waste, acid, etc.
The main object of the present invention is to provide an improved process for the production of electrolytic zinc powder from zinc waste and the anode being used as raw material unlike the use of separate metallic sheet in the conventional electrolytic cell.In an objective of the present invention is to produce an improved electrolytic process for the production of zinc powder from zinc dross generated by steel galvanising plants.
In an objective of the present invention is to produce high purity zinc containing negligibly small contamination with iron and lead.
Novelty of the the present invention lies in the use of zinc wastes containing container which also act as anode and in-situ releases the zinc ion during the electrolysis. The electrolyte used is sodium zincate which does not facilitates iron to deposit on cathode.
Accordingly, the present invention provides an improved process for the production of
zinc powder from zinc dross waste generated by steel galvanising plant which comprises:
(i) crushing zinc dross to size in the range of 2-3" and keeping in a porous bag for use as
anode.
(ii) dipping anode and cathode in a electrolytic bath of sodium zincate of pH in the range
of8-12.
(iii) passing current to the said electrolytic bath in the range between 5 to 15 Amps and
voltage in the range of 3 to 5V for a period ranging between 2-6 hours and temperature 35-
40°C.
(iv) removing zinc powder from cathode, washing and drying thereafter by known method
to obtain pure zinc powder.characterised in that the crushed zinc dross kept in porous bag
used as anode.
In an embodiment of the present invention used zinc dross used as raw material and anode comprises of zinc in the range of about 80-95%w/w of zinc dross.
In an embodiment of the present invention the zinc dross used comprises:
Zn in the range of about 80-95%
Fe the range of about 3-10% Pb the range of about 0.5-2% Si the range of about 0-2.5% . In an embodiment of the present invention, the porous bag is selected from polypropylene or stainless steel or like metallic porous bag.
In yet another embodiment of the present invention, the sodium zincate electrolytic bath have the composition of zinc in the range of 15-60 g/1 and sodium hydroxide in the range of 100-250 g/1.
In another embodiment of the present invention, wherein the cathode is selected from stainless steel, zinc and aluminium .
DETAILED DESCRIPTION OF THE INVENTION
In the process of present invention small pieces of zinc dross are taken into a porous bag, which do not react with the electrolyte. This bag is used as an anode. Fig 1 shows the schematic diagram for the zinc powder production from the zinc dross. Electrolyte are taken in a mild steel cell. Two porous anodes of size 6 cm x 16 cm cylinder, containing zinc dross are placed in the cell. The stainless steel cathode is inserted in between them .The distance between the two anodes are maintained to 16 cm. The anode and cathode are connected with the rectifier to pass the electricity. During the passing of electricity the electrolyte is ionised under the influence of current and the depletion of zinc in the electrolyte is compensated by the release of zinc ion from the anode. The packing density of zinc dross inside the porous bag is maintained constantly by known technique. Electric connection is made by inserting a rod made of titanium, steel and like in the bag. The conducting rod is placed in the bag in such a way that it touches to the bottom of anode cell and standing as axial position of the cylindrical bag.
Therefore, the present invention makes use of zinc wastes containing container, which act as anode and in-situ releases the zinc ion during the electrolysis. The electrolyte used is of sodium zincate, where iron dissolution is minimum and the zinc powder is mostly free from iron. In the alkaline medium, soluble zincate is formed according to the following reactions.
NaOH→Na+ + OH"
Zn + 20H→ ZnO2 + H2
ZnO + 20H→ Zn02 + H2O
Zn02 + 2H2O → Zn2+ + 40H-
Zinc dross is readily attacked by sodium hydroxide solution giving soluble zincate whereas both ferrous and ferric hydroxides are precipitated. During the electrolysis of sodium zincate solution, the reaction at cathode are:
Zn2+ + 2e →Zn
2H+ + 2e → H2 In the electrodeposition, zinc nuclei are formed at the cathode surface and then grow to small particles as the electrolysis continues. The zinc dross in the bag system releases the Zn2+ to counterbalance the metal ion continuously balancing the anodic and cathodic process.
40H → 2H2O + O2 + 4e The electrolyte is chemically analysed with respect to zinc ion every one hour during the process of electrolysis and the deficiency if any is balanced by addition of fresh zinc dross into the anodic bag. The contact of the zinc dross is ensured by suitably packing the same in the anode container with a rod.
During the electrolysis zinc sludge is produced in the bottom of the cell, which is the only solid waste.
The following examples are given by way of illustration and should not construed to limit the scope of the invention.
Example -1
In an electrolyte containing 16 g/L zinc and 200 g/L sodium hydroxide was taken in the mild steel electrolytic cell. Zinc dross as anodic feed stock was taken in a suitable stainless steel wire mesh container. Galvanised sheet was taken as cathode. The electrode spacing was kept for 6 cm. The temperature of bath was kept 35°C. To start the experiment the current was supplied by DC rectifier. The cathodic current density of 5 amp/dm2 was used in the experiment. The cathode was taken out and the powder was stripped. With the help of water the powder was thoroughly washed, dried and weight was taken. The purity of the powder was analysed by atomic absorption spectrophotometer and found 99%.
Example - 2
In this experiment the electrolyte contained 20 g/L zinc and 180 g/L NaOH was taken in a container. Anode made of zinc dross and cathode made of stainless steel was kept in the cell. The electrode spacing was kept for 7 cm. The temperature of bath was kept 40°C. With the help of DC rectifier the current was passed. At a current density of 7 amp/dm2 the experiment was done for 5 hrs. After completion of the experiment the zinc powder was taken out from the cathode. It was then washed and dried. The purity was checked and found 99.2%.
Example - 3
The electrolyte contained 45 g/L zinc and 250 g/L NaOH and zinc dross was taken in a stainless steel wire mesh container to act as anode. Stainless steel was taken as cathode. With the electrode spacing of 6.5 cm, the experiment was done. The temperature of bath was kept 45°C. The current was passed with the help of DC rectifier. At 10 amp/dm2 current density and with continuous filtration of electrolyte the experiment was done. After 6 hrs of experiment the zinc powder was taken out from the cathode. It was washed thoroughly with water, dried and weighed. The purity of zinc powder was found 99.5%.
ADVANTAGES
1. In the process of the invention, the anode used as raw material unlike the use of separate metallic sheet in the conventional electrolytic cell.
2. All industrial wastes containing zinc can be used in the porous container.
3. The energy consumption in the process is less than the conventional process of zinc electrowinning.
4. High purity zinc containing negligibly small contamination with iron and lead is produced.
5. The process is simple to operate and hence lower plant cost.
6. The cell productivity and the cathodic efficiency are higher in view of balanced anodic and cathodic processes.
7. The consumption of power is lower i.e 1.23-2.5 kWh/kg zinc produced than the
conventional method 3.3-3.5kWh/kg









We claim:
1. An improved process for the production of zinc powder from zinc dross waste generated by
steel galvanising plant which comprises:
(i) crushing zinc dross to size in the range of 2-3" and keeping in a porous bag for use as
anode, (ii) dipping anode and cathode in a electrolytic bath of sodium zincate of pH in the range of
8-12, (iii) passing current to the said electrolytic bath in the range between 5 to 15 Amps and voltage
in the range of 3 to 5V for a period ranging between 2-6 hours to deposit zinc powder in
the cathode, (iv) removing zinc powder from cathode, washing and drying thereafter by known method to
obtain pure zinc powder, characterised in that the crushed zinc dross kept in porous bag
used as anode.
2. An improved process as claimed in claim 1 wherein the used zinc dross used as
raw material and anode comprises of zinc in the range of about 80-95%w/w of
zinc dross.
3. An improved process as claimed in claim 1 wherein the used zinc dross used comprises;
Zn in the range of 80-95% Fe in the range of 3-10% Pb in the range of 0.5 - 2% Si in the range of 0 - 2.5%
4. An improved process as claimed in claim 1 wherein the porous bag is selected from polypropylene or stainless steel or like metallic porous bag.
5. An improved process as claimed in claim 1 wherein the sodium zincate electrolytic bath have
the composition of zinc in the range of 15-60 gpl and sodium hydroxide in the range of 100-250 gpl.
6. An improved process for the production of zinc powder from zinc dross waste generated by
steel galvanising plant substantially as herein described with reference to the examples.

Documents:

416-DEL-2004-Abstract-(05-10-2010).pdf

416-DEL-2004-Abstract-(20-08-2010).pdf

416-del-2004-abstract.pdf

416-DEL-2004-Claims-(05-10-2010).pdf

416-del-2004-claims.pdf

416-DEL-2004-Correspondence-Others-(05-10-2010).pdf

416-DEL-2004-Correspondence-Others-(20-08-2010).pdf

416-DEL-2004-Correspondence-Others-(25-08-2010).pdf

416-del-2004-correspondence-others.pdf

416-del-2004-correspondence-po.pdf

416-DEL-2004-Description (Complete)-(05-10-2010).pdf

416-del-2004-description (complete).pdf

416-DEL-2004-Form-1-(20-08-2010).pdf

416-del-2004-form-1.pdf

416-del-2004-form-18.pdf

416-DEL-2004-Form-2-(05-10-2010).pdf

416-del-2004-form-2.pdf

416-DEL-2004-Form-3-(20-08-2010).pdf

416-del-2004-form-3.pdf

416-del-2004-form-5.pdf


Patent Number 244377
Indian Patent Application Number 416/DEL/2004
PG Journal Number 50/2010
Publication Date 10-Dec-2010
Grant Date 03-Dec-2010
Date of Filing 11-Mar-2004
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SANJAY PRASAD METALLURGICAL LABORATORY, JAMSHEDPUR, JAHARKHAND, INDIA.
PCT International Classification Number C01G 9/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA