Precipitation of Antimony and Bismuth from Copper Refinery Electrolyte Using PbO2

Abstract

The invention is suitable for use in metal and nonmetal mining and in general, in any area of operation containing Sb and Bi in solution in an electrolyte based mainly on sulfuric acid. The system consists in contacting solid lead dioxide (PbO2), which can be activated superficially with an oxidizing agent, for example hydrogen peroxide (H2O2), to increase its reactivity, with an acid electrolyte that contains Sb and Bi in solution, with the purpose of causing a decrease in the content of soluble Sb and Bi present in the liquid phase, forming insoluble compounds based on Pb—Sb and Pb—Bi. With this invention the presence of Sb and Bi in solution is reduced by at least 70%.

Description

TECHNICAL FIELD OF THE INVENTION

The invention has its field of application in metal and nonmetal mining, and in general in any action field in which Sb and Bi are present in solution in an electrolyte based mainly on sulfuric acid.

BACKGROUND OF THE INVENTION

This invention is an extension of Patent No. 519-2005 Registry No. 45.037, called: “Method for decreasing the content of soluble antimony in an electrolyte, which comprises putting in contact lead oxide with an acid electrolyte that contains antimony in solution, leading to insoluble Pb—Sb based compounds.” Getting a high quality copper cathode implies among other things a cathode free of impurities. The copper electrorefining process is aimed at eliminating these impurities from the copper anode (99.5% purity) through a process of disolving it in an electrolyte by means of chemical and electrochemical reactions. This process yields as a product a cathode (99.99% purity). (W. G. Davenport, M. King, M. Schlesinder and A. K. Biswas, Elsevier Science Ltd, ISBN 0-08-044029-0; Noguchi, Yano, Nakamura and Ueda, Metallurgical Review of MMIJ, vol. 11. No 2, 1994, pag. 39-41; Noguchi, Iida, Nakamura and Ueda, Metallurgical Review of MMIJ, vol. 8, No 2, 1992, pag. 83-97). The concentration of the impurities that were contained in the now dissolved anodes starts increasing in the electrolyte; antimony is one of these impurities, and as its concentration increases in the electrolyte it favors the formation of floating slime and/or solids in suspension which are one of the main causes of cathode contamination (G. Cifuentes, S. Hernandez, P. Navarro, J. Simpson: “Control of the slimes Properties in copper electrorefining”. E.P.D .(Extraccion and Processing Division), 1999, pag 645-648).

The most important impurities from the standpoint of their effect on the quality of the copper are Ag, As, Sb, Bi, Pb, Se, Te and Ag. To control these impurities it is necessary to establish the effects of the parameters involved in electrorefining. These parameters are anode composition, additives, purification of the electrolyte, and current density, together with the critical acidity, in addition to other factors to be considered such as the morphology of the deposit, passivation of the anode and the characteristics of the slime particles (W. Charles Cooper, Copper '87 (vol. 3), ed. By W. C. Copper, G. E. Lagos and G. Ugarte, Universidad de Chile, 1988; Navarro y F. J. Alguacil, Revista de metalurgia, Madrid, 1994, pag, 213-226).

The importance of the management of these parameters and factors is due to the fact that the composition of the anode, for example, together with the addition of organic components to the electrolyte in electrorefining has an influence on the electrocrystallization of copper at the cathode. Another factor is the composition of the electrolyte, in which it is of interest to establish the critical acidity that allows a minimum of impurities in solution to be established with the purpose of preventing them from harming the electrorefining process itself.

Among the impurities in electrorefining, those that have been most widely studied are arsenic, antimony and bismuth because of their direct effect on lowering the physicochemical properties of copper. If we consider the volume of electrolyte discarded to keep the concentration of these impurities within the allowed levels (for example 50 m³/day for the Potrerillos refinery of CODELCO-CHILE, 24 m³/day for the Ventanas refinery of ENAMI), we need to establish some techniques that allow the recovery and/or the stabilization of these impurities with the purpose of cleaning the discarded electrolyte so that it can be recirculated through the electrorefining process. Some of these techniques are described below.

Conventional Decoppering Process

The recovery of copper from the discarded electrolyte is made by means of electrowinning processes that take place in successive steps that yield cathodes of different quality.

The process is carried out in copper releasing cells in which anodes of the type 93% Pb, 6% Sb and 1% Ag are used, and the cathodes are traditional electrorefining mother blank sheets. The decoppering process involves a reduction of the Cu²⁺ concentration in the first step, which produces commercial cathodes and the concentration drops from 45-50 g/L to 15-30 g/L, and in the second step cathodes with a higher impurity level than in the previous case are obtained, and they are sent for anode smelting and the concentration drops from 15-30 g/L to 8-12 g/L. Finally, a purification step is carried out in which the product that is obtained is an unpleasant granular deposit called arsenic slime that is returned to smelting or is stored and sold. In this step copper concentration decreases from 8-12 g/L to 0.1-0.8 g/L.

Solvent Extraction for Sb and Bi

Solvent extraction is one of the most widely used methods for the removal of As, Sb and Bi. The application of this method and of ion exchange has increased greatly in recent times because their application makes it possible to separate, purify and concentrate the above mentioned impurities.

Both technologies stand out because of their selectivity with respect to copper, an important factor if it is considered that both copper and antimony are marketable, together with their low cost compared to other technologies or processes. The solvent extraction process consists in decoppering the discarded electrolyte to levels of 10-30 g/L (Jorge A. Saenz-Diez E. Degree work, Departamento de Ingenieria Metalúrgica, Universidad de Santiago de Chile, 1998, pag. 17-22). This solution is then cooled to 25-30° C. and it is fed to an extraction step in which the electrolyte contains the antimony in solution, and when it comes in contact with the organic phase the following antimony extraction reaction takes place as represented by equation (1):

3H₂L_(org)+Sb_(aq)³⁺Sb(HL)_3(org)+3H_(aq)⁺  (1)

where L is the base organic phase that exchanges H⁺ for other types of cations, Sb³⁺ in this case. The organic phase (H₂L_(org) extracts the antimony in an acid medium, contributing protons that increase the electrolyte's acidity, which is negligible if we consider the concentration of antimony in the electrolyte.

In the reextraction step, Equation (2), the organic phase (Sb(HL)_3(org)) loaded in the previous step comes in contact with a highly acidic aqueous solution, but in a hydrochloric medium that is different from that of the extraction, which is sulfuric. The extraction reaction is given by

Sb(HL)_3(org)+3HCl_(aq)+SbCl_3(aq)+3H₂L_(org)  (2)

This reaction shows that the loaded organic phase does not present greater difficulties for being regenerated or unloaded, leaving the antimony free in the advancing solution in a highly acidic medium, whose proton contribution is determined by the hydrochloric acid, which shifts the reaction to the right.

Obtaining Sb as Final Product

The solution coming from the reextration step can be subjected to a process of precipitation with sodium sulfydrate, Equations (3) and (4), at a concentration of 250 g/L according to the following reactions:

NaHS_(aq)+HCl_(aq)H₂S_(aq)+NaCl_(aq)  (3)

3H₂S_(aq)+2SbCl_3(aq)Sb₂S_3(s)+6HCl_(aq)  (4)

The Sb precipitation process delivers as reaction products antimony trisulfide, which is used as raw material for preparing antimony trioxide, and hydrochloric acid, which is recirculated to the reextraction step. In the presence of Bi the procedure is similar, yielding a product rich in Sb and Bi.

Removal of As, Sb and Bi with Activated Charcoal

The industrial use of activated charcoal is essentially as an absorbent and catalyst. Its use in acid solutions like copper refining electrolytes has had good results in the extraction of impurities such as antimony and arsenic.

The “Sumitomo Nihama” copper refinery in Japan implemented the use of this technique to control the antimony present in the electrolyte. The results of this application are summarized in two different tests that were carried out (Alex O. E. Quezada I. Degree work, Departamento de Ingenieria Metalúrgica, Universidad de Santiago de Chile, 2000, pag. 34-38).

Test 1

A 50-mm diameter transparent polycarbonate column is filled with granular activated charcoal 98% of grain size 10 to 32 mesh (U.S. mesh sizes), and the electrolyte is fed from the bottom in a countercurrent system at a temperature of 50° C. and at a rate of 0.5 to 8 SV (SV: electrolyte volume/column bed volume). The concentrations of each of the elements present in the electrolyte are given in Table 1:

TABLE 1
Characteristics of the electrolyte used in test 1.
	[Cu]	[Sb]	[As]	[Bi]	[Ni]	[H2SO4]
	(g/L)	(g/L)	(g/L)	(g/L)	(g/L)	(g/L)
	45.5	0.58	6.4	0.6	18.2	202.2

The electrolyte is discharged from the top of the column and at each feed rate it is sampled and analyzed for antimony and other elements. The results obtained were the following: In the case of antimony the reduction of its concentration in the electrolyte was 95% for a rate of 0.5 SV, from approximately 50% for a rate of 8 SV. In the case of bismuth and arsenic the descrease was from 15 to 5% and from 10 to 1%, respectively, at the rates of 0.5 and 8 SV. The concentrations of nickel, copper and sulfuric acid did not undergo any kind of variation with respect to their initial and final concentrations.

Test 2

This test investigates the influence of the elements present in the electrolyte on the adsorption of antimony by activated charcoal, The composition of this electrolyte is given in Table 2:

TABLE 2
Characteristics of the electrolyte of test 2.
	[Cu]	[Sb]	[As]	[Bi]	[Ni]	[H2SO4]
	(g/L)	(g/L)	(g/L)	(g/L)	(g/L)	(g/L)
	5	0.6	5	0-5	0-18	200

The conditions under which this second test is carried out are the following: 10 g of activated charcoal in 1 L of electrolyte are stirred during 60 minutes at a temperature of 30° C. The results obtained differ with respect to those of test 1, and it is seen that lithe adsorption of antimony takes place when there is no arsenic in the electrolyte, which means that the latter has a direct relation on the adsorption of the antimony.

As a result of this second test Table 3 is shown:

TABLE 3
Influence of the elements present in the electrolyte
on the adsorption of Sb by activated charcoal.
	Bi	0 g/L		0.5 g/L
	Ni	0 g/L	18 g/L	0 g/L	18 g/L
Sb Adsorption (%)
	As
	0 g/L	2	2	3	2
	5 g/L	60	59	67	—

The advantages of this method are the following:

• • There is no consumption of sulfuric acid
  • There is no mixture of strange metal ions in the electrolyte, since the treatment consists only in making contact between the electrolyte and the activated charcoal.
  • Elimination of the releasing tanks or reduction of their capacity. This should become possible when the discharge rate of the electrolyte is determined by the antimony, provided the electrolyte is treated with activated charcoal.
  • The antimony can be recovered easily, for example by means of the use of activated charcoal loaded with antimony as reducing agent in the silver furnace.

One of the main disadvantages of this method is its slow kinetics.

BRIEF SUMMARY OF THE INVENTION

This invention presents a precipitation method that shows the conditions under which the antimony and bismuth present in the electrorefining electrolyte, in the presence of lead dioxide, precipitates forming compounds with lead, among them, in the case of Sb, the so-called bindheimite (Pb₂Sb₂O₇), compounds that have the particularity of being chemically stable in the presence of potable and distilled water under normal conditions. For Bi the formulation of the product would be similar with compounds of the type 2PbO.Bi₂O₅.

To carry out what was described above a literature search was required on the behavior and problems of antimony and bismuth in the electrorefining process, together with the solution alternatives. In this relation, what is submitted in this document is a novel and viable alternative solution for the control of antimony and bismuth in solution.

To establish the conditions under which this compound precipitates it was necessary to carry out tests of PbO₂ consumption, extraction efficiency of antimony, stability, reaction kinetics, traditional chemical as well as X-ray diffraction and fluorescence analyses, and particle size distribution, in addition to observation of the precipitates formed under the electron scanning microscope.

The tests were made with electrolytes from a national copper electrorefining plant, with Cu⁺², H₂SO₄, antimony, and bismuth concentrations of the order of 40 g/L of Cu⁺², 180 g/L of H₂SO₄, 300 ppm of Sb⁺³, and 140 ppm of Bi⁺³, with the addition of PbO₂, to determine the precipitation kinetics.

From the results obtained the formation of bindheimite as one of the Pb—Sb compounds present in the precipitates can be established, as well as that of Pb—Bi compounds. It is established that the theoretical ratio is 1:2 (1 mol of Sb₂O₃ reacts with 2 moles of PbO₂ to produce 1 mol of Pb₂Sb2O₇); however, from the work done, this ratio increases to higher values, but since the mass of lead dioxide can be used in more than one reaction cycle, the mole ratio obtained in practice tends to decrease to values closer to the theoretical value. In the case of Bi the mole ratios follow the same correlation

It was established that the reaction between PbO₇, Sb, and Bi in solution has fast kinetics, achieving within 5 to 10 minutes of contact a decrease greater than 80% of the Sb present in the treated electrolyte.

A more detailed explanation of the invention is provided in the following detailed descriptions and appended claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents the percent extraction efficiency of antimony (Sb) from a synthetic electrolyte as a function of the added mass of lead dioxide (PbO₂), which had been previously activated with H₂O₂.

FIG. 2 presents the percent extraction efficiency of antimony (Sb) for a synthetic electrolyte as a function of the number of steps of reuse of the same added mass of lead dioxide (PbO₂), with the lead dioxide previously activated with H₂O₂.

FIG. 3 presents the percent extraction kinetics of soluble antimony (Sb) for a synthetic electrolyte as a function of test time for an added mass of lead dioxide (PbO₂), with the lead dioxide previously activated with H₂O₂.

FIG. 4 presents the percent extraction efficiency of antimony (Sb) for a real electrolyte as a function of the number of steps of reuse of the same mass of lead dioxide (PbO₂) added, with the lead dioxide previously activated with H₂O₂.

FIG. 5 presents the extraction kinetics of bismuth (Bi) for a real electrolyte. The mass of lead dioxide (PbO₂) added was 10 g, with the lead dioxide previously activated with H₂O₂.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description and explanation of the preferred embodiments of the invention and best modes for practicing the invention.

Example of Application

A synthetic electrolyte with a concentration of 40 g/L of Cu²⁺, 180 g/L of H₂SO₄, and 200 ppm of Sb was considered, to which were added different amounts of PbO₂ previously activated with hydrogen peroxide (H₂O₂) if necessary. The masses of PbO₂ considered were 15, 10, 5, 2.5, 1.25 and 0.625 g for each of the synthetic solutions prepared (approximately 170 cm³ of each), which were allowed to react in a reactor with stirring during a period of 30 minutes at a temperature of 65° C., then removing and labelling a sample of each solution for its chemical analysis with the purpose of determining the drop of the Sb concentration in the electrolyte.

Extraction Efficiency of PbO₂

For the first experiment, in which the extraction efficiency of Sb by PbO₂ was established, the results of the chemical analysis are given in Table 4 together with FIG. 1, which is a graph of the extraction efficiency of Sb vs. the added mass of lead dioxide.

TABLE 4
Results of the chemical analysis of the first experiment.
	Mass of	Initial Sb	Initial Sb	Final Sb	Final Sb
	PbO2	concn. in	mass in	concn. In	mass in	Sb consumpn.	Sb extraction
	added	electrolyte	electrolyte	electrolyte	electrolyte	in electrolyte	efficiency
	(g)	mg/L	mg	mg/L	mg	mg	%
Synthetic sol. 1	15	176.9	21.4	1	0.12	21.28	99.43
Synthetic sol. 2	10	171.5	20.75	3.57	0.41	17.18	97.92
Synthetic sol. 3	5	172.8	20.90	47.7	5.77	15.13	72.39
Synthetic sol. 4	2.5	159	19.23	107.6	13.01	6.21	32.32
Synthetic sol. 5	1.25	142	17.18	103.3	12.49	4.68	27.25
Synthetic sol. 6	0.625	160	19.63	130.8	15.8	3.83	19.51

Extraction Efficiency vs. Number of Steps

To establish the number of steps in which it is possible to use a given mass of PbO₂ the following experiment is carried out. A synthetic electrolyte of the same characteristics as those of the previous experiment (40 g/L of Cu²⁺, 180 g/L of H₂SO₄, and 200 ppm of Sb), at a temperature of 65° C. is considered. The mass of PbO₂ used is 10 g, and it is made to react with a given number of synthetic solutions up to the point at which the PbO₂ stops reacting with the antimony present in each of the remaining solutions.

In this second test the extraction efficiency of lead dioxide was established, related to the number of steps in which it is possible to use a given amount of it. Table 5 and FIG. 2 show the results of this experiment.

TABLE 5
Results of the chemical analysis of the second experiment.
	Initial Sb	initial Sb	Final Sb	Initial Sb	Consumpn.
	concentration in	mass in	concentration in	mass in	of Sb in	Efficiency of
No. of	electrolyte	electrolyte	electrolyte	electrolyte	electrolyte	Sb extraction
steps	(mg/L)	(mg)	(mg/L)	(mg)	(mg)	(%)
1	176.7	21.38	1	0.12	21.26	99.43
2	176.7	21.38	12.1	1.46	19.92	93.17
3	176.7	21.38	12.7	1.54	19.84	92.81
4	176.7	21.38	12.8	1.55	19.83	92.75
5	176.7	21.38	33.5	4.05	17.33	81.04
3	176.7	21.38	34.5	4.17	17.21	80.47
7	176.7	21.38	46.6	5.64	15.74	73.63
8	176.7	21.38	52.9	6.41	14.98	70.06
9	176.7	21.38	39.3	7.83	13.55	63.38
10	176.7	21.38	84	10.16	11.22	52.46
11	176.7	21.38	106	12.83	8.55	40.01
12	176.7	21.38	113	13.67	7.71	36.05

From FIG. 2 and the data of Table 5 it is established that in the first reaction to which the PbO₂ is subjectd with the electrolyte, its percentage reaction with the antimony is high, almost 100% (99.43%) as expected. However, in the following reactions it is seen that the reaction percentage decreases slightly unto step number 4, where the variation of the reaction is approximately 6.68%; the variation then becomes more noticeable, and after steps 11 and 12 the reaction starts remaining almost constant. The reaction between the antimony and PbO₂ keeps taking place, but it is very small compared to the first ones, indicating that the number of steps in which a given mass of PbO₂ can be used will depend on the percentage of antimony that it is desired or possible to extract.

From the experiments made it can be stated that a given mass of lead can be used more than once, depending on the percentage of antimony that it is desired to extract. Considering approximately 80% antimony extraction, according to the experiments made the cycles over which lead dioxide can be used are six.

It should be pointed out that these two experiments were carried out at a temperature of 65° C., which is characteristic of electrorefining processes.

From what was done in the first and second experiments, the consumption of lead dioxide can be established theoretically for an industrial situation. For example, at the “Ventanas” refinery the amount of discarded electrolyte for impurity control is 24 m³/day, with an Sb concentration characteristic of the electrolytes of electrorefining plants of 200 ppm (0.2 g/L). The balance is made and it is determined from the empirical stoichiometry (1:290) that the amount of lead dioxide needed to treat that volume of solution for antimony will be approximately 1,822 kg of lead dioxide (approx. 7,620 moles), but since this material can be reused in more than one cycle, depending on the percentage of antimony extraction that is desired or requiered, then its consumption decreases. For example, if the desired extraction percentage of antimony is approximately 80%, the number of cycles in which lead can be used is six, and the consumption of lead dioxide decreases from 1,822 kg to 303 kg per cycle.

Extraction Kinetics

After what was presented above, the reaction kinetics must be established. To that end the following experiment is carried out:

A synthetic solution with the characteristics of those used previously is prepared and an amount of PbO₂ is added to it. The amount of solution, the same as in the previous experiments, is 170 mL and the mass of PbO₂ is 15 g. The PbO₂ is added to the solution, which has an antimony concentration of 178 mg/L, and the mixture is placed in a reactor with stirring, taking a sample of the solution that will be sent for chemical analysis at 5, 10, 15, 20, 25 and 30 minutes. The results are shown in Table 6 and FIG. 3, where FIG. 3 is a graph corresponding to the reaction kinetics between antimony and the lead dioxide.

TABLE 5
Results of the kinetics test.
MINUTES [Sb] mg/L
5       6.8
10      5.2
15      4.3
20      3.8
25      3.1
30      1.5

This experiment indicates that the reaction between the PbO₂ and the antimony is fast, since the variation of the Sb concentration in the electrolyte occurred between 5 and 30 minutes, which is very short, indicating that almost all the reaction takes place in the first 10 minutes of stirring.

Test with Real Electrolyte

Finally, a test was carried out with real electrolyte from a national electrolytic copper refinery. In this test the results of the previous tests ran with synthetic electrolytes were used to determine if the results obtained for them are repeated using the real electrolyte.

A real electrolyte sample was subjected to chemical analysis to determine its Sb concentration, and it was found that the Sb concentration is 159.9 mg/L. Then 170 mL of solution and 15 g of PbO₂ superficially activated with H₂O₇ are taken and the number of times that it is possible to use this lead mass is established. The results are shown in Table 7 and FIG. 4:

TABLE 7
Results of the chemical analysis.
	Initial Sb	Initial Sb	Final Sb	Final Sb	Sb
	concn. in	mass in	concn. in	mass in	consumption	Eficiency of
Number of	electrolyte	electrolyte	electrolyte	electrolyte	in electrolyte	extracted Sb
steps	(mg/L)	(mg)	(mg/L)	(mg)	(mg)	(%)
1	159.9	23.99	40.6	6.09	17.9	74.62
2	159.9	23.99	101.8	15.27	8.72	36.34
3	159.9	23.99	130.5	19.575	4.415	18.41
4	159.9	23.99	135.8	20.37	3.615	15.06
5	159.9	23.99	142.6	21.39	2.6	10.84
6	159.9	23.99	143.2	21.48	2.51	10.46
7	159.9	23.99	145.3	21.78	2.21	9.21

The results are shown in Table 7, but more clearly in FIG. 4, which is a graph of the relation between the number of steps and the percentage of antimony extraction. The test is carried out with real electrolyte, and the efficiencies obtained with the real electrolyte are not very similar to those obtained with the synthetic electrolyte. The difference is due to the impurities contained in the real electrolyte, which are not present in the synthetic electrolyte. But there is no doubt that important percentajes of Sb extraction are obtained.

Test with Real Electrolyte 2

Another test was carried out with real electrolyte from a national electrolytic copper refinery. In this test the results of the previous tests with both synthetic and real electrolytes were used.

A real electrolyte sample was subjected to chemical analysis and it was found that the Sb concentration is 300 mg/L and that of Bi is 140 mg/L. Then 170 mL of solution and 10 g of PbO₂ superficially activated with H₂O₂ are taken and the extraction kinetics is determined. The results are shown in Table 8 and Graph 5:

TABLE 5
Results of the chemical analyses.
Time Bi
0    14
5    80
15   40
30   17

Although embodiments and examples of the invention have been shown and described, it is to be understood that various modifications, substitutions, and rearrangements of compounds, elements, components, and method (process) steps as well as other uses for the invention can be made by those skilled in the art without departing from the novel spirit and scope of the invention.

Claims

1. A method for decreasing the content of soluble antimony and bismuth in an electrolyte CHARACTERIZED because it comprises putting in contact lead dioxide with an acid electrolyte that contains antimony and bismuth in solution, yielding insoluble compounds based on Pb—Sb and Pb—Bi.

2. A method for decreasing the content of soluble antimony and bismuth in an electrolyte according to claim 1, CHARACTERIZED because the lead dioxide is in the form of particles.

3. A method for decreasing the content of soluble antimony and bismuth in an electrolyte according to claim 1, CHARACTERIZED because the lead dioxide can or cannot be previously activated superficially with an oxidizing agent, for example hydrogen peroxide (H2O2), to improve its extraction capacity.

4. A method for decreasing the content of soluble antimony and bismuth in an electrolyte according to claim 1 CHARACTERIZED by the formation of insoluble Pb—Sb based compounds and Pb—Bi compounds.

5. A method for decreasing the content of soluble antimony and bismuth in an electrolyte according to claim 1 CHARACTERIZED by allowing the reuse of the particulate system for the precipitation of antimony more than two times.

6. A method for decreasing the content of soluble antimony and bismuth in an electrolyte according to claim 1 and 5, CHARACTERIZED by allowing a reduction of at least 70% of the dissolved antimony and bismuth present in acid electrolytes.