Method for measuring impurity concentration and removing impurities from electrolytic solution for electrowinning of zinc

Concentrations of cobalt and copper of electrolysis solution for electrowinning of zinc can be measured any time during electrowinning operation by continuously sampling the solution, diluting it, adding coloring reagent to the flow of the solution and spectrophotometrically analyzing the solution. The cobalt and copper as deleterious impurities can be removed continuously and automatically by measuring their concentrations by the above method and adding precipitation reagents for them in an amount calculated by a microcomputer on the basis of said analysis.

FIELD OF THE INVENTION 
This invention relates to a method for measuring the concentrations of 
impurities in the electrolytic solution for electrowinning zinc and a 
system for automatically purifying the electrolytic solution on the basis 
of such measurement. 
BACKGROUND OF THE INVENTION 
In the electrowinning of metals having oxidation potential greater than 
hydrogen overvoltage, presence of impurities in the electrolytic solution 
causes marked impairment of electrolysis efficiency and sometimes it makes 
the electrolysis impossible. Therefore, usually there is provided a step 
for purifying the electrolysis solution wherein impurity metal ions are 
removed. In the case of electrowinning of zinc, the zinc sulfate 
electrolysis solution is purified by removing copper, cobalt, cadmium, 
etc. Of these, copper is precipitated by addition of zinc powder and 
cobalt is precipitated by addition of zinc powder and arsenic oxide 
(As.sub.2 O.sub.3) and the precipitates are removed. 
It is necessary to determine the concentrations of the impurities to be 
removed prior to purification. For the measurement of low concentration 
impurity metals, (a) colorimetric method using a coloring reagent, (b) 
measurement of redox potentials, (c) atomic absorption analysis, (d) 
Inductively coupled Plasma Atomic Emission Spectrometry (ICP) emission 
spectrophotometry, etc. are generally employed. In the case of zinc 
sulfate solution, however, manual chemical analysis is solely resorted to 
for the measurement of concentrations of copper and cobalt ions, since the 
zinc sulfate solution is a relatively viscous, acidic or weakly acidic 
solution with super-saturated zinc and is liable to clogging of conduits 
with the deposition of salts and automatic and continuous analysis is 
impossible. That is, samples are taken for the determination of 
concentration of copper and cobalt each time and the samples are subjected 
to classic colorimetric analysis or titration. 
In the classic colorimetry, color of a sample is compared with a reference 
by human eye. It takes a long time and the results are not so accurate. 
Thus, it is an obstacle for automating the electrowinning process. 
There has been an attempt to overcome this defect of the prior art. It is 
called "flow injection method". That is, there is provided a flow of a 
reagent mixture. The sample taken from the electrolysis solution is added 
to this flow when measurement is desired and the concentration of the 
object impurity is measured spectrophotometrically for instance. 
This method has the following defects. 1. The sample is dispersed in a 
reagent solution and thus the sample is highly diluted and thus the 
measurement of a very slight amount of the impurity is difficult. 2. 
Expensive coloring reagent is wasted. 
This invention is intended to overcome the above-mentioned various problems 
and provide improved method of measurement of the impurities in the 
electrolysis solution for electrowinning of zinc and further to provide an 
automatic purifying system for electrowinning electrolysis solution for 
electrowinning of zinc. 
SUMMARY OF THE INVENTION 
This invention provides a method for measuring concentration of cobalt in 
the electrolytic solution for electrowinning of zinc instantly at any 
desired time, which comprises continuously taking out the zinc sulfate 
electrolytic solution from the electrolytic apparatus; continuously 
diluting said solution; continuously adding to the flow of the solution a 
buffer solution and a chelating agent for masking metal ions other than 
cobalt ions; adding a coloring reagent for cobalt to the thus mixed 
continuous flow when measurement is desired; adding a reagent for 
decomposing the complexes of metals other than cobalt if necessary; 
allowing said flow to pass through a reaction zone and finally allowing 
said flow to pass through a spectrophotometric means, whereby the cobalt 
concentration is determined. 
A preferred coloring reagent for cobalt is 
1-nitroso-2-naphthol-3,6-disulfate sodium (hereinafter called "nitroso R 
salt"). 
A preferred chelating reagent is a citric acid salt such as diammonium 
citrate (0.5M). 
A preferred buffer reagent is ammonium acetate (2M). 
When the above preferred reagent for decomposing the complexes of the 
metals other than cobalt is used, a mineral acid is used. Preferably 
hydrogen peroxide is used in combination with the mineral acid. 
A preferred reaction zone is a spiral tube provided in a heating medium. 
This invention also provides a method for measuring concentration of copper 
in the electrolytic solution for electrowinning of zinc instantly at any 
desired time, which comprises continuously taking out the zinc sulfate 
electrolytic solution from the electrolytic apparatus; continuously 
diluting said solution; continuously adding to the solution a buffer 
solution and a chelating agent for masking metal ions other than copper; 
adding a coloring reagent for copper to the thus mixed continuous flow 
when measurement is desired; adding a reagent for decomposing the 
complexes of the metals other than copper if necessary; allowing said flow 
to pass through a reaction zone and finally allowing said flow to pass 
through a spectrophotometric means, whereby the copper concentration is 
determined. 
A preferred coloring agent for copper is bathocuproine disulfonic acid 
disodium, neocuproine hydrochloride, etc. Preferably an aqueous solution 
of ascorbic acid is used for reduction of copper ions. 
When the above preferred coloring reagents are used, the reagent for 
decomposing the complexes is not required. 
A preferred reaction zone is the same as in the case of the measurement of 
cobalt concentration. 
This invention further provides a system for purifying the zinc sulfate 
electrolytic solution for electrowinning zinc, which comprises: providing 
a first electrolytic solution cleaning zone and a first separation zone 
and a second electrolytic solution cleaning zone and a second separation 
zone in series; continuously taking out said electrolytic solution from 
the electrolytic apparatus and supplying it into said first cleaning zone 
to let it pass through the first separation zone and the second cleaning 
and separation zones; continuously taking out a portion of said flowing 
electrolytic solution from the downstream of said first separation zone 
and leading it to a first analysis zone; wherein the solution is diluted, 
a buffer solution and a chelating solution for masking metal ions other 
than cobalt ions are added to said electrolytic solution, a coloring 
reagent for cobalt ions is added into the thus mixed continuous flow when 
measurement is desired and the cobalt concentration is 
spectrophotometrically measured; sending the analysis information to a 
information-processing unit wherein the amounts of zinc powder and arsenic 
oxide to be added to the first cleaning zone are calculated; sending the 
calculated information to hopper means for the first cleaning zone so as 
to supply the calculated amounts of zinc powder and arsenic oxide to said 
first cleaning zone; continuously taking out a portion of said flowing 
electrolytic solution from the downstream of said second separation zone 
and leading it to a second measurement zone; wherein a buffer solution and 
a chelating solution for masking metal ions other than copper ions are 
continuously added to said electrolytic solution, a coloring reagent for 
copper ions into the thus mixed continuous flow when measurement is 
desired and spectrophotometrically measuring the copper concentration; 
sending the analysis information to said information-processing unit 
wherein the amount of zinc powder to be added to the second cleaning zone 
is calculated; sending the calculated information to a hopper means for 
the second cleaning zone so as to supply the calculated amount of zinc 
powder to said second cleaning zone. 
The above-mentioned preferred conditions for analysis can optionally be 
applied to this system. 
Preferably, measurement of impurity metals is carried out periodically in 
accordance with the command from the information-processing unit. 
If there is no necessity to separately collect cobalt and copper, 
purification can be carried out in one precipitation tank.

DESCRIPTION OF SPECIFIC EMBODIMENT OF THE INVENTION 
Now the invention will be specifically described with reference to the 
attached drawings. 
FIG. 1 shows the method and apparatus of the analysis in accordance with 
the present invention. 
The apparatus for the analysis substantially comprises a long tube system 
having an inside diameter of 1 mm with several instruments incorporated 
therein. 
Zinc sulfate electrolysis solution is continuously taken from the 
electrolysis bath by means of a pump 1 and a conduit 1a. Water (containing 
0.25M H.sub.2 SO.sub.4) for dilution is continuously added to the solution 
through a pump 2 and a conduit 2a and a reagent solution containing a 
buffer reagent and a chelating reagent is added to the electrolysis 
solution by means of a pump 3 and a conduit 3a. These can be plunger 
pumps. To this flow of the mixed solution, a coloring reagent for the 
object metal is added to the solution at the station 4. The thus mixed 
solution is passed through a reaction zone 5, at least a part of which can 
be heated. In the reaction zone which is a part of a measurement zone, 
another reagent can be added by means of a pump 6 and a conduit 6a. The 
solution which has passed the reaction zone is passed through a 
spectrophotometric means 7, wherein the concentration of the object metal 
is spectrophotometrically determined. 
The apparatus represented by FIG. 1 is indicated as an analysis zone as a 
whole in FIG. 2. 
FIG. 2 shows for the electrolysis-solution-purifying system of the present 
invention. 
Zinc sulfate electrolysis solution is taken from the electrolysis bath 
through a conduit 10 and transferred to a first purifying zone (tank) 11, 
wherein the solution is stirred by a stirrer not shown. The solution is 
overflown to a second purifying zone (tank) 13 through a first separator 
12 which may be a filter. Arsenic oxide and zinc powder are continuously 
fed into the purifying tanks 11 and 12 respectively. A portion of the 
solution is taken from the conduit after the first separator 12 and sent 
to a first analysis zone 20, wherein the cobalt concentration is 
spectrophotometrically determined. The result of the analysis is sent to 
an information processing unit 50, wherein the amounts of zinc powder and 
arsenic oxide to be added to the first purifying tank are calculated and 
the resulting information is sent to valve means 40 and 42 of an zinc 
powder hopper 30 and an arsenic oxide hopper 32 so that necessary amounts 
of zinc powder and arsenic oxide are added to the first purifying tank 11. 
From the conduit after the second separator 14, a portion of the solution 
is taken and sent to a second analysis zone 21, wherein the copper 
concentration is spectrophotometrically determined. The result is sent to 
the information processing unit 50 and the amount of zinc powder to be 
added to the second purifying tank 13 is calculated. The resulting 
information is sent to a valve 41 of a zinc powder hopper 31 so that a 
necessary amount of zinc powder is added to the second purifying tank. 
Preferably, analysis is carried out periodically in accordance with the 
command from the information-processing unit. 
Further the invention will be illustrated by way of working examples. Basic 
Experiment 
Standard solutions of zinc sulfate and nitroso R salt were prepared. Zinc 
sulfate solutions of various concentrations were colorimetrically measured 
with nitroso R salt and the results were compared the results with respect 
to the actual electrolysis solution for electrowinning of zinc. It was 
established that the measurement curve for standard solutions well 
corresponds to the curve for actual electrolysis solution. 
Separately, the cobalt concentration of the actual electrolysis solution 
was measured by atomic absorption analysis and the results were compared 
with the results of the present method. Examples of such comparison are as 
follows: 
______________________________________ 
Samples Present Invention 
Atomic Absorption 
______________________________________ 
A 0.12 ppm 0.11 ppm 
B 0.068 ppm 0.070 ppm 
______________________________________ 
It is understood that the present method is practically useful. 
EXAMPLE 1 
Measurement of Cobalt Concentration 
Using an apparatus represented by FIG. 1, concentration of cobalt in an 
electrolysis solution for zinc electrowinning was measured. 
An electrolysis solution was taken from the electrolytic bath at a rate of 
0.3 ml per min. and diluted to 5 times. To the flow of the diluted 
solution, a 2M ammonium acetate-0.5M diammonium citrate solution was 
continuously added at a rate of 1.5 ml per min. And 60 .mu.l of a 1% 
solution of nitroso R salt was added by means of a syringe pump (a syringe 
with a stop valve). The thus mixed solution was passed through the 
reaction zone heated to 80.degree. C. during which 2M nitric acid solution 
containing 0.5% hydrogen peroxide solution at a rate of 2 ml per min. in 
order to decompose complexes of metals other than cobalt. 
The solution is led to a spectrophotometer, Ratio-beam U-1000 manufactured 
by Hitachi, Ltd., wherein light absorption at 520 nm was measured. The 
electrolysis solution contained 0.12 ppm of cobalt. 
EXAMPLE 2 
Measurement of Copper Concentration 
Using the same apparatus, copper concentration of the same electrolysis 
solution was measured. 
The electrolysis solution was taken from the electrolytic bath at a rate of 
0.3 ml per min. and diluted to 5 times. To the flow of the diluted 
solution, a 2M ammonium acetate-0.5M diammonium citrate solution was 
continuously added at a rate of 1.5 ml per min. Further a 0.05% solution 
of bathocuproine disulfonic acid disodium was added. The thus mixed 
solution was passed through the reaction zone at room temperature. 
The solution is led to a spectrophotometer, Ratio-beam U-1000 manufactured 
by Hitachi, Ltd., wherein light absorption at 525 nm was measured. The 
electrolysis solution contained 0.5 ppm of copper. 
EXAMPLE 3 
Purification of Electrolysis Solution 
The same electrolysis solution was purified in accordance with the system 
of the present invention using an apparatus represented by FIG. 2. 
The approximate capacity of the cleaning tanks 11 and 13 was 150 m.sup.3. 
The analysis sections 20 and 21 were the same as described above with 
respect to the measurement of concentrations of cobalt and copper. 
The separators 12 and 14 were filter presses. 
The valve 40, 41 and 42 were screw feeders. 
The information processing unit used was a "NEC PC9801" personal computer. 
The electrolysis solution was drawn into the first cleaning tank 11 at a 
rate of 200 l/min and the residence time in the tank 11 was about 40 min. 
The hopper 30 contained arsenic oxide and the hopper 31 and 32 contained 
zinc powder. 
Analysis was carried out every 10 minutes in accordance with the command 
from the information-processing unit 50. Thus the amounts of arsenic and 
zinc powder to be added were well regulated, nearly continuously. 
Prior to the present invention, the measurement of the impurity metals was 
carried out only every one hour because of the time-consuming manual 
analysis. Thus the regulation of the amounts of zinc powder and arsenic 
oxide was manually conducted intermittently only once in an hour. 
Therefore, purification could not be follow the fluctuation of the 
impurity concentration and there were overs and shorts in supplying the 
precipitation reagents.