FR2616159A1 - METHOD FOR ADJUSTING THE QUANTITY OF ZINC POWDER USED IN THE PRECIPITATION OF IMPURITIES OF A ZINC SULFATE SOLUTION FOR ELECTROLYTIC REFINING OF ZINC - Google Patents

Abstract

The present invention relates to a method for adjusting the amount of zinc powder used in the precipitation of impurities from a solution of zinc sulfate. This method is characterized in that the amount of added zinc powder is adjusted by means of redox potential measurements. Application to the electrolytic refining of zinc.

Description

The method of the present invention relates to the elimination of

  impurities of zinc sulphate solutions for the electrolytic refining of zinc and in particular the adjustment of the amount of zinc powder used in the removal of impurities. The removal of impurities, such as copper, cobalt, nickel and germanium, as well as cadmium, is carried out by agglutination thereof with zinc powder and the amount of zinc powder used is optimized

  by means of redox potential measurements.

  The main raw materials used in zinc electrolytic processes are zinc concentrates, which are first oxidatively calcined. The calcined product is dissolved in a recycle acid solution containing sulfuric acid, which is recycled by electrolytic precipitation. The nonsoluble components are separated from the zinc sulphate solution created in the dissolution process. The solution is then sent to a purification of the solution, in which are removed all elements nobler than zinc. After the purification of the solution, this one

is sent to electrolysis.

  The crude solution of a zinc production process contains a number of more noble elements than zinc, the content of which varies according to concentrates and other components. The most important of these are copper, cadmium, cobalt, nickel, arsenic, antimony, germanium and thallium. Since these elements are nobler than zinc, they tend to be precipitated on the cathode during electrolysis. This is undesirable, because they impure the precipitated zinc and some of these elements cause

  secondary reactions (release of hydrogen).

  Since the elements mentioned above are more noble than zinc, they can be separated from the solution by agglutination with zinc metal and this method is used almost exclusively in zinc production - except for the method of purification of zinc. the solution, in which elements nobler than zinc are eliminated from

  the zinc electrolyte by extraction with 3-naphthol.

  Although the general agglutination agent used in the purification of the solution is metallic zinc, certain auxiliary agents are usually employed such as arsenic or antimony. If antimony is used, the purification steps are usually continuous-action steps, so the first step involves the removal of cadmium and copper, the second stage involves the removal of cobalt and nickel , and the second possible step is primarily a step

  additional for the previous procedure.

  There are basically two different methods that use arsenic as an auxiliary agent for zinc. In the first method, copper, cobalt and nickel are removed from the zinc electrolyte in the first solution purification step, either in a batch process or in a continuous process. The second step is that of eliminating cadmium and the third step is, if that is

  necessary, used as a complement step for the process.

  According to the second method of purifying the solution employing arsenic as an auxiliary agent for zinc, the purification of the solution takes place in three stages, among which in general, the first and third steps are continuous and the middle step is a automatic batch process. In the first stage, most of the copper is separated from the zinc electrolyte. In the second stage, the rest of the copper is separated with cobalt, nickel and germanium. In the third stage, cadmium is mainly separated. The second step (the batch process) in a three step solution purification process employing arsenic as an auxiliary agent is generally carried out as follows: the zinc electrolyte is first charged into the reactor. When the reactor is, for example, half full, the mixture is started and the introduction of zinc powder can begin. The introduction of the powder is first rather fast to provide a sufficient content in the reactor. At the end of the filling of the reactor, the feed is slowed down, but still continued until the total amount of zinc calculated for the batch is added. After a given period, the Co content of the solution is analyzed and if it is proved that the cobalt precipitated sufficiently, the batch is ready. If the result of the analysis is poor, the introduction of powder is continued until a proper precipitation of the cobalt is obtained. The created precipitate is not removed after each precipitation, but several precipitations are carried out successively and the

  precipitate is only removed from time to time.

  The dosage of zinc powder has been a big problem. In general, a "sufficient" amount of powder has been added to obtain a good end result. Miming minor disturbances normally induce a greater use of powder and it took a lot of work to return to previous lower additions. In other words, there was no suitable indicator of whether

  The introduction of powder was sufficient.

  It has been known for a long time that precipitation follows the equation k x t - In go Ct where: k is the coefficient of the precipitation rate t is the precipitation duration CO is the initial content

Ct is the content at time t.

  According to this equation, the precipitation takes place when the conditions in the reactor are correct, the quantity of zinc powder is sufficient, etc. However, it is noted that an increase in powder additions beyond the point of sufficiency does not accelerate the precipitation. On the contrary, excessive use of the powder may even slow down the reaction,

  because alkaline zinc sulphate is formed.

  FI Publication 66027 discloses a method for purifying the solution for zinc electrolyte, wherein the amount of zinc powder required in removing the copper is adjusted to roughly match the amount of

  stoichiometric required to remove copper from the solution.

  The addition of zinc powder can be adjusted by the redox potential of the electrolyte solution. The redox potential is adjusted to control the zinc powder additions so that the potential of the electrolyte is maintained in the range of +200 to -600 mU. The redox scale used defines the degree of copper removal and limits the precipitation of other metals. The solution disposed of copper, is then directed towards the elimination of

cobalt.

  In the publication of Sawaguchi et al., Zinc Electrolyte Purification at Ijima Zinc Refinery, MMIJ / RusllMM Joint Symposium 1983, Sendai, p 217229, it is indicated that to sufficiently lower the germanlum level in the electrolyte solution in the second step of the solution purification, a potential adjustment is used to adjust the germanium content.When the potential is accordingly adjusted in the range of -610 to -640 mU, the germanium level can be maintained

below 10 ppm.

  In the publications mentioned above, measurement of the redox potential has been employed to adjust the degree of removal of the metal to be removed from the solution. This is of course an important factor from the point of view of the quality of the final product. Another factor that affects zinc production costs is the amount of zinc powder used in the purification of the solution. As is evident from the publication Fl 66027, for example in copper removal, the initial amount of added zinc powder roughly corresponds to the stoichiometric amount, after which the powder is added according to the needs of the situation. It is true that this publication indicates that the additions are adjusted using the redox potential, but on the other hand, the given scale (+200 to -600 mU) reveals that the interdependence between additions and

potential remained obscure.

  According to the method of the present invention, zinc powder additions particularly in the purification of the zinc electrolyte solution, can be adjusted to remain in the optimum range by redox potential measurements. The essential features of the invention are

  apparent in the following claims.

  In the second stage of purification of the solu-

  In the so-called cobalt removal, the remaining copper after removal of the copper is precipitated from the solution with cobalt, nickel and germanium. The table below shows the quantities of elements entering the second. step

  purification applied to the solution. The residual contents

  of the solution obtained after the second step must be extremely weak: Element Initial content Final content Cu 50 mg / l <0.1 mg / I Co 10 - 50 mg / i <0.2 mg / I Ni 10 - 50 mg <0.1 mg / kg Ge 0.1 - 3 mg / I <0.02 mg / I As noted above, the zinc metal powder and Rs203 are used in the precipitation. The precipitation process conforms to the following reaction equations: (1) Cu ++ + Zn> Cu + Zn ++ (2) 6Cu ++ + Rs203 + 9Zn> 2Cu3Rs + 9Zn + "(3) 2Me ++ + s03203 + 5Zn> 2MeRs + 5n ++

Me = Co, Hi.

  The precipitation of germanium is unknown.

  The dissolution of the zinc powder can take place as a secondary reaction: (4) Zn + H2SO> ZnSO4 + H2 (5) xZn + ZnSO4 + (x + y) H20> ZnSO4, xZn (OH) 2, YH2O $ + xH2 t The amount of arsenic is easily adjusted according to the initial levels. As a result, the use of an amount that is too small or too large leads to difficulties in precipitation or to a high final

arsenic.

  It has now been surprisingly demonstrated that by adjusting the amount of Zn powder added by means of the redox potential, optimal precipitation conditions can be maintained.

  without using excessive amounts of Zn powder.

  At the same time, the measurement also reveals possible disturbances in powder additions. The invention is also described with reference to the accompanying drawings, where the features

  of the invention are graphically illustrated.

  Figure 1 illustrates the removal of cobalt from the electrolyte solution with different values of redox potential as a function of time; Figure 2 illustrates nickel removal in the same manner as above; Figure 3 illustrates the elimination of germanium from

same way as above.

  The drawing shows that the maximum of the precipitation of the

  cobalt and nickel is already reached with the potential of -575 mU.

  The maximum precipitation of germanium falls in the range of -600 d -625 mU. The redox potential was measured with a platinum electrode and the reference electrode employed was an electrode

calomel.

  It has been found in further research that by adjusting the powder additions by means of potential measurements, the amount of added zinc powder can be substantially reduced to as much as half that of 261615e. used previously, while the impurity rate remains the same. This means that the production capacity of a unit can be essentially increased, in which case the profit achieved can be calculated according to the profit, if the electrolysis is the bottleneck of the process. Simply reducing the cost of producing zinc powder also means a remarkable profit. According to the new method of adjustment, in the second stage of solution purification, additions of zinc powder to the reactor are adjusted to a certain level by means of redox potential measurements during the reactor filling period. . This amount of powder introduced is chosen so that the Cu ++ entering the reactor with the solution does not dissolve the cobalt arsenide or nickel arsenide precipitate already present in the reactor, but the copper precipitates. On the other hand, the addition of zinc powder should be such that the Zn powder does not dissolve and the hydrogen arsenide is not created, although the solution also contains arsenic. If hydrogen arsenide is created, it is harmful to itself because of the risks it presents to the environment, but moreover, it naturally leads to an increased consumption of zinc powder. It has been shown that by employing a potential adjustment, the amount of hydrogen arsenide discharged with the exhaust gas is remarkably lower than that of the prior art. This is because now the potential does not reach such a low level that allows the creation of hydrogen arsenide. In practice, adjusting the redox potential in the range of -480 to -550 mU relative to the calomel electrode has been found to be good

solution in this step.

  When the reactor is filled, the copper that remained in the solution after the first purification step of the

  solution, is also eliminated according to the description above.

  The addition of zinc powder is then adjusted so that the precipitation of cobalt, nickel and germanium begins. In

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  In practice, this potential range is -570 to -650.U compared to the calomel electrode. Each impurity has its own range of potential and the amount of old precipitate present in the

  reactor exerts an effect on the optimal range.

  By employing the redox potential measurement, it is therefore possible to adjust the zinc powder additions to maintain the desired potential and precipitate the metals, but at the same time avoid excessive use of Zn powder. When the contents of various impurities of the solution to be introduced into the reactor are known, as well as the quantity of precipitate present in the reactor after the previous batches, it is possible to define experimentally the precipitation duration, after

  which is stopped the introduction of powder.

  Adjustment of the redox potential was described above in the second solution purification step, when the process operates batchwise. However, the adjustment of the

  Redox potential can also be achieved in a continuous process.

  As a result, the removal of cobalt can be carried out in a continuous action, or the redox potential adjustment can be

  applied in other steps of the purification of the solution.

  In the description above, the invention has been

  described primarily with reference to a process employing arsenic as an auxiliary agent. However, it should be noted that the method can also be applied to processes employing other auxiliary agents and behaves perfectly according to the invention in such cases. The optimum values of the redox potential may vary slightly from those given in the specification above, but they do not vary substantially.

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REUENDICRTIONS

  A method for adjusting the amount of zinc powder used in the precipitation of impurities of a zinc sulphate solution for the electrolytic refining of zinc, characterized in that the amount of zinc powder added is

  adjusted by means of redox potential measurements.

  2. Method according to claim 1, characterized in that the amount of added zinc powder is adjusted by means of redox potential measurements carried out in the elimination step

  cobalt from the purification of the solution.

  3. Method according to claim 1, characterized in that during the introduction of the zinc powder carried out to precipitate the copper, the redox potential is adjusted so as to remain in the range of -480 to -550 mU relative to to one

calomel electrode.

  4. Method according to claims 1 and 2, characterized in that during the introduction of the zinc powder carried out to precipitate cobalt, nickel and germanium, the redox potential is adjusted so as to remain in the

  range from -570 to -650.U compared to a calomel electrode.

  5. Method according to claim 1, characterized in that the amount of added zinc powder is adjusted by means of

  of redox potential measurements in a batch process.

  6. Method according to claim 1, characterized in that the amount of added zinc powder is adjusted by means of

  of redox potential measurements in a continuous process.

  The method according to claim 1, characterized in that the amount of zinc powder added is adjusted by means of redox potential measurements in a process employing

arsenic as an auxiliary agent.

  8. Method according to claim 1, characterized in that the amount of zinc powder added is adjusted by means of redox potential measurements in a process employing

antimony as an auxiliary agent.