US2371829A - Zinc compounds - Google Patents

Description

Patented Mar. 20, 1945 UNITED STATES PATENT OFFlE Ernst Kuss and Hans Holm, Duisburg, Germany; vested in the Alien Property Custodian No Drawing. Application luly 8, 1939, Serial No.

283,459. In Germany July 19, 1938 17 Claims.

The present invention relates to a process for the manufacture of zinc compounds by electrolyzing zinc containing liquids with mercury cathodes and working up the zinc amalgam obtained to zinc compounds, which process is particular- 1y adapted for the manufacture of pure zinc compounds from zinc containing liquids being contaminated by metal compounds other than zinc compounds.

It is known .that zinc compounds of varying composition find extensive application in industry, particularly as pigments, as medicaments, in the textile and preserving industry and so on. Frequently the highest requirements as regards purity are placed upon such products so that the processes of manufacture must either start from expensive materials such as purest zinc metal, or be burdened by cumbersome and expensive purification operations.

In accordance with the present invention zinc compounds are manufactured by electrolyzing zinc containing liquids with mercury cathodes and working up the Zinc amalgam obtained to zinc compounds. In accordance with the invention in a simple manner highly valuable zinc compounds are produced from commercial liquors, containing zinc when the initial liquors which may contain the alkali metal and alkaline earth metal salts, magnesium and aluminium compounds and also heavy metal salts in small quantities, are electrolyzed with mercury cathodes and the zinc amalgam obtained worked up to zinc compounds. Particularly suitable initial liquors are chloride solutions since they allow of the simultaneous production of chlorine as secondary product, The amalgams obtained have suitably so low a concentration of zinc that they are of good mobility at the working temperatures. Their decomposition and thus the manufacture of the desired zinc product can take place for example by the action of acids or in such a manner that the amalgams are connected up as anodes and electrolytically decomposed. Suitably the amalgam is allowed to circulate between electrolysis portion and decomposition apparatus in uninterrupted course and the speed of decomposition so regulated that the amalgam is only in part decomposed and is led back again into the electrolysis while stillcontaining zinc.

The technical advance of the process lies in the possibility of being able to produce from impure zinc liquors of complex composition in a simple manner extraordinarily pure zinc compounds as can otherwise only be produced with the application of purest zinc metal as starting material. The course by way of the amalgam has proved to be an automatic purification stage of universal nature and of high efliciency; it gives about the same effect as an electrolytic metal refining, without having the sensitivity towards certain impurities which is otherwise characteristic of zinc electrolysis. In the production of the amalgam all accompanying metals less noble than zinc are left behind, in the amalgam decomposition all more noble accompanying metals. If electrolytes or decomposing acids are employed in which zinc ions are difificultly soluble there is obtained in some circumstances a further purification effect in such a manner that the smallest traces of impurities which have escaped stoppage in the amalgam formation and amalgam decomposition, are retained in solution. It is thus possible in simple working operations from commercial zinc liquors to obtain zinc products which are frequently completely pure spectroscopically and contain no tracesof undesired anions,

A further advantage of the application of zinc in the form of amalgam compared with the ap-' plication of zinc metal in the manufacture of insoluble zinc compounds, for example basic zinc carbonate, consists in the fact that amalgam anodes do not incline to passivation.

The circulated amalgam naturally becomes enrichened in those impuritieswhich are more noble than zinc so that it must from time to time be purified; the interposition of a simple prepurification before the electrolysis step may therefore frequently be of advantage. In most cases the impurities separate as solid or pasty depositions on the mercury, since their'amalgams are difllcultly'soluble in mercury; they can be collected by immersion weirs and then passed to further working up. The purification can take liquor purification prior to the electrolysis stage,

say a zinc dust purification, in which, however, comparatively only small quantities of zinc dust are necessary and the separation need not be so complete as for example in the case of the customary zinc electrolysis.

. The process can be worked in various ways and for the production of many very pure zinc compounds; for the production of zinc sulphate,

for example, the zinc amalgam produced'by electrolysis is decomposed with sulphuric acid and the liquors thus obtained worked up to zinc sulphate. The decomposition of the amalgam takes place suitably by acids of such concentration that zinc sulphate is only relatively, difiicultly soluble therein. The zinc sulphate is then produced in the form of a precipitate directly in the acid without an evaporation process .for the recovery of the salt being necessary. In

addition the sulphate then no longer contains seven molecules of water of crystallisation but only about one; the speed of reaction is considerably increased. The zinc sulphate formed can easily be suspended, for example by stirring, in the decomposition acid and in this manner transferred to a. deposition vessel in which it forms a sediment and for further working up can be drawn off.

The concentration of the acid for decompositicn must be maintained within certain limits for the reason that too dilute acids only react slowly, whereas too concentrated acids are reduced to hydrogen sulphide and also attack the mercury of the amalgam; also the reaction temerature must not be too high. Suitably the process is carried out with a sulfuric acid of the density 1.4 and at 70-75 C.; however, the reaction can also be carried out at other temperatures and acid densities. The decomposing acid can be carried in circulation between the reaction and the deposition vessel, its composition being continuously controlled by density measurements and maintained at the desired value by addition of water or sulfuric acid. The

decomposition of the zinc amalgam is suitably not carried out completely since otherwise also the mercury will be attacked and atomised; in addition the zinc salt then contains mercurous sulphate in large quantities. The incompletely de-zinced mercury is again led back to the electrolysis.

In order to attain a high speed of reaction the amalgam can be distributed in the decomposition vessel in thin films say in such a manner that it is allowed to trickle down over vertical walls. Also by means of catalysts, say by platinum nets, which are in electrical connection withthe amalgam, the reaction speed canbe increased. Of particular influence is, however, the movement of the amalgam; it can for example be produced bystirring or in such a manner that the decomposition is carried out in a channel for example provided with wave breakers,

which is rapidly traversed by the amalgam.

In order to carry out the process various apparatus can be employed; thus for example continuous horizontal cells can be arranged in which in uninterrupted mercury circulation the zinc of the initial liquors is converted into amalgam and this subjected in a return channel to the action of sulfuric acid. Suitably the average amalgam concentration of the circulation is regulated by the speed with which the circulation de-zinced by acid between decomposer and electrolysis portion. to traverse a wash zone separated by inane;- n weirs, in which the amalgem in contact with water, zinc sulphate solu-' tion or other suitable washing agent is freed from entrained solid zinc sulphate.

The salt precipitated in the decomposing acid and then withdrawn is for example centrifuged and in this manner separated from the major portion of the acid; it always contains however some adhering acid which can be driven off by short heating for example to 600 C. together with the water of crystallisation. The sulphate thus produced is in powder form and pure white; it exhibits surprising purity even when the initial liquors. were considerably contaminated. The present process is thus particularly suitable for the dlrect working up of contaminated zinc liquors, for example of liquors from the chlorinating roasting, the more so since in the electrolytic amalgam production the current yield in contradistinction to all other aqueous zinc electrolyses is scarcely influenced by impurities.

For the manufacture of purest zinc chloride the zinc amalgam produced electrolytically is decomposed with solutions containing hydrogen chloride, for example aqueous hydrochloric acid, and the pure zinc chloride liquors thus obtained evaporated. The initial liquors can for example be sulphate solutions; with particular advantage, however, the process is applied also to chloride solutions. The chlorine produced as secondary product possesses a high degree of purity and the hydrogen produced in the decomposition of the amalgam with hydrochloric acid is also very pure so that by the mutual combustion of these gases again very pure hydrogen chloride is produced which can again be treated with amalgam.

In this case also it is advantageous to conduct the amalgam between electrolysis cell and decomposition vessel in uninterrupted circulation whereby the amalgam returning to the electrolysis cell from the decomposition vessel can have a zinc content only a little less than the amalgam flowingout of the electrolysis cell into the decomposition vessel. Suitably the decomposition vessel is itself constructed as a return channel through which the amalgam is led from the end of the electrolysis cell back to the beginning again.

It is advantageous to carry out the decomposition of the amalgam at elevated temperatures and with concentrated hydrochloric acid; it is, however, also possible to work in the cold if the decomposing acid has a suflicient concentration, for example is saturated with hydrogen chloride. It is possible to carry out the decomposition in such a manner that during the reaction hydrochloric acid gas'is conducted into th decomposing acid. It is, however, also possible to work with dilute acids if the reaction-is supported by catalysts, i or example'by platinum wire nets immersed in the acid, which are in electrical contact with the amalgam, The working up of the zinc chloride liquors thus obtained can take place in known manner say by evaporation in enamelled pans.

As is knownthe complete separation of the water from zinc chloride meets with considerable difiiculties; it can only be carried out by electrolysis of the melt or by its decomposition with metallic zinc and entails considerable expense. For the manufacture of anhydrous zinc chloride according to the invention therefore the decomposition of the amalgam is efl'ected by means of solutions of hydrogen chloride gas in non-aqueous organic solvents which are capable oi dissolving both the zinc chloride and the hydrogen chloride in considerable quantities.

There are suitable for this purpose for example ethyl ether or methanol. The solvents are evaporated from the zinc chloride and recovered for further use. For removal of the last residues of the organic solvent it is necessary to melt the zinc chloride, whereby a small portion of the solvent is decomposed with separation of carbon; the

melt can be bleached in known manner by additrolyte and converted into oxides in the manner.

known per se.

For example zinc amalgams with a zinc content of 2-6% are reacted in cells in which they are connected as anodes using cathodes consisting say of zinc or iron; the electrolyte consists for example of practically saturated sodium bicarbonate solution. The decomposition takes place in such a manner that at the anode a zinc compound containing carbonate is produced, while at the cathode hydrogen and caustic soda solution are formed. According to the invention the electrolyte serving for the decomposition of the amalgam is continuously regenerated by passage of carbon dioxide, say from valueless waste gases, and passed back into the cycle; it is suitable to rinse the cathode itself with carbonic acid. Advantageously the electrolyte is maintained in acondition of rapidfiow in order to remove the insoluble zinc compounds continuously from the cell. These can be collected in a deposition vessel and in known manner be filtered, washed, dried and calcined at suitable temperaures; if necessary the hot oxide thus obtained is introduced into water, filtered again and then dried; it can also be passed through suitable mills and sifted. In this manner zinc oxides are obtained of extraordinary purity, in particular also products with all the properties of an excellent white pigment.

It is suitable to combine each cell for the production of the amalgam with a decomposition cell to a closed aggregate in which the amalgam continuously circulates without addition or removal of mercury being necessary. The current yields in the manufacture of the amalgam and in its decomposition are, however, not quite the same; in most cases the current yield in the amalgam decomposition is somewhat higher. It therefore appears in the first place necessary to operate the two cells with different current strengths and from this further dificulties arise for the linking up of a large number of aggregates. In such a case two different sources of voltage are necese sary for the two electrolytic processes or resistances must be incorporated which render a part of the energy supplied useless; the amalgam circulating between formation and decomposition cells has to be electrolytically interrupted at two places and the two associated cells carefully insulated from one another.

In accordance with a further feature of the present invention in spite of the different current yields a plurality of aggregates can be operated with one and the same current source without loss of energy and in a simple manner when the amalgam flowingthrough the two cells of an aggregate is conducted without electrical interruptions, that is to say maintained at the same potential, and the poles of the voltage source, or the adjacent aggregates connected to the two other electrodes of the aggregate, that is to say to the anode of the formation and the cathode separated by weirs in which it is freed from small I of the decomposition cell and when one of the two cells combined in the aggregate, namely that with the better current yield, is short circuited from time to time. This short circuiting can take place according tothe amalgam concentration prevailing in the aggregate, by hand, or also by an automatically-operating, correspondingly adjusted time switch; it has the purpose of maintaining the amalgam concentration, which on account of the different current yields continuously varies, and in fact as a rule decreases, constant within certain'limlts.

The invention is illustrated by the following examples without being restricted thereto:

' Example 1 ing much common salt which contains in addition in large quantities calcium and magnesium chloride, further some sulphate and small quantitles of copper, cadmium, cobalt, nickel and iron; the zinc content amounts to 65 grams per litre. This liquor is electrolyzed in -'a horizontal cell provided with graphite anodes; the operating temperature amounts to C. the current density i200 amps. per sq. metre. The electrolytically formed amalgam then likewise passes in the form of a horizontal stream but with ten times the streaming velocity through an extended channel-like reaction vessel in which wave breakers .are incorporated for production of a very turbulent fiow. Here sulfuric acid of density 1.4 and temperature C. flows in counter-current to the amalgam. After traversing the channel the amalgam is raised by means of a bucket conveyor to its original height and in this manner continuously passed in circulation between cell and decomposer. At the commencement of the decomposition channel the zinc concentration of the amalgam amounts to 4%, at the end 3.8%. Between decomposition channel and bucket conveyor the amalgam traverses a washing chamber quantities of entrained sulfate by means of a zinc sulfate solution containing weak sulfuric acid, which after concentration is renewed. The

decomposition acid leaves the channel in a rapid stream and contains the zinc sulfate formed suspended in fine small crystals; it is now passed through a thickener and cleared whereupon it passes through an automatic density regulator back to thereaction channel. The zinc sulfate is removed from the thickener and supplied to a centrifuge in which it is well centrifuged; finally it is heated in'a rotating drum to 600 C. the

traversing speed being adjusted to a period of treatment of 20 minutes. The zinc sulfate produced is anhydrous, of neutral reaction and spectroscopically pure.

Example 2 A liquor produced as a result of the chlorinating roasting of a roasted pyrite containing zinc,

with '70 grams per litre of zinc, which together with much common salt, calcium and magnesium chloride also contains small quantities of copper, cadmium, cobalt and iron and also some suirate, is electroiyzed at 80 C. and with a cur rent density of 1400 amps. per sq. metre with a mercury cathode and graphite anodes. The mercury cathode flows in the form of a wide band slowly under the anode; it leaves the cell with a zinc content of 4% and passes into a wide return channel in which it is covered with a layer of saturated aqueous hydrochloric acid. The decomposition channel which is closed to the exterior possesses an exit pipe for the evolved hydrogen and a supply pipe for hydrogen chloride gas. The amalgam leaves the decomposition channel with a zinc content of 3.8% and passes back into the electrolysis cell. From the channel is continuously drawn off a zinc chloride liquor containing about 350 grams per litre of zinc, which is evaporated to dryness in enamel pans and finally fused. The zinc chloride produced is spectroscopically and polarographically pure and contains no trace of sulfate or other foreign anions.

Example 3 A liquor obtained as a result of the chlorinating roasting of a roasted pyrite containing zinc, with 68 grams of zinc per litre, which contains together with much common salt, calcium and magnesium chloride, also small quantities of copper, cadmium, cobalt and iron, is electrolyzed at 80 C. with a current density of 1400 amps. per sq. metre with a mercury cathode and graphite anodes and a 5% amalgam produced. This is conveyed to a second cell and here arranged as anode with a cathode consisting of iron; as electrolyte is employed a liquor with 84 gram per litre of sodium bicarbonate, under the cathode is passed a slow stream of carbonic acid gas. The operating temperature herelikewise amounts to 80 C. but the current density to only 1000 amps. per sq. metre. The electrolyte is passed with brisk flow over the amalgam surface so that the crude pigment formed is entrained; it'is passed into a deposition vessel from which the clear liquor is again passed into the cell. The amalgam is replaced by a fresh batch after its content of zinc has sunk to 1%. The crude pigment is withdrawn from the deposition vessel, washed by repeated decantation, filtered, dried and ignited at Example 5 A cell for the formation of zinc amalgam and a decomposition cell are connected by channels in such a manner that an uninterrupted mercuryasrneao so on. The basic zinc compound produced is worked up to oxide in the manner set forth in Example 4.

We claim:

1. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercurycathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam to decompose the amalgam only partially and to form a zinc compound, and separating the zinc compound obtained from the amalgam.

2. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with a chemical reactant to decompose the amalgam only partially and to form a zinc compound, and separating the zinc compound obtained from the amalgam.

3. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with an acid to decompose the amalgam only partially and to form a. zinc the zinc salt obtained from the amalgam.

4. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with sulfuric acid to decompose the amalgam only partially and to form zinc sulfate, and separating the zinc sulfate obtained from the amalgam.

5. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with asulfuric acid solution in which zinc sulfate is diflicultly soluble, said treatment being to decompose the amalgam only partially-and to form zinc sulfate, and separating the zinc sulfate obtained from the amalgam.

6. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, leading a sulfuric acid solution in turbulent flow over the amalgam to decompose said amalgam only partially and to form a suspension ofzinc sulfate, causing the sus pended zinc sulfate to settle, separating the settied zinc sulfate from the supernatant liquid and drying it.

7, In the process of claim '6 the step which comprises heating to about 600 C. the zinc suisalt, and separating fate to eliminate the sulfuric acid adhering wit.

8. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with hydrogen chloride to decompose the amalgam only partially and to form zinc chloride, and separating the zinc chloride obtained from the amalgam.

9. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amal-' gam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with an aqueous solution of hydrogen chloride to decompose the amalgam only partially and to form zinc chloride, and separating the zinc chloride obtained from the amalgam.

10. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with a solution of hydrogen chloride in an organic solvent to decompose the amalgam only.

partially and to form zinc chloride, and separating the zinc chloride obtained from the amalgam.

11. Process for the manufacture of a zinc compound which comprises electrolyzing with a mer-' cury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam obtained from the liquid, treating the amalgam with an etherealsolution of hydrogen chloride to decompose the amalgam only partially and to formzinc chloride, and separating the zinc chloride obtained from the amalgam.

I 12. Process for the manufacture of a zinc co pound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amal- 'gam is obtained, separating the amalgam obtained from the liquid, subjecting the'amalgam to electrolysis to decompose the amalgam only partially and to form a zinc compound, and separating the zinc compound obtained from the amalgam.

13. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgam is obtained, separating the amalgam'obtained from the liquid, subjecting the amalgam to electrolysis with zinc amalgam as the anode and an aqueous solution containing an alkali metal salt of carbonic acid as electrolyte, said electrolysis being to decompose the amalgam only partially and to form a carbonic acid compound of zinc, and separating the carbonic acid compound of zinc obtained from the amalgam.

14. Process for the manufacture of a zinc compound which comprises electrolyzing with a mercury cathode an impure zinc containing aqueous liquid, whereby an impure, zinc-containing amalgem is obtained, separating the amalgam obtained from the liquid, subjecting the amalgam to electrolysis with zinc amalgam as the anode and an aqueous solution containing sodium bicarbonate as electrolyte, said electrolysis being to decompose the amalgam only partially and to form a carbonic acid compound of zinc, and separating the carbonic acid compound of zinc obtained from the amalgam.

15. In the process ofclaim 13 the step which comprises calcining to zinc oxide the carbonic acid compound of zinc obtained.

16. In the process of claim 13 comprises introducing carbon dioxide into the electrolyte during the decomposition of the zinc amalgam.

17. In the process of claim 1 the steps which comprise interrupting the decomposition of the amalgam after the amalgam is only partially decomposed and'reintroducing the remaining zinc amalgam into the first step of operation.-

ERNST KUSS. HANS HOHN.

the step which