AU677864B2

AU677864B2 - Purification of aqueous solutions

Info

Publication number: AU677864B2
Application number: AU77624/94A
Authority: AU
Inventors: Cornelis Petrus Geyer
Original assignee: Council for Scientific and Industrial Research CSIR
Current assignee: Council for Scientific and Industrial Research CSIR
Priority date: 1993-11-04
Filing date: 1994-11-03
Publication date: 1997-05-08
Anticipated expiration: 2014-11-03
Also published as: CA2134586A1; AU7762494A

Description

M-~UMi 1 28J/l91 Regulatlln 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT *8 @8 8@8S Application Number: Lodged: Invention Title: PURIFICATION OF AQUEOUS SOLUTIONS The following statement Is a full description of this Invention, Including the best method of performing It known to us :0 THIS INVENTION relates to the purification of aqueous solutions containing metal ions, such as electrowinning solutions. More particularly the invention relates to a process suitable for the purification of zinc electrowinning solutions, and for the production of electrochemical cell-grade manganese dioxide.

According to the invention there is provided a process for the purification of an aqueous solution containing metal ions other than manganese cations and which contains divalent manganese (Mn 2 cations as an impurity therein, which process comprises the steps, at a hydrogen ion concentration of 1 x 10 7 2 moles/?, of: oxidizing the Mn 2 cations to Mn 4 cations to precipitate MnO 2 from the solution; and catalyzing the oxidizing of the Mn 2 cations by carrying out the oxidizing in the presence of catalytically active manganese dioxide, The process is suitable for purifying electrowinning solutions (although it can apply generally to purifying aqueous solutions containing metal ions such as those of cobalt, nickel, copper, lead, zinc or the like, typically forming aqueous streams arising from hydrometallurgical processes, 00 including feed solutions to electrowinning circuits), which electrowinning solutions typically have a suitable hydrogen ion concentration in the above range, but if the solution has an unsuitable hydrogen ion concentration, outside the above range, the method may include the step of adjusting its hydrogen ion concentration by adding a suitable alkali or acid thereto, to bring said concentration into, and if necessary maintain said concentration in, the above range. More preferably, the hydrogen ion concentration range is 1 x 10 1 moles/e.

It is expected that the invention will have substantial utility in the purification of aqueous zinc electrowinning solutions in which the concentration of zinc (Zn 2 cations is in the range 1 200 g/e, typically 150 g/C, eg about 100 g/C, and in which the concentration of manganese (Mn 2 cations is <100 g/e, typically 10 40 g/e, eg about 15 g/f. Such solutions will typically be zinc sulphate solutions, but may be solutions of other salts of zinc, for example zinc chloride, in which case the manganese will typically be present as a solution of such other salt.

In a particular embodiment of the invention, the aqueous solution may thus be an electrowinning solution wherein hydrogen ion concentration range is 1 x 10" 1 moles/e, preferably an aqueous zinc electrowinning solution in which the concentration of zinc (Zn 2 cations is 1 200 g/l, the concentration of manganese (Mn 2 cations in the solution being 100 g/fe.

In particular, the solution may be an aqueous zinc sulphate solution, the process including adding sulphuric acid (HzSO 4 to the solution to acidify the solution in response to any drop in the hydrogen ion concentration below 1 x 10" 7 moles/e, to maintain said hydrogen ion concentration at a value of 1 x 107-1 moles/C.

The oxidizing may be carried out chemically, by adding an oxidizing agent to the solution, suitable oxidizing agents including ozone, perchloric acid, suitable chlorates such as sodium chlorate, lead dioxide, suitable persulphates or suitable permanganates in suitable proportions. In other words oxidizing the Mn 21 cations to Mn 4 1 cations may be by adding an oxidizing agent to the solution, the oxidizing agent being selected from ozone, perchloric acid, chlorate salts, lead dioxide, persulphale s~its, permanganate salts and mixtures thereof. Instead,the oxidation may be carried out electrochemically, by applying an electric potential between electrodes immersed in the solution, the potential having a suitable value.

It is expected that routine experimentation will be employed to select an optimum or attractive chemical or electrochemical oxidation step, bearing practical and economic considerations in mind.

Whatever the oxidation step, however, the invention provides for the presence of catalytically active manganese dioxide in the solution to catalyze the oxidizing step, the proportion thereof again being selected, taking into account practical and economic considerations, to provide an optimum or attractive rate of oxidizing. For electrowinning solutions having Mn 2 ion concentrations within the above limits, the proportion of catalytically active manganese dioxide may be such that the mass ratio of active manganese WP.. dioxide to the manganese dioxide to be derived, by the above oxidation, from the solution is typically about 4:1. Thus, in a particular embodiment, the proportion of catalytically active manganese dioxide used to catalyze the oxidation of the Mn 2 cations may be such that the catalytically active manganese dioxide is present in the solution in a mass ratio of the manganese in the catalytically active manganese oxide:Mn 2 cations in the solution of 1:3. More particularly, the concentration of the Mn 2 cations in the solution may be 100 g/e, said ratio being 3:1 -5:1.

By catalytically active manganese dioxide is meant manganese dioxide 'which has been prepared by heating a precursor thereof, such as a suitable salt or hydroxide, for example manganese carbonate, at an elevated temperature under an oxidizing atmosphere for a sufficient period to convert the precursor to manganese dioxide, followed by leaching, eg with a suitable acid to leach residual unreacted precursor from the manganese oxide product.

The oxidizing atmosphere is preferably air, the temperature being 150 450 0 C, eg about 300*C, the heating being for a period of 8 120 hours, eg 96 hours, The leaching may be by means of a mineral acid such as sulphuric acid, nitric acid or hydrochloric acid, which may be at a concentration of 1 100%, typically 15 65%, eg about 30% by mass.

Thus, the method of the present invention may include the prior step of preparing the catalytically active manganese dioxide by heating a precursor of manganese dioxide selected from manganese salts, manganese hydroxide and mixtures thereof, at a temperature of 150-450 0 C under an oxidizing atmosphere for a period of 8-120 hours, to produce a mixture of manganese dioxide and said precursor, the heating being followed by leaching the precursor from said mixture by means of an aqueous leaching solution comprising a mineral acid. In this case the acid may be selected from sulphuric acid, nitric acid, hydrochloric acid and mixtures thereof, the aqueous leaching solution having an acid concentration of 1-65 by mass, Preferably the catalytically active manganese dioxide has a particle size such that 80% by mass thereof is, 120 m, eg 80% <40 Am, which can be obtained by using a precursor of suitable particle size, its average .4S" particle size also being within the above limits, although manganese dioxide having a particle size smaller than that whereby 80% by mass thereof is 10 Itm will not usually be used. In particular, the catalytically active manganese dioxide may thus be particulate, having a particle size such that by mass thereof is 120 /m in size, the average particle size of the particles being 120 jm, and the proportion of particles having a particle size of 10 ,tm being :80% by mass.

The oxidation may take place at a temperature in the range 10 110 0 C, typically being above ambient at 50 100 0 C, eg 90 0 C, and will typically be continued until the Mn 2 ion concentration in the solution is acceptably low, being <2 g/e, typically 1 2 gle, and, after completion of the oxidation, solids in the solution (precipitate and catalyst) may be removed by filtration. In a preferred embodiment, the oxidizing of the Mn 2 cations to Mn 4 cations takes place at a temperature of 50 -100°C, and is continued

I

until the Mn 2 ion concentration in the solution is <2 g/f, the process including the step, after the oxidation, of separating solids from the solution.

The invention extends to manganese dioxide whenever produced by the process described above, The invention will now be described, by way of example, with reference to the following non-limiting illustrative Examples: EXAMPLE 1 Cata'/tically active manganese dioxide was prepared by ,heating high density MnCO, precursor (dry tap density 2 g/cm 3 having a particle size of 2 100 Am, and an average particle size of 40 um in a muffle furnace under air for 96 hours at 320 0 C. The product was leached at 90 0 C with 27% by mass

H

2

SO

4 to remove unreacted MnCO 3 The resulting material (enriched MnO 2 was used as the catalyst during the oxidative removal of Mn 2 ions by precipitating MnO from a simulated zinc electrowinning solution. The catalyst had a dry tap density of 1,3 g/cm 3 and essentially the same particle size as the precursor. (The tap density is obtained by manually tapping a measuring cylinder containing the material to constant volume, and dividing the mass of material by the volume after tapping).

A synthetic solution simulating the feed stream to a zinc electrowinning circuit was prepared with a hydrogen ion 7 concentration of 0,5 moles/e and the following composition by mass: ZnSO 4 24% MnSO 4 4,4%

H

2 S0 4 2,8% 68,8% 9,1 g of the catalyst was added to 100 g of solution, and the solution was then heated to 90 0 C. 7,3 g of NaClO 3 was then added slowly to the solution. The reaction time wa: 3 hours, after which the solids were removed by filtration. The filtrate contained 41 ppm (parts per million by mass) of Mn, indicating very good Mn removal,

*O

EXAMPLE 2 Example 1 was repeated, using a zinc eletrowinning solution containing Mn 2 cations obtained from the Zincor (Zinc Corporation of South Africa Limited) zinc electrowinning plant 9 in Springs, South Africa, which contained 9,3% by mass of Zn 2 cations and 1,2% by mass of Mn 2 cations. The catalyst was added in an amount of 6,4 g/100 g of solution, followed by 5,2 g of the NaCIO3. The filtrate contained 30 ppm of Mn, which is a value below the required maximum level for zinc electrowinning plants of the type in question.

A feature of the invention, as described in particular with reference to the Examples, is that it provides an effective way of reducing the Mn 2 ion content of electrowinning solutions, such as zinc sulphate electrowinning solutions, in which the Mn 2 ions are an impurity which can cause an 8 undesirable decrease in the current efficiency during zinc electrowinning, particularly when the Mn 2 ion concentration is at levels of >4 g/e, Furthermore, it is a material advantage of the invention that the purified manganese dioxide is a high density product (the catalyst contained therein also being of a high density), suitable for use as an active cathode material in electrochemical cells, eg those having lithium as active anode material.

The manganese removed from the electrowinning soluton is thus not merely converted into a waste product, but is indeed converted into a valuable byproduct.

19.. When use of the catalyst is omitted, the product has a substantially lower density and is less suitable for this purpose. In this regard it is to be noted that, during the acid leaching both the density and tap density of the MnO, produced by heating the MnCO 3 precursor are decreased, but during the oxidation of Mn" 2 ions some of the precipitated MnO 2 is deposited in .1d" pores in the active leached catalytic Mn.Oz and on its sutace, thereby increasing its density and tap density.

$*oo: h*

Claims

1. A process for the purification of an aqueous solution containing metal ions other than manganese cations and which contains divalent manganese (Mn" cations as an impurity therein, which process comprises the steps, at a hydrogen ion concentration in the solution of 1 x 10 7 2 moles/e, of: oxidizing the Mn 2 cations to Mn 4 cations to precipitate Mn02 from the solution; and catalyzing the oxidizing of the Mn 2 cations by carrying out the oxidizing in the presence of catalytically active manganese dioxide. 0

2. A process as claimed in claim 1, in which the aqueous solution is an iJ electrowinning solution wherein hydrogen ion concentration range is 1 x 10 7 1 moles/e.

3. A process as claimed in claim 2, in which the solution is an aqueous zinc sulphate solution, the process including adding sulphuric acid (HIS0 4 4 to the solution to acidify the solkdon in response to any drop in the hydrogen ion concentration below 1 X 10" moles/e, to maintain said hydrogen ion concentration at a value of 1 x 10' 7 -1 moles/e, 4* 0

4. A process as claimed in any one of the preceding claims, in which oxidizing the Mn+ cations to Mn" cations is by adding an oxidizing agent to the solution, the oxidizing agent being selected from ozone, perchloric acid, chlorate salts, lead dioxide, persulphate salts, permanganate salts and mixtures thereof, A process as claimed in any one of claims 1 3 inclusive, in which the oxidizing is carried out electrochemically, by applying an electric potential between electrodes immersed in the solution.

6. A process as claimed in any one of the preceding claims, in which the proportion of catalytically active manganese dioxide used catalyze the oxidation of the Mn 2 cations is such that the catalytical ntanganese dioxide is present in the solution in a mass ratio of the 6,unese in the catalytically active manganese oxide:Mn cations in the solution of 1:3.

7. A process as claimed in claim 6, in which the concentration of Mn 2 cations in the solution is 100 g/e, said ratio being 3:1 -5:1. e* 0o

8. A process as claimed in any one of the preceding claims, which includes the prior step of preparing the catalytically active manganese dioxide by heating a precursor of manganese dioxide selected from manganese salts, manganese hydroxide and mixtures thereof, at a temperature of 150-450°C under an oxidizing atmosphere for a period of 8-120 hours, to produce a mixture of manganese dioxide and said precursor, the heating being followed by leaching the precursor from said mixture by means of an aqueous leaching solution comprising a mineral acid.

9. A process as claimed in claim 8, in which the acid is selected from sulphuric acid, nitric acid, hydrochloric acid and mixtures thereof, the aqueous leaching solution having an acid concentration of 1-65% by mass. A process as claimed in any one of the preceding claims, in which the catalytically active manganese dioxide is particulate, having a particle size such that 80% by mass thereof is 120 rni in size, the average particle size of the particles being 120 Am, and the proportion of particles having a particle size of <10 Am being .80% by mass.

11. A process as claimed in any one of the preceding claims, in which the oxidizing of the Mn 2 cations to Mn 4 cations takes place at a temperature of 50 -100°C, and is continued until the Mn 2 ion concentration in the solution is <2 g/e, the process including the step, after the oxidation, of separating solids from the solution,

12. A process as claimed in claim 1, substantially as described herein with reference to the examples.

13. Manganese dioxide, whenever produced by the process of any one of the preceding claims, DATED this 3rd day of November 1994. CSIH WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWWOD ROAD HAWTHORN. VIC. 3122. e. 6 4. 0 I ABSTRACT The invention provides a process for the purification of an aqueous solution containing metal lons and which contains divalent manganese cations as an impurity. The process comprises, at a hydrogen ion concentration in the solution of I x 10-1- 2 moles/e, oxidizing the divalent manganese (W cations to Mn" cations to precipitate Mn02 from the solution, The oxidising of the MnO2 cations is catalyzed by carrying out the oxidation in the presence of catalytically active manganese dioxide, feet I