CA1075195A - Arsenic removal from electrolytes - Google Patents

Abstract

"ARSENIC REMOVAL FROM ELECTROLYTES"

ABSTRACT OF THE DISCLOSURE
A method is provided for removing arsenic from arsenic and copper containing electrolytes by electrolysis while mini-mizing the formation of arsine gas, through the application of a periodically reversed or interrupted current during such electrolysis. The method is particularly suitable for the purification of copper refinery electrolyte.

Description

~'7~5~5 This invention relates to a novel method of rernoving arsenic from arsenic and copper containing electrolytes by electrolysis while minimizing the foxma-tion of toxic arsine gas.
More particularly, the me-thod provides for the application of a periodic reverse current during electrolysis leading to deposi-tion of arsenic, copper and eventua:Lly other metallic elemen-ts present in the electrolyte onto the cathode while substantially reducing the formation of arsine gas which would normally be formed at the cathode under the same electrolysis conditions, but with the conventional application of direct current.
The novel method is particularly suitable for the puri Eiciation of copper refinery electrolyte.
The use of periodic reverse current has been well known in the electroplating industry for a good number of years. For example, in U.S. Patent 1,534,709, issued to F. A. Holt on April 21, 1925, there is described a method of conducting electrolytic operations in which periodic reversal of the current i5 used to depolarize the electrodes during the electroplating of copper from an acid bath at high current density. U.S. patents Nos.

2,451,341 of October 12, 1948 and 2,575,712 of November 20, 1951, both in the name of G. W. Jernstedt, describe other methods oE
electroplating of metals selected from the group consisting oE
copper, brass, silver; zinc, tin, cadmium and gold with the use of periodic reverse current.
It i5 also known to use periodic reverse current in the electrolytic refining o copper as described, for example, in .
British Patent specification No. 1,157,686 in the name of Uedodobiven Kombinat "Georgi Damianov", published on July 9, 1969, and U.S. patents N~os. 3,824,162 of July 16, 1974 to Kenichi Sakii et al and 3,864,227 of February 4, 1975 to Walter L. Brytczuk et al.
Furthermore, there are also known processes Eor apply-~ ing periodic reverse current for the electrowinning o-E copper ''' , . ',. . " ' ' ' ~i7~ 5 (Canadian Patent No. 876,284 oE July 20, 1971 to Donald A. Brown et al) and for the electroextraction of zinc (Canadian Patent No. 923,845 of April 3, 1973 to Ivan D. Entshev et al.).
The present applicant has now found a new and a very surprising application of periodic reverse current for the purpose of removing arsenic from arsenic and copper containing electroly~es while minimizing the formation o~ toxic ar~ine gas which is a constant health hazard in such operations.
It is well known,for example, that, during electro~
reining of impure copper, the impurities present in the anode are either dis~olved into the solution as soluble compounds or precipitated in the form of insoluble compounds. To avoid con-tamination of the cathode copper~ it is essential to control the concentration of undesired soluble impurities by puriication of the electrolyte. Such electrolyte purification is carried out by passing a part of the tankhouse solution through the so-called liberator cells containing insoluble anodes, such as anodes made o~ lead or lead alloys, whose main purpose is to control the copper le~el o~ the electrolyte. After partial decopperization of the electrolyte, the solutian is directed into purification cells, which are electrowinning cells where copper i5 depleted to low levels and, meanwhile, arsenic, antimony, bismuth and possible other impurities are co-deposited onto the cathode, thus providing a means o controlling the concentration of these impu-; rities in the electrolyte. During this co-deposition, arsenic is reduced at the cathode to its metallic form and at low copper concentrations to its hydride form, thus liberating the toxic arsine gas. The ~iberation of this arsine gas presents a major problem for every copper refinery in the world since it consti-tutes a constant h alth hazard to its workers. It is known that arsine gas is extremely toxic and an exposure thereto in a con-~ ; conaentration o 250 ppm for thirty minutes is fatal while exposure ; to concentrations as low as 10 ppm can cause poisoning symptoms 75~S~5 in a ~ew hour~ (cf. American Conference of Governmental IndustrialHygienists: Threshold Limit Values ~or 1964, ~MA Arch. Environ.
Health 9:5~5 (1964)). It i5, there~ore, extremely important to minimize the evolution of arsine yas in all operations involving electrodeposition of arsenic from electrolytic solutions. A good agitation of the electrolyte as well as application of low current densities and high electrolyte temperatures have been ~ound to de-crease the rate of arsine gas formation. However, these method6 alone are not sufficient in themselves and, consequently, they are normally accompanied by a strong ~entilation system to avoid dan-gerous concentrations of the toxic arsine gas close to the purii-cation cell. Obviously, such ventilation system merely transports the toxic ga~ ~rom one place to another, namely from the workroom to the atmosphere and this may ~e found unacceptable by khe ever stricter anti~pollution regulations implemented by the various go-vernmental authorities. Furthermore, ventilation systems are pro-ne to breakage and require a great deal of maintenance.The process of the present invention minimizes the formation o~ the arsine gas at the ~ource, namely at the cathode and, consequently, to a great extent; obviates the disadvantages encountered heretofore~
Basically, therefore, the present inven~ion provides a method of removing aræenic from arsenic and copper containing elec-trolytes in which copper aoncentration reaches such low levels as to liberate toxic arsine gas, which comprises carrying out an electrodepoqition of the arsenic on a cathode by applyin~ through the electrolyte a direct current and periodically reversing the polarity o~ the current so a~ to minimi e the formation of arsine ~ ~ qas at the cathode during such electrodeposition.
;~ The electrolyte is pre~erably an acidic electrolyte, ;30 such as, for example, an aqueous solution co~taining sulphuric acid and copper ion therein. This electrolyte is also prefer bly maintained at a temperature between about 50 and 75C during the electrodeposition and is also preferably circulated at an adequate

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rate ~hich is usually in the range of about 40 to 70 U~S. gallons per m.inute for cells having a cathode surface area of about l,000 square feet each. Lower or higher rat~s could also be suitable and the novel process is certainly not restricted by the preferred flow rates mentioned above.
The initial arsenic concentration of the electrolyte can vary within a wide range; for example, it can extend from less than l gram per litre to about 30 grams per litre. This is the normal range for arsenic containing electrolytes occurring in in-dustry. Furthermore, the anode used in such electrodeposi.tion is pre~erably an insoluble anode, for instance, made oE lead or lead alloys, while the cathode is usually made of a metal such as cop-per or stainless steel.
The current density normally applied during such. elec-: trodeposition would vary between about 5 and about 30 amps. per square foot, the forward current being applied during periods of 5 ~ 30seconds wh.ile the reverse current during periods of l - 4se-conds alternating with. the forward current application. The ratio of the duration of reversed to forward current application is usually between 2- and llO.
In its most preferred embodiment, thepresent application .
provides a method of purification of copper refinery electrolyte, which. comprises passing the electrolyte through electrolytic cells containing insolu~le anodes, applying a direct current through the-~ .
~ se cells~so as to.co-deposlt copper, arsenic, antimony and bismuth ::~ present in the electrolyte onto ca~hodes in these cells, andperio-~ dically reversinc3 the polarity of the current such as to minimize ;~ formation of arsine gas during the co-deposition of copper andar-senic onto the cathodes. Under these conditions the electrolyte :; 30 entering the cells in which the polarity is periodically reversed : will usually contain about 6to a~out 12 grams perlitre of Cu and ~.
; ~ about 4 to about 8 grams per litre of arsenic and the co-deposi-~ tion o~ copper and arsenic will be permit-ted to proceed until the :
:

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electrolyte leaving the cells contains between about 0.3 and about 1 gram per litre of Cu and between about 1 and about 2 grams per litre of ~5 .
In this operation, each of the cells employed has a cathode surface area of a~out 1,000 square feet and contains about 1,400 U.S. gallons o electrolyte. The flow rate of the electrolyte through these cells is preferably maintained between about 40 and about 70 gallons per minute during the co-deposition of copper and arsenic onto the cathodes which are preferably made of copper starting sheets. The temperature of the electrolyte is also preferably maintained between about 50C
and a~out 75C and the current density ~etween about 10 and about 25 amps. per square foot.
It is also possible to vary the current density during the co-deposition of copper and arsenic. Thus, the initial current density i9, preferably, maintained near the lower limit of about lO amps. per square foot and, after a few hours of operation, it can be increased to near the higher limit of about 25 amps. per square foot, without producing any substan-ial increase in the arsine gas evolution.
Again, the forward polarity may be applied for periods of 5 to 30 seconds while the reverse polarity for periods of 1 to

4 seconds with the ratio o~ reverse to forward polarities being be~ween ~ and r~.
In addition to arsenic and copper, the electrolyte entering the cells will usually contain small amounts (about 0.1 to~about 0.4 grams per litre) of Sb and of Bi and the electrolyte leaving these cells will have reduced each of these elemPnts to about 0.01 - O.O'j grams per litre.
The invention will now be described with reference to ~:
, the ~ollowing non--limitative examples which illustrate the pre-ferred~operating conditions as well as the advantages of the ~ -novel process.

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~0~5~L5~5i Examples l to ll _ _ Eleven experimentalex~mples of thepurification ofelectro-lyte under periodic reverse current (P.R.C.) and direct current I (D.C.~ electrolysis conditions were carried out on a laboratory ! scale in a cell of a 40 litre volume using copper starting ¦ sheets as the cathodes and lead-antimony insoluble anodes.
The electrolyte feed rate into the cell was 21 ml/min and the electrolyte was circulated in said cell at a rate of 800 ml/min while the temperature of the electrolyte was main-tained at 65C.
The first eight examples were carried out under P.R.C.
conditions having the following characteristics:
Forward current (If) = 62 amps. (corresponding to 21 amps.
per square foot current density).
Revexse current (Ir) = 36 amps. (corresponding to 12.2 amps.
per square foot current density).
Forward time (Tf) = lO seconds.
Reverse time (Tr) ~ 2 seconds.
The last three examples, namely examples 9, 10 and ll, were carried out under D~Co conditions with the direct current ~I) = 30 amps. (corresponding to 10 amps. per square foot current density).
The results obtained under these experimental conditions were then extrapolated to a full scale plant application for ~ighteen operational cells, each having about 1,000 square feet in cathode surface area and containing about 1,400 U.S. gallons of electrolyte, and five days of sixteen hours plus two days of ; twenty four hours per week of normal operation.
The actual results of the experiments are given in Table I~hereafter and the extrapolated full scale plant results are given in~Table II hereafter.
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__. _ _____ _ The above tables, and particularly, the results tabulated in the last columns thereoE, showing the arsine gas emission amply demonstra-te the substantial decrease in the arsine gas formation and emission when using P.R.C. conditions as compared to D.C.
conditions.
These results are even more striking when one considers that higher current densities have been used under P.R.C. conditi-ons than under the D.C. conditions and it is weIl known that evolu-tion of arsine gas increases with t:he increase in current density.
The effects of various P.R.C. conditions, as well as tem-perature, circulation rate, duration of electrolysis and the like on the novel process were studied and some of these effects are represented in graphical form in the attached drawings in which:
Figure 1 shows a graph illustrating the variation of arsine concentration in exhaust gas as a function of duration of electrolysis for various P.R.C. conditions for eighteen cells when operating with fresh copper cathodes.
Figure 2 shows a graph illustrating the effect, under spe-cific P.R.C. conditions, of electrolyte temperature on the arsine gas concentration in the stack.
Figure 3 shows a graph illustrating the effect, under spe-cific P.R.C. conditions, of electrolyte circu~ation rate on the - arsine gas concentration in the stack, for eighteen cells using fresh cathodes.
Reerring now to Fiyure 1, it shows, in graphical form, that various conditions of forward current densities and reverse current densities as well as forward times and reverse times of electrolysis lead to different amounts of arsine gas evolutions which have been expressed as pounds of arsine per day in the table and as ppm of arsine in the exhaust air flowing at 20,000 cfm for an eighteen cell plant operation with the use of fresh copper cathodes.
The graph has been drawn using the data of the following Table III, which were obtained experimentally.

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This graph clearly shows that, after a few hours of operation, in all these cases, -the arsine evolution decreases very drastically, which is believed to be due to the fact that, as the process progresses, a powdery deposit of coppex and arsenic, as well as o-ther impurities, is obtained on the fresh cathodes, substantially increasing their e~fective surface and thereby substantially decreasing the effective current density at the cathode to such low levels that, at one point, there is no arsine evolution at all. As already mentioned above, when the current density decreases, the arsine evolution also decreases and, at a certaln point, it is entirely eliminated, as this i.s illustrated in Figure 1. Obviously, the cathode must be re- ..
placed after a certain duration of electrolysis and, therefore, at that stage, the arsine will again begin to evolve for the first several hours of the process. In an eighteen cell puri-fication plant, the replacement of the cathodes can be done at predetermined intervals and in such manner that only part of the cathodes will be replaced at each particular time, thus even further decreasing the total amoun-t of arsine emission per day.
. ~ 20 Obviously, a man of the art can readily select the best conditions for his own plant or purification system, which will . give him the most satisfactory results while minimizing the arsine gas emissions in the system or decreasing them to a de-sired value, depending on the amount of arsenic and/or copper that needs to be removed.
For the operations studied by the present applicant, the best conditions exist when If = Ir = 10 asf and where Tf = 10 sec. and Tr = 2 or 3 sec.
Referring to Figure 2, the effect of the electrolyte temperature has been shown to be quite significant under the .
predetermined P.R.C. conditions where If = 10 asf, Ir = 10 asf, Tf = 10 sec. and Tr = 3 sec. employed i.n an 18 cell system with fresh cathodes. From this graph, it will be seen that, wh~n the ~75~

electrolyte is at 65 or 70C, there is a much smaller evolution of arsine in an air flow in the stack of 60,000 cfm than at 40C.
Thus, the preferred temperature range for the electrolyte is between about 50 and 70C.
Referring to Fiyure 3, it shows that the effect of the electrolyte circulation may be significant for specific P.P~.C.
conditions where I~ = 10 asf, Ir = 10 asf, T~ = 10 sec. and Tr = 3 sec., again employed in an eighteen cell system with fresh cathodes. When circulation is 5 U.S. gallons per minute, the arsine evolution is higher than when it is 60 U.S. gallons per minute by a ratio of about 10:1. Thus, at 5 U.S. gallons per minute, about 0.5 lbs per day of arsine for eighteen cells will be evolved while at 60 U.S. gallons per minute, only about 0.05 lbs. per day of arsine will be evolved in a stack having an air flow of 60,000 cfm.
Again, a man of the art should have no difficulty in adjusting his specific conditions of temperature and electrolyte circulation to the desired values of arsine elimination and to -his desired requirements generally.
Other effects have also been studied and, for example, the effect of the reverse pulse duration, in seconds, for experi-mental conditions, such as those employed in previous examples, has been tested.
The following Table IV illustrates the results of these tests where arsine gas emission is given for eighteen commercial cells operating at 10 amps.per square foot with fresh cathodes.

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- 107 S~ 5 From this table, it is obvious that different forward and reverse pulse durations with difEerent current densities will give different results which can be adjusted to the desired operating conditions. Again, the smallest arsine evolution appears to be when the forward and reverse current densities are at 10 amps. per square foot and the forward pulse is 10 seconds while the reverse pulse is 2 or 3 seconds.
The applicant has also studied the effect of increasing the current density during the last stage of purification under P.R.C. conditions. It has general:Ly been observed that the in-crease of the current density after three or four hours of electrolysis from ten amps. per square foot to 15 or 20 amps.
per square foot for the remainder of the electrolysis cycle (which is sixteen hours in the present case) will not cause any significant increase of arsine emission rate since the cathodes will be relatively old and covered with a powdery deposit which decreases the effective current density at the cathodes to a substantial degree.
It should also be mentioned that the novel method can be applied to various electrolysis systems. For example, a con-tinuous feed and withdrawal system with recirculation of eIectrolyte can be employedO Also, the so called "cascade"
system where the electrolyte is passed thEough a plurality of cells in series can be used. Finally, a batch system in which the electrolyte remains in the cell under agitation until the desired levels of copper and arsenic are achieved can also be employed. In the case o~ a batch type operation, electrolysis experiments have been carried out using small scale cells (700 ml in volume) and large scale cells t40 litres in volume) to compare the amount of arsine gas evolved under both D.C. and P.R.C~ electrolysis conditions. The electro-lyte in these cel:Ls, which contained 6 to 10 gpl copper and about ;; ~ 6 gpl arsenic,was agitated and decopperized to low , .
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concen-trations. Th~ electrolyte temperat~lre was maintained at 60 to 65C. The agitation, in both cases, (small and large scale) was maintained such as to correspond to a flow rate of the electrolyte of 60 U.S. gallons per minute in a commercial cell having about 1,000 sq. ft. of cathode surface area.
The P.R.C. conditions were:
1. For the small scale experiment:
I~ = 21 asf Ir = 17 asf Tf = 10 sec.

Tr = 2 sec.
2. For the large scale experiments:

E r 21 asf Tf = 10 sec. Tr = 2 sec.; and If = 15 asf Ir = 10 asf Tf = 10 sec. Tr = 2 sec.
The results are shown in the following Table V where the arsine emission rates under P.R.C. and D.C. conditions are given. It can again be consluded from these results that the application of P.R.C. electrolysis during decopperization causes a drastic xeduction of arsine gas evolution.
TABLE V

,,, ......... , . .. _ . . . _ . _ .... . _ . _ _ . _ _ _ .
. CellArsine Gas Evolution Rate Type of Electrolysi Capacity ( ~ amp.hr.) at .
0.52-0.57 ~pl Cu 0.3-0.34 ~pl Cu _ ~ _. _. _ __ P.R.C. 21 asf 0.7 lltres 0.513 2.05 D.C. 2l a I ~ ._ . __ 39 4 P.R.C. 21 as-f 40 litres 0 0.042 : P.R.C. 15 asf . 0 0 : D.C. 21 asf 7.0 9.7 D.C. 10 asf _ 4.7 . ~ ~ .

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i Finally, a full scale purification plant ba~ed on this process has been built at Canadian Copper Refiners Limi-ted in Montreal East. This plant comprises twenty seven liberator I cells which regulate the copper levels in the electrolyte, in I which the copper is plated out o~ the tankhouse electrolyte to about 30 gpl Cu. About 70% of the electrolyte is then returned to the tankhouse and 30~ is treated further in nine cells that deplete copper to about 9 gpl Cu.
The electrolyte from these nine cells depleted to about 9 gpl Cu is then fed into eighteen purification cells forming the purification process under periodic reverse current corl-ditions described above. Each purification cell has a cathode surface area of about 1,000 sq. ft. In these cells, the electro-lyte is recirculated at the rate of 50 U.S. gallons per minute per cell and the copper depletion proceeds under P.R.C. con-ditions to about 0.4 gpl.
I The operating data of a test run carried out in this plant ! were as follows:
Operating Data Current Density: Forward 14.7 asf - Reverse 10.6 asf Pulse Duration : Forward 10 sec. Reverse 2 sec.
Air Flow in Stack: 60,000 cfm Arsine in Stack - Avg. oper. conditions: 20 ppb (parts per billion) - New cath. in one tier:200 ppb (parts per billion) Circulation rate : 50 U~S.G.P.M. per cell Temperature : 60 - 65C
Feed Solution Analysis: Cu 9.4 gpl ~; 30 As 6.18 gpl Sb 0.38 gpl , Bi 0.28 gpl Rate of Introduction of Feed Solution: 24 U.S.G.P.M.

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Outlet Solu-tion An~lysis; Cu 0.36 gpl As 1.70 gpl Sb 0.08 gpl Bi 0.02 gpl Two automatic arsine detectors are used to monitor the workroom environment and the stack emissions. A third detector acts as a spare. The arsine monitor in the stack is set to cut off the power from both the liberator and the purification recti-fiars when the stack emission reaches a predetermined value of arsine. The arsine evolution has been less than 1 lb. per day and the upper limit in the stack has been set at 1.5 ppm.
The plant has been test run for some time and has opera-ted satisfactorily providing a purification of the solution with removal of about 1,000 lbs. of arsenic per day,which is deposited on the cathodes together with copper, ~ismuth and antimony. These cathodes are then removed and sent to the smelter for further treatment~ such as recovery of the copper.
This is believed to ~e a remarkable achievement which provides a su~stantial improvement over the known prior art in the field of copper refinery electrolyte purification by minimi-zing the arsine emissions and thereby the health and/or pollution hazard that they provoke. A substantial contribution has, there-fore, been made in the art of arsenic electrodeposition generally -~ and in the art of copper refinery electrolyte purification in particular.
From the foregoing results, it is, however, obvious that ; the invention is not limited to the specific conditions given in , the above exampIes, but rather provides a new principle of elec-~;~ trodeposition of arsenic, eventually in combination with other 3Q~ elements, while minimizing arsine emission by the use of periodic reverse current.

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~7~ 5 SUPPLEMENT~RY DISCLOSURE

It has ~een found that the method in accordance with this invention proceeds not only with the application of periodically reversed current (PRC), but also with the appli-cation of periodically interrupted current (PIC) in order to substantially reduce the formation of arsine gas durin~ electro~
lytic removal of arsenic from arsenic and copper containing eleatrolytes.
Thus, the present invent:ion also includes a method o~
removing arsenic from arsenic and copper containing electrolytes in which copper concentration reaches such low levels as to liberate toxic ar~ine gas, which comprises carrying out an electrodeposition o~ the arseni on a cathode by applying through the electrolyte a direct current and periodically interrupting said current so as to substantially reduce the ~ormation o~
arsine gas at the cathode during said electrodeposition.
Furthermore, in its preferred embodiment, the present invention also includes a method of purification of copper refinery electrolyt~ which comprises passing said electrolyte through electrolytic cells containing insoluble anodes, applying a direct current through said ~ells so as to co-deposit copper, arsenic, antimony and bismuth pr~sent in the electrolyte onto cathodes in said celIs, and periodically interrupting said current in said cells so as to substantially reduce the : formation of arsine gas during co-deposition of copper and : arsenic onto the cathodes.
As in the case o~ PRC, the current density applied under PIC electrolysis normally varies between a~out 5 and about 30 amps per ~quar~e foot, however, the forward current may be applied during pe.riods of 2 - 30 seconds, preferably 2 - 15 30 second~, and the .interruptions (zero current pulse durations~
are normally 1 - 6 seconds, preferably l - 3 seconds, while the , :L~7S~S
ratio of zero to forward current pulse durations is usually between 3 and 1O. The preferred current density range under PIC
conditions is between about 10 and about 20 amps. per square foot.
The invention will now further be described with reference to the following additional non-limitative examples which illustrate the various operat:ing conditions of the novel method as well as its advantages.
Examples 12 to _ Fifteen experimental examples of the purification of electrolyte under direct current (DC) electrolysis conditions, periodically reversed current (PRC) electrolysis conditions and periodically interrupted current (PIC) electrol~sis conditions (two examples with DC, two examples with PRC and eleven examples with PIC) were carried out on a laboratory scale using 1500 ml of ;

bleed-off solution obtained from Canadian Copper Refiners Limited, in Montreal East, which was recirculated through a 750 ml elec-trolysis cell. Fresh bleed~off solution containing about 10 gpl Cu was continuousl~ fed to the recirculating stream at a rate of 0.6 ml/min to maintain constant solution composition. The re-circulation flow rate was of 120 ml/min with the exception ofexamples 25 and 26 which were aimed at studying the effect of the flow rate. The electrol~sis cell contained two lead anodes and one copper cathode centrally located between the two anodes.
Each example was started using a fresh cathode surface and lasted for three hours to evaluate the effect of cathode age on the rate of arsine gas e~olution.
The results of the experiments are outlined in ~ables ~; VI and VII which ~ive, for each example, the operating conditions, the rates of copper removal and copper deposition current efficlencies, the rates of arsine gas formation for three differ-ent periods during each example (in mg AsH3/amp.hr.) and the total amount of arsine gas formed (Table VI), as well as the rates of arsenic, antimony and bismuth removal (Table VII).

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The above tables, and particularly the results show-ing the arsine ~as emissions, amply demonstrate the substantial de crease in the arsine gas formation and emission when using PRC
or PIC conditions as compared to DC conditions.
The com~arative effectr, of arsine gas emission for the DC, PRC and PIC conditions were studied for two difEerent current density values and they are represented in graphical form in the attached drawings in which:
Figure 4 shows a graph illustrating the variation of arsine gas emission as a function of electrolysis duration for some DC, PRC and PIC examples, at a curren-t density of 10 asf; and Figure S shows another graph illustrating the variation oE arsine gas emission as a function of electrolysis duration for some DC, PRC and PIC examples, at a current density of 15 asf.
Referring to these Figures 4 and 5, they illustrate the comparative emissions of arsine gas AsH3 in mg/amp.hr. for DC, PRC and PIC conditions, at 10 asf and 15 asf respectively and at an electrolyte temperature of 50C, as a function of electro-lysis duration. From these figures, it is clear that both PRC
and PIC conditions present very substantial improvement over ~C
electrolysis. Moreover, it can be seen from Figure S that, under ~;~ given conditions, arsine evolution under P~C conditions will be greater at the beginning of the electrolysis than arsine evolution under PIC conditions, but as electrolysis progressPs arsine evolution under PRC will fall more drastically than under PIC.
Thus, the lowest or minimum arsine gas emissions are obtained with the use of PRC, particularly after the electrolysis has been carried out Eor a certain period of time, however, ovPrall ; emissions during, let us say, 3 hours of electrolysis, could be more or less equivalent for both PRC and PIC conditions, as illustrated in Fic3ure 5.

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Some other effects wnder PIC conditions have also been studied and they can be summarized as follows.
l. Effect of Electrolyte Temperature The effect of variations in the electrolyte temperature on the rate of arsine gas emission is indicated by the results of examples 23 (40C), 22 (50C), and 24 (65C), all carried out at a current density of 15 asE using forward and zero current pulse durations of 5 and :L second respectively. The results indicate that the rate of arsine gas emission decreased from 158.3 to 86.7 and 41.9 mg/3 hr when the electrolyte temper-ature is increased from 40 to 50 and 65C respectively.
2. Effect of Electrolyte Recirculation Rate . . _ A comparison of the results of examples 26, 22 and 24, all carried Otlt at a current density of 15 asf and at a temperature of 50C, with forward and zero current pulse durations of 5 and l seconds respectively, indicates that the arsine gas emission decreases from 112.1 to 82.7 and 80.3 mg/3 hr when the recirculation flow rate is increased from 40 to 120 and 180 ml/
min. respectively.
3. Effect of Variations of Forward and Zero Current Pulse Durations . .
The relevant examples are exam~les 16 to 20 inclu-sively, which were all carried out at 10 asf and 50C.
A comparison of total arsine gas emission obtained during examples 19 and 20 indicates that, for a constant forward current pulse duration of 3 seconds, increasing the duration of the zero current pulse from l second (Ex. 19) to 2 seconds (Ex.20 results in a decrease in arsine gas emission from 43.2 to 29.3 mg/3 hr. `
Doubling both forward current and zero current pulse durations has little or no effect on the rate of arsine gas emission, as shown by comparison of the results of examples l9 and 18 (Tf/To:3/l to Tf/To:6/2) and 16 and 17 (Tf/To:5/l) to Tf/To:10/2).`~

For a constant zero current pulse of 2 seconds, in-creasing the forward current pulse duration from 3 seconds (~x.~.0) to 6 seconds (Ex. 18) and 10 seconds (Ex. 17) has resulted in increased rates of arsine gas emission from 29.3 to 32.5 and 45.5 mg/3 hr respectively.
4. Effect_of Current Density The effect of varying the current density of the current-on pulse under PIC conditions can be obtained by com-paring the results of examples 20 and 21 for a kiming of 3 seconds 10 current-on and 2 seconds current-off, and of examples 16 and 22 for a timing of 5 seconds current-on and 1 second current-off.
In both cases, the rate of arsine gas emission is significantly higher when the current density is increased to 15 asf (29.3 to 72.~ and 45.0 to 86.7 mg/3 hr).
The efficiency of copper, arsenic and antimony removal also tends to be favoured by operation at 10 asf than at 15 asf.
The efficiency of bismuth removal is low and unaf~ected by vari-ation of the current density.
Generally, when comparison is made between results ob-tained under PRC and PIC with regard to various operating con-ditions as well as arsine gas emissions, the following observa-tions can be made:
The range of suitable temperatures, the effect of electrolyte circulation, the ranges of suitable initial and final concentrations of Cu, As, Bi and Sb, and the effect of cathode ageing, are all similar for both PRC and PIC applications. The cathode ageing effect on the rate of arsine gas evolution is, :
however, not as pronounced for PIC as it is for PRC electrolysis.
Thus, the lowest emissions of arsine gas are obtained under PRC
conditions, particularly after several hours of operation, whereas slightly higher emissions (but which are still substantiall~ lower than those obtained under DC conditions) are produced under PIC

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s conditions. The PIC system, however, has one important advan~age over the PRC sys-tem, in that it requires only one source of power supply and uses less elec-tric power, -thereby making it simpler and less expensive, while still producing acceptable rates of arsine gas emission.
Finally, sirnilarly to PRC, the PIC method is applicable to varlous electrolysis systems, including the continuous feed and withdrawal system, "cascade" system and the batch system.

, . . . ..
.
. ~ :

Claims (40)

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. In a method of removing arsenic from electrolytes containing arsenic and copper within an electrolytic cell having cathodes and insoluble anodes, wherein the copper concentration of the electrolyte reaches such low levels that toxic arsine would be formed at the cathodes during electrolysis, and which comprises applying a direct current through said cell so as to co-deposit arsenic and copper present in the electrolyte onto the cathodes, the improvement comprising periodically reversing the polarity of the current so as to minimize the formation of the toxic arsine gas as the cathodes.

2. Method according to claim 1, wherein said electro-lyte is an acidic electrolyte.

3. Method according to claim 2, wherein said electro-lyte is an aqueous solution of sulphuric acid containing arsenic and copper ions.

4. Method according to claims 1, 2 or 3, wherein said electrolyte is maintained at a temperature between about 50 and 75°C during said electrodeposition.

5. Method according to claims 1, 2 or 3, wherein said electrolyte is circulated during said electrodeposition.

6. Method according to claims 1, 2 or 3, wherein the initial arsenic concentration of said electrolyte is up to about 30 grams per litre.

7. Method according to claims 1. 2 or 3, wherein the direct current applied through the electrolyte has a current density of between about 5 and 30 amps. per square foot and said direct current is applied for periods of 5 to 30 seconds and its polarity is reversed for periods of 1 to 4 seconds, with the ratio of reverse to forward current polarity durations being between 1/2 and 1/10.

8. Method according to claims 1, 2 or 3 wherein the cathode is made of copper.

9. Method of purification of copper refinery electro-lyte which comprises passing said electrolyte through electro-lytic cells containing insoluble anodes, applying a direct current through said cells so as to co-deposit copper, arsenic, antimony and bismuth present in the electrolyte onto cathodes in said cells, and periodically reversing the polarity of the current in said cells to as to minimize formation of arsine gas during the co-deposition of copper and arsenic onto the cathodes.

10. Method according to claim 9, wherein the electro-lyte entering the cells contains about 6 to 12 grams per litre of Cu and about 4 to 8 grams per litre of As and the co-deposition of copper and arsenic is permitted to proceed until the electrolyte leaving said cells contains between about 0.3 and 1 gram per litre of Cu and between about 1 and 2 grams per litre of As.

11. Method according to claims 9 or 10, wherein cells each having a cathode surface area of about 1,000 sq. ft. are used and the flow rate of the electrolyte through these cells is maintained between about 40 and 70 U.S. gallons per minute during the co-deposition of copper and arsenic onto the cathodes.

12. Method according to claims 9 or 10, wherein the temperature of the electrolyte in the cells is maintained between about 50 and 75°C.

13. Method according to claim 9, wherein the current density is maintained between about 10 and 25 amps. per square foot.

14. Method according to claim 13, wherein initially the current density is maintained near the lower limit of the range and is then increased to near the higher limit of the range for the remainder of the electrodeposition cycle.

15. Method according to claims 13 or 14, wherein the forward polarity is applied for periods of 5 to 15 seconds and the reverse polarity for periods of 1 to 4 seconds with the ratio of reverse to forward polarity durations being between 1/2 to 1/10.

16. Method according to claims 9 or 10, wherein the insoluble anode is made of lead or lead alloys and the cathode is made of copper or stainless steel.

17. Method according to claims 9 or 10, wherein the electrolyte entering the cells in which the polarity is period-ically reversed also contains about 0.1 to 0.4 gpl of Sb and of Bi and the electrolyte leaving said cells contains about 0.01 to 0.05 gpl of Sb and of Bi.

18. Method according to claims 9 or 10, which is applied to a continuous feed and withdrawal system with recirculation of the electrolyte.

19. Method according to claims 9 or 10, which is applied to a cascade system where the electrolyte is passed through a plurality of cells in series.

20. Method according to claims 9 or 10, which is applied to a batch system where the electrolyte remains in the cell under agitation until the desired levels of copper and arsenic are achieved.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

21. A method of removing arsenic from arsenic and copper containing electrolytes, in which copper concentration reaches such low levels as to liberate toxic arsine gas, which com-prises carrying out a co-deposition of arsenic and copper on a cathode by applying through the electrolyte a direct current and substantially reducing the formation of arsine gas at the cathode by effecting one of the following operations:
(a) periodically reversing the polarity of said current;
(b) periodically interrupting said current.

22. In a method of removing arsenic from electrolytes containing arsenic and copper within electrolytic cells having cathodes and insoluble anodes, wherein the copper concentration of the electrolyte reaches such low levels that toxic arsine gas would be formed at the cathodes during electrolysis, and which comprises applying a direct current through said cells so as to co-deposit arsenic and copper present in the electrolyte onto the cathodes, the improvement comprising periodically inter-rupting said direct current so as to substantially reduce the formation of the toxic arsine gas at the cathodes.

23. Method according to claim 22, wherein said electro-lyte is an acidic electrolyte.

24. Method according to claim 23, wherein said electrolyte is an aqueous-sulphuric acid solution containing at least arsenic and copper ions.

25. Method according to claims 22, 23 or 24, wherein said electrolyte is maintained at a temperature between about 50°C
and 75°C during electrolysis.

26. Method according to claims 22, 23 or 24, wherein said electrolyte is circulated during electrolysis.

27. Method according to claims 22, 23 or 24, wherein the initial arsenic concentration of said electrolyte is up to about 30 grams per litre.

28. Method according to claims 22, 23 or 24, wherein the direct current applied through the electrolyte has a current density of between about 5 and about 30 amps per square foot and said direct current is applied for periods of 2 to 30 seconds and is interrupted for periods of 1 to 6 seconds, with the ratio of zero to forward current durations being between 2/3 and 1/10.

29. Method according to claims 22, 23 or 24, wherein the cathodes are made of copper.

30. Method of purification of copper-refinery electrolyte which comprises passing said electrolyte through electrolytic cells containing insoluble anodes, applying a direct current through said cells so as to co-deposit copper, arsenic, antimony and bismuth present in the electrolyte onto cathodes in said cells, and periodically interrupting said current in said cells so as to substantially reduce the formation of arsine gas during co-deposition of copper and arsenic onto the cathodes.

31. Method according to claim 30, wherein the electro-lyte entering the cells contains about 6 to about 12 grams per litre of Cu and about 4 to about 8 grams per litre of As and the co-deposition of copper and arsenic is permitted to proceed until the electrolyte leaving said cells contains between about 0.3 and about 1 gram per litre of Cu and between about 1 and about 2 grams per litre of As.

32. Method according to claim 30, wherein the temperature of the electrolyte in the cells is maintained between about 50°C
and about 75°C.

33. Method according to claims 30, 31 or 32, wherein the current density is maintained between about 10 and about 20 amps.
per square foot.

34. Method according to claim 30, wherein the forward current is applied for periods of 2 to 30 seconds and the inter-ruptions are made for periods of 1 to 6 seconds, with the ratio of zero to forward pulse durations being between 2/3 and 1/10.

35. Method according to claim 34, wherein the forward current is applied for periods of 2 to 15 seconds and the inter-ruptions are made for periods of 1 to 3 seconds.

36. Method according to claims 30, 31 or 32, wherein the insoluble anodes are made of lead or lead alloys and the cathodes are made of copper or stainless steel.

37. Method according to claims 30, 31 or 32, wherein the electrolyte entering the cells also contains about 0.1 to about 0.4 gpl of Sb and of Bi and the electrolyte leaving said cells contains about 0.01 to about 0.05 gpl of Sb and of Bi.

38. Method according to claims 30, 31 or 32, which is applied to a continuous feed and withdrawal system with re-circulation of the electrolyte.

39. Method according to claims 30, 31 or 32, which is applied to a cascade system where the electrolyte is passed through a plurality of cells in series.

40. Method according to claims 30, 31 or 32, which is applied to a batch system where the electrolyte remains in the cell under agitation until the desired levels of copper and arsenic are achieved.