|Title of Invention||
"A PROCESS FOR THE RECOVERY OF ZINC FROM ZINC BEARING INDUSTRIAL WASTE"
|Abstract||A process for the recovery of zinc from zinc bearing industrial waste comprising sieving the said waste material, separating the heavy iron particles from the said waste material by mechanical treatment, leaching the said raw material with sulphuric acid of desired concenteration optionally adding hydroxide of iron, filtering the said solution to remove undissolved material, precipitating iron from said solution by addition of ammonium hydroxide, hydrogen peroxide,seperating the finest ZnO from said waste material in the presence of compressed air, optionally adding flocculating agent to settle the precipitate, purifying the said solution by means of adding copper in the form of copper sulphate or copper dust while heating the solution and adding pure zinc dust while continuously agitating the said solution, adding ammonium hydorxide and passing reducing gas , filtering the said solution and finally electrolyzing the said filtered solution to collect solid zinc.|
|Full Text||The present invention relates to a process for the recovery of Zinc metal from zinc
bearing industrial waste .
BACKGROUND OF THE INVENTION:
Many Zinc related industries produce zinc bearing residue e.g. drosses and chloride containing galvanizers ash and cyclonic ash from galvanizing plants, residue from zinc oxide plants, electrolytic zinc plants residue, sludge and residues from zinc sulphate plants, blue powder from lead smelters, residue from electroplating plants and the like.
As per the prior available art the existing processes are not capable of treating a varied \ range of materials as all the existing processes are designed to treat only a particular type of material of a definite origin and composition. Also, there exists no process that can treat raw materials with a chloride content of upto 3-5% in the manner. All the existing processes reduces the chloride contents of the material being used before its hydrometallurgical tretament, essentially by roasting.
The existing methods for the hydrometallurgy and electowinning of zinc which are in
practice are used exclusively either primary or secondary source of zinc. However, there
is no single process in practice which can be extended to treat zinc bearing residues of
such varying origins and compositions. Also, existing processes often required expensive ;
plant machinery, such as rotary furnances for treating chloride bearing residues or high
pressure and high temperature equipment and reactors to prepare and purify the leach
The other drawback of the existing electroytic zinc plants is their adverse effect on the environment. This is partly caused by the difficulty of these plants in managing the effluents produced during processing.
The object of the invention is that, zinc of a sufficient high purity is produced with relative ease, even on large commercial scale, which is not possible in the existing available processes.
The main embodiment of the present invention relates to treatment and recovery of the zinc from a wide range of locally produced zinc bearing industrial waste, including materials with a moderate chloride content as herein described.
Another objective of the present invention is to administer a complete mechanical, hydrometallurgical and electrolytic treatment of the residues /materials used.
The objective of the subject invention is to recover the maximum possible quantity of zinc from the raw material i.e the waste material as herein described.
The other objective of the present invention is to administer various methods of purification so as to remove impurities that are generally found to be present in different raw materials thus recovering zinc of atleast 99.9% purity even on sufficiently large commercial scale.
In the present invention the process in the recovery of zinc from the raw material that can be of secondary or even certain primary sources. They may contain moderate amounts of chlorides, zinc ferrites, large amounts of iron as well as other elements, metallic particles and oxides.
The present process is suitable for treating zinc residues with a zinc oxide content and with compositions lying within the following ranges:
• Total zinc: at least 8-12%
• Fe : upto 70- 80%
• Chlorides: upto 8-15%
• Zn ferrites: upto 50%
• Pb,Cu, Cd, Ni, Ge,As, Sb etc.: Upto 2-5% (individually or in combination)
Such waste materials include galvanizers ash, cyclonic exhaust ash from galvanising industries, zinc plant residues, zinc sulphate plants residues, blue powder flume from lead blast furnaces, zinc oxide plant residues etc.
A combination of different material can be used so as to bring the overall composition of the material within these ranges. This is especially useful when chloride containing materials or materials with low zinc oxide content are to be used.
According to the present invention, a process for the recovery of zinc from zinc bearing industrial waste as herein described comprising sieving the said waste, separating the heavy iron particles from the said waste, leaching the said waste with sulphuric acid of desired concentration as herein described to obtain a solution having pH of 2.3 to 2.6, adding hydroxide of iron, as herein described filtering the said solution to remove undissolved material, precipitating iron from said solution by addition of ammonium hydroxide, hydrogen peroxide, finest ZnO separated from said waste in the presence of compressed air, adding flocculating agent as herein described to settle the precipitate, purifying the said solution by means of adding copper in the form of copper sulphate or copper dust while heating the solution and adding pure zinc dust while continuously agitating the said solution, adding ammonium hydroxide and passing reducing gas, filtering the said solution and finally electrolyzing the said filtered solution to collect solid zinc.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 relates to a flowsheet of mechanical treatment stages Figure 2 relates to a simplified flow sheet leaching stage Figure 3 relates to a simplified flow sheet of iron removal stage Figure 4 relates to a simplified flow sheet of general purification stage
Figure 5 relates to the structure of anodes and cathodes in the electolytic bath Figure 6 relates to a simplified flow chart of complete process
DETAILED DESCRIPTION OF THE INVENTION
The present process mainly comprises two stages.
As shown in figures 1- 4, the process of the subject invention involves the stages covered by the pre-electrolysis process steps.
STAGE -1 :PRE-ELECTROLYSIS:
The Pre-electrolysis stage involves first the mechanical treatment of the raw material, as shown in figure 1, which is followed by the hydromettalurgical treatment. The Pre-electrolysis is a four stage process as:
1. MECHANICAL TREATMENT
2. LEACHING STAGE
3. IRON REMOVAL STAGE
4. GENERAL PURIFICATION
The mechanical treatment of the raw material as shown in figure 1, forming the first stage of the process involves the "electrolysis stage" which involves the electrowinning of elemental zinc from the purified solution produced.
Electrolysis begins with the mechanical treatment of the raw material, to classify it into its main fractions. After this, the waste material is dissolved in a solution of dil.Sulphuric acid , to form a neutral solution. This is called the leaching stage. The objective of the leaching stage is to dissolve the maximum amount of zinc possible, into the solution. In
order to achieve this, the dissolution of other unwanted elements is unavoidable. The neutral solution has to then be sufficiently purified to the required levels, before it can be transferred to the electrolysis stage. The electrolysis of the purified solution is carried out between permanent metal (titanium- lead-carbon) anodes and appropriate cathodes. During this stage, the purity of the zinc metal recovered depends entirely on the purity of the zinc sulphate solution used. The correct conditions, i.e., electrical current, voltage, temperature, pH are to be maintained so that zinc is deposited with optimum efficiency.
The pre-electrolysis stage is a multi step process, comrising steps of: 1. MECHANICAL TREATMENT :
The raw material is first physically separated into its various fractions on the basis of their screening as shown in figure 1. These various fractions are then to be added to the leaching solution in a specific sequence :
i) SEPARATION OF RELATIVELY PURE ZINC OXIDE : The relatively pure zinc oxide coating the ash and within the flakes of the raw material is separated first, using a screen.
ii) CRUSHING OF ASH: Gently crushing the waste material to powder the soft flakes/chumps of ash to smaller pieces. Separate the particles of the raw material of a screen size of a smaller size. This portion of the raw material will be relatively richer in ZnO than the other particles, except the portion separated in step (i). Further, crush the coarser particles that remain to a size of 150 mesh. This fraction mainly contains metallic zinc, any zinc ferrites present and iron.
iii) MAGNETIC SEPARATION OF HEAVY IRON PARTICLES: Magnetically separate the heavy iron particles from all the different fractions of the waste material as
shown in figure 1. where these heavy iron particles is to be treated separately for the recovery of any zinc it contains, by leaching it with diluted Sulphuric acid.
2) LEACHING STAGE:
The leaching stage is carried out in four steps as shown in figure 2, in which different fractions of the raw material, separated so far are added to the diluted solution of sulphuric acid at the appropriate stages. The overall concenteration of zinc in the purified solution should be between 110 to 170 g/Ltr. The dil. sulphuric acid soloution with which the ash is leached will have an initial composition of sulphuric acid of 120-180 g/ltr. and 40-50 g/ltr. of zinc. At the end of this stage waste products will remain undissolved, while more than 95-98% of the zinc present in the ash will have been dissolved.The leaching stage is to be carried out in gas tight reactors with an arrangement for adequate agitation and heating. The steps of leaching stage are as follows:
i) Take said solution equalling 75% of the total volume of the required leach solution.To
this add the powdered coarser fraction of the said raw material, in small lots, any
conventionally used surface agent can also be added to assist dissolution. Good agitation
has to be maintained to prevent the solidifying of the particles at the bottom of the
reactor. This fraction mainly contains the metallic contect of the ash to form its basic
Zn + H2SO4 > ZnSO4 +H2
ii) At a pH of 3.0 to 3.5 add more solution of diluted sulphuric acid till pH 1.5- 2.0 is reached. Now add the finer ZnO rich component of the raw material in smaller lots whilst maintaining good agitation. The pH of the solution will rise comparatively faster also this fraction of the raw material.will contain some metallics and ferrites, however the rate of dissolution of these elements will be low due to the rise in pH. To the solution is added concenterated sulphuric acid equalling 3-6% of the volume taken and maintain good agitation. The addition of the concenterated sulphuric acid to the solution will increase its concenteration to about 200g/ltr.creating a stoichiometeric excess of acid in relation to
zinc in the solution. Due to this the dissolution of the metallics will increase rapidly and there will be slight increase in the temperature.
iii) The solution is heated to 80-85 C maintaining the temperature for 25-30 minutes.
The solution has to be agitated to keep the ash particles in suspension, as solidifaction of
the particles of the raw material, can greatly reduce the rate of dissolution of zinc. At this
temperature and under these conditions, the zinc ferrites present will be attacked by
sulphuric acid, the rate of the dissolution of the metallic zinc will also increase. The build
up in pressure due to the production of steam and hydrogen gas will also increase the rate
of dissolution of zinc into the solution. The solution is retained for a period of three to
five hours or overnight. The dissolution of the zinc ferrites thus takes place in the manner.
ZnO.Fe2O3+ H2SO4 → ZnSO4 +Fe203+H2O
IV)After keeping the solution for four hours or overnight, agitate the solution and add dil sulphuric acid till 90% of the required volume is made up. If the pH is below 2.0, add Fe(OH)3 and undissolved zinc dust residues stored from the previous batches till a pH value of 2.5 is reached. If required add stored wash water. This is the optimum pH value for the next stage. The solution is then filtered to remove particles of upto 2.5 microns to produce a clear solution. The filtered solution is transfered to the next stage.
3. IRON REMOVAL STAGE:
This stage is also accomplished by a multi process steps as shown in figure 3, comprising
I) AGITATION OF THE SOLUTION: Check that the pH of the leach solution is at 2.3 to 2.5 after transferring it to the iron removal stage. The solution is agitated well which enables it to dissolve compressed air into it at high pressure.
ii) ADDITION OF ZnO SEPARATED FROM THE ASH IS STEP 2: A small amounts of ZnO separated from the raw material earlier is added, the ferrous iron present
will be directly oxidized to filterable ferric hydroxide. In order to achieve this good agitation and dissolution of the oxygen present in the compressed air is required. Continue to add ZnO and precipitate the ferric hydroxide till pH reaches 4.0 to 4.5.
iii) ADDITION OF HYDROXIDE: Continue the dissolution of air with agitation, add a small amount of ammonium hydroxide to produce ammonium chloride, some of which will be evolved as gas. This is formed due to the presence of any ferric chloride that may be in the solution. Ferric Hydroxide will be precipitated both from the ferric chloride and ferric sulphate present.
iv) ADDITION OF HYDROGEN PEROXIDE: Now add hydrogen peroxide in the required quantity so as to oxidize the remaining ferrous iron to the ferric state, while continuing the dissolution of air. Continue to add zinc oxide, so that all the iron present is precipitated as ferric hydroxide. Add a conventional flocculating agent to settle the precipitates and filter the solution to remove particles of upto 12-18 microns in size. The ferric hydroxides can be hydrocyloned to retrieve as much liquor as possible. Transfer the neutral solution to the next stage,which at this point will have an iron content of less than 0.01 g/ltr. Also, other III rd group elements and elements such as As and Ge will also be removed during this operation. The ferric hydroxide precipitated can be stored and washed with spent liquor to remove all the zinc entrapped in it.
4 GENERAL PURIFICATION:
This stage comprises the steps as shown in figure 4, comprising :
i)ADDITION OF COPPER SULPHATE: Check the pH of the solution. Add solution of dil sulphuric acid to it in small lots till a pH value of 4.0 to 3.8 is reached. Begin to heat the solution and add copper sulphate at the rate of 0.2% by weight of the raw material dissolved.
ii) ADDITION OF ZINC DUST IN STEP i): At 70-75 ° C add zinc dust equalling 30 g per Kg of raw material dissolved. Agitate the solution moderately. Maintain the temperature for 30 minutes, add more dil. sulphuric acid if the pH rises above 4.0. The unwanted elements present in the solution i.e. Ilnd and IVth group elements such as Cu, Pb, Ni,Cd, Co, Sb are removed by cementation with zinc dust, by which they are separated from their solutions. The efficiency in this process is increased due to the presence of extra copper in the solution as a result of the addition of copper sulphate. At 70 °C most of the elements present in the solution are removed in an excess of zinc dust. Retain the solution for at least 30 minutes.
iii)ADDITION OF AMMONIUM HYDROXIDE: Add ammonium hydroxide to the solution with agitation at the rate of 50ml per Kg of raw material dissolved. This will produce some ammonium chloride gas on account of the zinc chloride content in the solution, however, some ammonium chloride will remain in the solution. Zinc hydroxide will be formed as white precipitate, due to the neutralization of zinc sulphate and zinc chloride.
iv) TREATING THE SOLUTION WITH HYDROGEN PEROXIDE: Agitate the solution and add a small amount of dil sulphuric acid to redissolve the zinc hydroxide forrhed. Continue to add spent liquor till pH is 4.5. This will help the dissolution of zinc dust to continue. Now pass a small quantity of hydrogen sulphide gas through the solution, the solution will turn light greenish - brown due to the precipitation of the sulphide of any unwanted Ilnd group elements remaining in solution. The ammonium chloride in the solution and the hydrogen gas produced due to the dissolution of zinc dust will prevent the precipitation of ZnS. Allow the precipitates to settle and filter to remove particles of upto 20 microns in size. The residue will contain Cu, Pb, Cd, Ni, Co etc. and their sulphiides and undissolved zinc. Hence, it can be either reintroduced at the leaching stage or sent for the recovery of these metals. By the end of this stage the purified zinc sulphate solution may contain only traces of these elements. Transfer the purified solution to the electrolytic stage.
STAGE II :ELECTRQLYSIS
The electrolysis stage involves the electrowinning of elemental zinc from the purified zinc sulphate solution. The electrolysis of this purified solution is carried out in a suitable bath between metal (Ti-Pb-Carbon) anodes and (aluminium sheet) cathodes as shown in figure 5. The anodes are slightly smaller then the cathodes. The anode are of a new design unique to this process as opposed to the lead anodes used in the conventional electrowinning of zinc. These anodes consists of thin carbon sheets held by a titanium -lead frame. These anodes are quite resistent to the corrosive effect of any chlorides that may be present in the electrolyte and also cause practically no contamination in the zinc deposited at the cathodes. The homopolar electrode distance can be from 6-9cm.
A direct current is to be passed through the bath at a cathodic current density of 4.5 A/dm2 to 4.8A/dm2 , the cell voltage is between 0.33 to 0.35 V. As a high direct current is passed through the electrolyte the temperature of the electrolyte may rise. This should be cooled by a suitable means to keep the temperature between 35 and 38 °C, at this temperature hydrogen evolution decreases and local action increases.Zinc metal from the purified zinc sulphate solution is deposited on the aluminium cathode in a plate-like form, while suplhuric acid is formed at the anodes. Due to the formation of the sulphuric acids this process will reach a stage when the efficiency of the deposition of zinc will be greatly reduced due to the redissolution of the zinc. Thus to overcome this effect ammonium hydroxide solution is added to the electrolyte so as to neutralise some of the free acid present in it. In this manner the pH of the electrolyte is kept between 3.0 and 3.5 , except at the end of this operation when the pH can be allowed to fall below 2.0. Reaction at Anode:
ZnS04 > SO4' + Zn2+
Reaction at Cathode:
H2O > H+ + OH"
Overal Cell reaction:
ZnSO4+H20 > Zn+H2SO4+O
Neutralization of Free Acid:
2NH4OH+ZnSO4 > Zn(OH)2+ (NH4)2SO4
Zn (OH)2 +H2SO4 > ZnS04 +2 H20
The ammonium hydroxide solution is introduced to the lower part of the bath by means of a narrow bore pipe in the form of steady stream of droplets. The outlets of this pipe should be behind the extreme anodes on the either side of the bath. As the ammonium hydroxide is introduced, zinc hydroxide is precipitated which is redissolved by the acid in the electrolyte.
The amrnoinium hydroxide solution serves another purpose, that of neuralizing any chlorides present in the electrolytes, to prevent the formation of perchloric acid. The ammonium liberated combines with the chlorine gas to give gaseous ammonium chloride.
2NH4O H+ ZnCl2 > 2NH4C1 + Zn (OH)2
As the source of this operation depends on the purity of the electrolyte, the use of these anodes coupled with the maintaining of the correct conditions ensures that very pure zinc is yielded by this process.
The process is stopped when the zinc concenteration in the electrolyte reaches 40-50g/ltr. The remaining solution as spent electrolyte is drained and returned to the leaching stage.
1) A process for the recovery of zinc from zinc bearing industrial waste as herein described comprising sieving the said waste, separating the heavy iron particles from the said waste, leaching the said waste with sulphuric acid of desired concentration as herein described to obtain a solution having pH of 2.3 to 2.6, adding hydroxide of iron, as herein described filtering the said solution to remove undissolved material, precipitating iron from said solution by addition of ammonium hydroxide, hydrogen peroxide, finest ZnO separated from said waste in the presence of compressed air, adding flocculating agent as herein described to settle the precipitate, purifying the said solution by means of adding copper in the form of copper sulphate or copper dust while heating the solution and adding pure zinc dust while continuously agitating the said solution, adding ammonium hydroxide and passing reducing gas, filtering the said solution and finally electrolyzing the said filtered solution to collect solid zinc.
2) The process as claimed in claim 1, wherein the said iron particles are separated
form the said waste by means of mechanical treatment as herein described.
3) The process as claimed in claim 2, wherein the said mechanical treatment is
carried by means of gentle crushing, sieveing the said waste thereby separating into fine
and coarse particles, again crushing the heavy particles and finally magnetic separation of
the iron particles.
4) The process as claimed in claim 1, wherein the said leaching stage comprises
dissloving the said coarse particles of said waste in the dilute sulphuric acid to get a pH of
the solution at 2 having initial composition of said solution of 120-180g/ltr of H2SO4 and
40-50 g/ltr of zinc.
5) The process as claimed in claim 4, wherein to the said leaching solution Cone.
Sulphuric acid is added while maintaining good agitation till a pH of 1.5-2.0 is achieved
heating the same at 80-85 degrees C for 25-35 minutes, and cooling at ambient temperatures for the dissolution of zinc ferrites .
6) The process as claimed in claim 5, wherein ferric hydroxide is added to the said
solution and said solution is filtered followed by the addition of small lots of said finest
ZnO with continuous agitation in the presence of compressed air.
7) The process as claimed in claim 6, wherein the said leaching solution is agitated
and mixed with dil Sulphuric acid till 90% of the required volume is made up by adding
8) The process as claimed in claim 7, wherein to said leaching solution ammonium
hydroxide is added followed by the addition of hydrogen peroxide, and a flocculating
agent as herein described.
9) The process as claimed in claim 8, wherein the said solution is purified by the
addition of copper in the form of copper sulphate or copper dust while heating the said
solution, adding pure zinc dust with continuous agitation, followed by the addition of
ammonium hydroxide and reducing gas.
10) The process as claimed in claim 9, wherein the said solution is filtered to remove
11) The process as claimed in claim 10, wherein the said solution is electrolysed
having perchloric acid resistant Ti-Pb-carbon anode and aluminium cathode.
12) The process as claimed in claim 11, wherein the-solidified zinc is collected at
13) A process for the recovery of zinc from zinc bearing industrial waste, substantially as herein described with reference to the foregoing description.
|Indian Patent Application Number||1/DEL/1998|
|PG Journal Number||10/2008|
|Date of Filing||01-Jan-1998|
|Name of Patentee||VISHAL RAJ GUPTA|
|Applicant Address||SAINIWAS, M-25 HOUSING BOARD COLONY, NAHAN-173001, HIMACHAL PRADESH, INDIA|
|PCT International Classification Number||B03D 1/02|
|PCT International Application Number||N/A|
|PCT International Filing date|