WO2007045055A2

WO2007045055A2 - Method for electroextaction of zinc

Info

Publication number: WO2007045055A2
Application number: PCT/BG2006/000019
Authority: WO
Inventors: Pencho Lesidrenski; Ivan Gruev; Nikola Nikolov; Atanas Apostolov; Slaveya Stoyanova; Nikola Petrov; Angel Grozev; Tsvetan Alexandrov
Original assignee: LEAD AND ZINC COMPLEX PEC
Current assignee: LEAD AND ZINC COMPLEX PEC
Priority date: 2005-10-20
Filing date: 2006-10-19
Publication date: 2007-04-26
Anticipated expiration: 2008-04-20
Also published as: WO2007045055A3; BG109329A

Abstract

A method for electroextraction of zinc from zinc sulphate solution obtained by leaching of zinc calcine in which a sulphite reagent is added continuously to a purified from impurities zinc sulphate solution to provide a concentration of SO3-2 in the solution of 300 to 1500 mg/1 and said zinc sulphate solution is subjected to electroextraction where the zinc is plated on the cathode and a part of the spent electrolyte is fed for dissolving new portions of calcine. Sulphurous acid or water solution of its salt is used as sulphite reagent. Preferably a solution of sodium bisulphite NaHSO3 is used.

Description

METHOD FOR ELECTROEXTRACTION OF ZINC TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for electroextraction of zinc from zinc sulphate solutions obtained in sulphuric acid leaching of zinc calcine. BACKGROUND OF THE INVENTION

A method of zinc electroextraction from sulphuric acid solutions of zinc sulphate is known, in which zinc sulphate solutions, purified from impurities (iron, arsenic, antimony, copper, cadmium, nickel, cobalt, etc.) are subjected to electrolysis with current density of 400-600 A/m² and electrolyte acidity of 110-180 g/dm³. As a result, the zinc is plated on the cathode and a part of the spent electrolyte is fed for dissolving new portions of calcine.

A disadvantage of the method is that when in the sulphate solutions there are impurities of metals with variable valency such as cobalt, antimony, arsenic, germanium, iron the cathode zinc yield is decreased, because of reverse dissolving, and the electricity consumption is increased.

It is found that when there are metal impurities in the sulphuric acid solution the reverse dissolving of zinc is prevented to a certain degree by using additives such as gum arabic, glue, molybdate, 2-butyne-l,4-diol, tetrabutyl- or tetraethylammonim chloride; a mixture of ethoxyacetylenic alcohol, triethylbenzylammonium chloride and polyethylene glycol and other surface-active organic compounds (US 5,194,125). Some of these additives are expensive or scarce and they are accumulated in the electrolyte. They do not affect the valency state of the electrolyte.

In the background art there is also a known method of high temperature extraction of zinc by dissolving of zinc ferrites, followed by precipitation in the presence of ammonium or alkali ions and taking the iron out of the hydrometallurgic cycle as jarosite cake (EP 451456; US 4274931; WO 98/06879). Usually sodium sulphate Na₂SO₄ is used as a reagent for the formation of the jarosites. The ZnSO₄ solution obtained after removing the impurities is subjected to electroextraction and as a result the zinc is plated on the cathode and a part of the spent electrolyte is fed for dissolving new portions of calcine. A disadvantage of the method is that the sodium sulphate, used for precipitation of jarosites, is expensive and scarce.

DISCLOSURE OF THE INVENTION

The present invention provides a method for electroextraction of zinc by which a higher current efficiency and lower energy consumption is achieved in the electrolysis of solutions with higher content of metal impurities with a variable valency such as iron, cobalt, antimony, arsenic, germanium.

The present method for electroextraction of zinc from zinc sulphate solutions obtained in sulphuric acid leaching of zinc calcine consists in that to the purified zinc sulphate solution fed for electrolysis a sulphite reagent is added continuously to provide a concentration of SO₃ ^"2 in the solution between 300 and 1500 mg/1 and said zinc sulphate solution is subjected to electroextraction, where the zinc is plated on the cathode and a part of the spent electrolyte is fed for dissolving new portions of calcine.

Sulphurous acid or water solution of its salt such as alkali sulphite or alkali bisulphite or ammonium sulphite or ammonium bisulphite or zinc sulphite are used as a sulphite reagent.

A preferred sulphite reagent of the present invention is a solution of sodium bisulphite NaHSO₃.

In a preferred embodiment of the present invention, the sodium bisulphite NaHSO₃ is obtained through absorption of sulphur dioxide contained in waste gases with solutions of sodium carbonate. Considerably lower concentrations OfNaHSO₃ in the solution are achieved in the absorption of SO₂ from poor waste gases of metallurgic and chemical productions (0.2 - 2.0 % by volume SO₂) compared to the requirements for the trade product (33 - 36 % NaHSO₃). Along with that, a relatively large amount OfNaHSO₃ is oxidized to Na₂SO₄.

The advantages of the method according to the present invention are that it provides a higher quality of the cathode zinc as well as improvement of the electrochemical process performance - a higher current efficiency (η_c, %) and a lower energy consumption (W, kWh/t Zinc) under equal other conditions of electrolysis. During the jarosite process in the sulphuric acid extraction stage the necessary amount of sodium ions for the jarosite precipitation is delivered by the spent electrolyte which W

is led to the cycle of the jarosite process. Thereby the use of the expensive and scarce sodium sulphate is avoided. The method according to the present invention also has a substantial ecological advantage because it allows full extraction of sulphuric dioxide from waste metallurgic gases. Furthermore, the liberation of sulphuric dioxide upon mixing the neutral zinc solution with the working electrolyte is reduced to minimum and deterioration of the sanitary conditions in the electrolysis area is avoided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 represents a flowsheet of sulphuric acid leaching of zinc calcine, in which the jarosite process is used, and electroextraction of zinc with addition of sodium bisulphite.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the present invention can be broadly described with reference to the flowsheet of FIG. 1. The method includes the following consecutively executed operations: Obtaining sodium bisulphite: Spent gases, containing sulphuric dioxide (for example monocatalytic systems for sulphuric acid, dust purified agglomeration gases or gases from the shaft furnace of lead production, etc.) are subjected, in a cyclic or continuous regime, to absorption with a water solution of soda (sodium carbonate Na₂CO₃) in a scrubber type absorber or a Venturi tube, obtaining sodium bisulphite with a concentration of 10-20 % NaHSO₃. Partial oxidizing of the bisulphite to sulphate occurs (8-10 % Na₂SO₄).

Dosing of the sodium bisulphite: The NaHSO₃ solution obtained is fed continuously from a collecting tank with a doser (a slide gate or a valve) and is mixed with the purified zinc sulphate solution flow fed for electrolysis. The bisulphite solution rate is controlled in accordance with the production load in the leaching scheme (m³/h of produced zinc sulphate solution) as well as in accordance with the NaHSO₃ concentration in the fed reagent so that the neutral zinc solution flow dispatched for electroextraction will maintain a concentration of SO₃ ^"2 not lower than 300 mg/1 and not higher than 1500 mg/1. The high reduction potential of the sulphite and bisulphite ions provides even in small concentrations a reduction of polyvalent metal impurities (Fe³⁺, Fe²⁺, Co³⁺, Co²⁺, As⁵⁺, As³⁺, Sb⁵⁺, Sb³⁺, Ge⁴⁺, Ge²⁺, etc.) to their low valency forms according to the following chemical reaction equations:

2Fe₂(SO₄)₃ + 2NaHSO₃ + 2H₂O = 4FeSO₄ + Na₂SO₄ + 3H₂SO₄;

2Co₂(SO₄)₃ + 2NaHSO₃ + 2H₂O = 4CoSO₄ + Na₂SO₄ + 3H₂SO₄; H₃AsO₄ + 2NaHSO₃ = 2H₃AsO₃ + Na₂SO₄ + H₂SO₄;

H₃SbO₄ + 2NaHSO₃ = 2H₃SbO₃ + Na₂SO₄ + H₂SO₄;

H₂GeO₃ + 2NaHSO₃ = 2H₂GeO₂ + Na₂SO₄ + H₂SO₄.

It is supposed that the restriction of the relative part of the high valency forms of the said deleterious impurities in the electrolysis cell is a prerequisite for improved quality of the cathode zinc (appearance of the cathode residue), for a higher current efficiency (η_c, %) and also lower energy consumption (W, kWh/t Zinc) during zinc electroextraction. The introduction of a reducer influences favourably the cathode polarization during the electrolytic process.

The main part of the fed NaHSO₃ is oxidized to a sulphate (Na₂SO₄) by the Mn⁷⁺ and Mn ⁺ available in the spent electrolyte (as MnO₄ ^" ions or fine particles of MnO₂ — slurry, respectively) already during mixing of the neutral zinc sulphate solution and the spent electrolyte, according to the following reactions:

10NaHSO₃ + 4HMnO₄ = 5Na₂SO₄ + 4MnSO₄ + H₂SO₄ + 6H₂O;

2NaHSO₃ + 2MnO₂ + H₂SO₄ = Na₂SO₄ + 2MnSO₄ + 2H₂O. The small amounts of NaHSO₃ which could remain, are oxidized fully in the electrolytic cell where because of the high concentration of the oxygen emitted by the anodes and the large quantity of finely dispersing anode slurry (MnO₂ - slurry) a very high oxidizing potential is maintained. In this mechanism of the ongoing oxidization and reduction processes, the emitting of sulphuric dioxide during mixing the neutral zinc solution with the spent electrolyte is reduced to minimum and deterioration of the sanitary conditions in the electrolysis workshop is avoided. In this connection, it is preferred the concentration of sulphite ions in the neutral zinc solution for electrolysis not to exceed 1500 mg/dm³ SO₃ ^"2. The obtained sodium sulphate with the spent electrolyte goes as a reagent to the jarosite process stage. The full oxidizing of the bisulphite (NaHSO₃) to sulphate (Na₂SO₄) is necessary because the presence of a reducer in the solutions, fed in the jarosite process stage is undesirable. EXAMPLES

The following examples illustrate the present invention without limiting it. EXAMPLE 1.

In a tank with a capacity of 15 m a lot of about 12 m solution of sodium bisulphite with a content of about 125 g/1 OfNaHSO₃ and 90 g/1 OfNa₂SO₄ is fed. The solution of sodium bisulphite is obtained through absorption of sulphuric dioxide from waste gases containing 0.15 - 0.20 % by volume SO₂ after a monocatalytic system for sulphuric acid production, with soda solution. The tank is placed over a collector with a capacity of 40 m³ intended for the purified solution of zinc sulphate. In the ZnSO₄ solution collector the sodium bisulphite is fed and mixed up with a rate of about 0.15 m³/h (about 6 m³ per twenty-four hours respectively), which at a rate of 30 mVh of the neutral ZnSO₄ solution corresponds to a concentration of about 480 mg/dm³ of SO₃ ^"2 ions in the solution. The tank is equipped with a steam heater for maintaining a temperature not lower than 3O⁰C in order to prevent crystallization of the solution. After feeding the lot Of NaHSO₃ solution (for 96 hours approximately), the tank is filled again.

The neutral ZnSO₄ solution with the added NaHSO₃ is led to the cellhouse according to the flowsheet in Fig.1.

The cellhouse comprises 222 electrolytic cells connected in a series (each cell of capacity 1.8 m³, with 20 cathodes and 21 anodes). The series of electrolytic cells works under current load of 10 000 A, which corresponds to a cathode current density D_c of about 400 A/m². The industrial test is carried out in the course of 360 hours (15 days and nights) with 150 - 160 g/1 Zn and 140 - 150 g/1 H₂SO₄ in the neutral ZnSO₄ solution and variable valency impurities content: 0.01 - 0.02 mg/1 of antimony (as Sb⁵⁺), 0.5 - 1.5 mg/1.of arsenic (as As⁵⁺), 0.3 - 0.5 mg/1 of cobalt (as Co³⁺), 0.003 - 0.007 mg/1 of germanium (as Ge⁴⁺), 10 - 20 mg/1 of iron (as Fe³⁺). The calculated average values over a twenty-four-hour period for the current efficiency (η_c, %) and specific energy consumption (W, kWh/t Zinc) are between 87.7 - 89.0 % (88.4% average) and 3121 - 3103 kWh/t (3112 kWh/t average) respectively. The obtained cathode zinc has a very good outer surface of the sheet without black dots on its inner surface. The removing of zinc residues from the cathodes is carried out without difficulty (the so-called "stripping" of the zinc is good).

The spent electrolyte which contains sodium sulphate obtained from the introduced sulphite reagent is led to the sulphuric acid leaching cycle and the jarosite process (FIG.l) where sodium ions are used in the formation of jarosite.

EXAMPLE 2.

Using the method described in Example 1 , an industrial test is performed in the course of 6 weeks. The addition of sodium bisulphite (with concentrations within 125 - 130 gΛ Of NaHSO₃ and 90 - 95 g/1 of Na₂SO₄) is changed as follows: 0.20 m³/h - the first week; 0.40 m³/h - the second week; 0.60 m³/h - the third week; 0.50 m³/h - the fourth week; 0.30 m /h - the fifth week; and 0.10 m /h - the sixth week, which corresponds to SO₃ ^"2 ion concentrations between about 1980 mg/1 (when the addition is 0.60 m³/h) and about 300 mg/1 (when the addition is 0.10 m³/h). The obtained results are similar to those in Example 1. The calculated average values of the current efficiency (η_c, %) and specific energy consumption (W, kWh/t Zinc) are 87.5 - 89.8 % (88.6 % average) and 3190 - 3015 kWh/t (3103 kWh/t average) respectively.

When the addition is larger than 0.5 m /h and the SO₃ ^' ion concentration in the neutral ZnSO₄ solution exceeds 1500 mg/1 of SO₃ ^"2, a certain gas emission with a sulphuric dioxide smell is observed when mixing the neutral solution with the electrolyte flow in a recycle, an indication of which is the complete decolorization of the solution at the entry of the electrolytic cells. When leaving the cells the solution regains its pink-violet color, which indicates a full oxidation of SO₃ ^"2 ions to SO₄ ^"2 ions, i.e. of sodium sulphite to sodium sulphate. The obtained cathode zinc is dense, with a good outer surface of the sheet, without black dots on its inner surface. No substantial differences are observed in the so-called "stripping of the cathode zinc", nor differences in the quality of the cathode zinc in the different amounts of additions - from 0.10 to 0.60 m³/h of NaHSO₃ solution, respectively concentrations of SO₃ ^"2 ions between 300 mg/1 and 1980 mg/1 in the purified solution of zinc sulphate subjected to electrolysis. COMPARATIVE EXAMPLE

As a basis for comparison, the so-called "passive industrial experiment" of the electrolysis process was carried out with the whole series of cells with similar parameters of electroextraction as in Example 1, but without addition of sodium bisulphite to the neutral ZnSO₄ solution.

The addition of sodium bisulphite is stopped and in the course of 15 days and nights a standard electroextraction is carried out through the set technological mode of production. The cathode current density Dc, the concentration of zinc and sulphuric acid in the neutral ZnSO₄ solution, as well as the content of impurities with variable valency are the same as in Example 1: 0.01 - 0.02 mg/1 of antimony (as Sb⁵⁺), 0.5 - 1.5 mg/1 of arsenic (as As⁵⁺), 0.3 - 0.5 mg/1 of cobalt (as Co³⁺), 0.003 - 0.007 mg/1 of germanium (as Ge⁴⁺), 10 - 20 mg/1 of iron (as Fe³⁺).

The calculated average values over a twenty-four-hour period of the current efficiency (η_c, %) and specific energy consumption (W, kWh/t Zinc) are 85.8 - 87.2 % (86.5 % average) and 3228 - 3194 kWh/t (3211 kWh/t average) respectively. The obtained cathode zinc is with good outer surface, but with black dots on the inner surface of many of the cathode sheets, which is an indication of, even though limited, reverse dissolving of zinc.

Comparison of the results from the given examples shows that through the addition of sodium bisulphite an average of 100 - 110 kWli/t Zn, respectively 3 to 3.5 %, lower consumption of electricity is achieved compared to electrolysis without additions of sodium bisulphate, with visibly better appearance of the cathode residues.

Claims

1. A method for electroextraction of zinc from zinc sulphate solutions obtained in sulphuric acid leaching of zinc calcine, characterized in that to the purified zinc sulphate solution fed for electrolysis a sulphite reagent is added continuously to provide a concentration of SO₃ ^"2 in the solution between 300 and 1500 mg/1 and said zinc sulphate solution is subjected to electroextraction, where the zinc is plated on the cathode and a part of the spent electrolyte is fed for dissolving new portions of calcine.

2. A method according to claim 1, characterized in that sulphurous acid or water solution of its salt such as alkali sulphite or alcali bisulphite or ammonium sulphite or ammonium bisulphite or zinc sulphite is used as sulphite reagent.

3. A method according to claims 1 and 2, characterized in that sodium bisulphite NaHSO₃ solution is used as sulphite reagent.

4. A method according to claim 3, characterized in that the sodium bisulphite NaHSO₃ is obtained by absorption of sulphuric dioxide, contained in waste gases, with sodium carbonate solutions.