NL1013590C2 - Method for the selective removal of metals from concentrated metallic streams. - Google Patents

Description

Method for the selective removal of metals from concentrated metallic streams.

Field of the Invention The invention relates to a process in which, using sulfide, a better separation can be achieved between metals which are close to each other in solubility as metal sulfide in the specific solution. Also, the dewaterability of the formed metal sulfide precipitates using the new method is much better than previously achieved.

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Background

Many processes are known in which metals are selectively recovered from concentrated streams in which multiple metals are present. Often used for the selective separation of metal sulfide, a separation being effected based on the differences in solubility of the different metal sulfides at different pHs.

In particular, the sulphide separation of copper and arsenic from spent electrolyte from electro-refining processes, such as those widely used to produce pure copper, has heretofore been experienced as very difficult and is not often used. The blowdown current is normally processed in a separate circuit consisting of the following process steps: 1. electropinning copper; 2. arsenic removal via electrowinning; Nickel sulfate crystallization.

The copper electrolysis and arsenic removal are closely linked: the moment the residual copper concentration becomes low (say a few grams per liter), arsenic begins to settle together with the copper in the form of a copper-arsenic alloy. After formation at the cathode, this alloy largely falls to the bottom of the cell. This means that the cells must be cleaned regularly.

In addition to being labor-intensive, electrolytic arsenic removal is life-threatening due to The possible formation of lethal AsE 4 gas when the copper concentration becomes too low (<0.1-0.5 g / L). Furthermore, the CuxAs residue from the cells must be further processed. There are different processes for this.

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Nickel sulfate is then crystallized from the then highly acidic copper and arsenic-free solution in evaporators or by subcooling. The crude nickel sulfate product is sold or further upgraded as such, for example for electrogalvanic applications. Condensate and mother liquor are typically recycled for the water and acid management of the copper factory.

Naturally, there are many variants of the classic process. For example, there are several copper refineries in the world that use arsenic by means of liquid-liquid extraction is separated from copper and nickel. Other refineries use ion exchange to maintain the concentrations of arsenic, antimony and bismuth. Falconbridge in Ontario, Canada, uses an ion exchange system precisely for the selective removal of sulfuric acid from the purge stream. Still other plants remove part of the copper in solution by crystallization of copper sulfate. Finally, there are only a few copper factories in the world where copper and arsenic are selectively precipitated from blowdown using sulfide. In Utah, USA, the Kennecott Company uses 100% H2S gas to achieve a copper-arsenic separation. Selectivities of only 70% are achieved.

Besides the separation of copper and arsenic from spent electrolyte, there are also other concrete applications in which the method according to the invention can have great advantages in terms of selectivity and dewaterability, such as the separation of nickel from nickel, cobalt and ferrous (bio) leach flows. and the recovery of zinc from magnesium bleeds from zinc electrolysis processes. This new method can also be used for the production of crystalline lead sulphide and copper sulphide.

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Precipitation processes for the separation of metals are known from the literature.

US 4,404,071 describes the precipitation of copper sulfide using hydrogen sulfide. In the continuous process, the redox potential measurement is used to monitor the H2S supply. Selectivities of between copper and arsenic of 70% are achieved.

According to US 4,432,880, it is possible to obtain lower metal concentrations in metal-contaminated water by means of sulphide precipitation than using hydroxide precipitation alone.

US 5,498,398 proposes the possibility of separating copper from arsenic, in which the metal is completely precipitated from the water in a first step. Then As, Sb, and Bi are oxidized with oxygen and redissolved in the water. The remaining precipitate 1013590 3 is mainly copper sulfide. Arsenic, antimony and bismuth are then again precipitated with hydrogen sulphide.

US 5,616,168 describes the precipitation of metal sulfide using sulfur and sulfur dioxide. Metal separation is achieved by adding controlled sulfur and sulfur dioxide to the solution.

Description of the invention

A problem with selective precipitation of metals with sulfide is that local supersaturation, in addition to the desired metal, also precipitates other metals, which no longer dissolve easily. This problem can be partially solved by increasing the mixing in the precipitation step. However, especially with metals, which are difficult to separate due to their solubility, the mixing that can be achieved by, for example, a stirrer is often not good enough to achieve really high selectivities (> 95 15%). The solution to this problem, in combination with a high concentration of inoculum in the precipitation step, is to feed the sulfide slowly and evenly in a distributed manner to the liquid. This can be achieved by introducing the H2S in the form of a gas into the precipitation step, the H2S concentration in the gas being kept low in a controlled manner and adjusted to the precipitation rate in the liquid. As a result, local supersaturation can be greatly minimized, making high selectivities feasible. The dewaterability of the crystalline sulphide precipitates formed thereby is also greatly improved.

The required H 2 S is added to a gas stream which is recycled over the precipitation step, the H 2 S concentration being the gas preferably kept between 0.5 and 25%. More preferably, the concentration is kept between 1 and 15 vol5, most preferably between 2 and 10%. Nitrogen gas can for instance be used as carrier gas. Preferably, the H 2 S is produced in a bioreactor where sulfate, sulfur or other oxidized sulfur compound is reduced to H 2 S. For bacteria and reaction conditions in such a process, see, e.g., WO 97/29055. In this case, the H 2 S is stripped from the bioreactor with the same gas used as the support for the precipitation. As a carrier gas, therefore, hydrogen gas, which is used as an electron donor for sulfur / sulfate reduction, or methane and carbon dioxide produced in the bioreactor, can be used as carrier gas.

Figure 1 shows the process diagram of the sulphide precipitation. The water (1) contaminated with metal ions [for example electrolyte] is mixed in a mixer (MT1) with inoculum 10135 gπ 4 recycled. This material is then gassed in the precipitator (Prec. 1) with controlled dilute H2S gas (7). In the precipitator, the metal selectively precipitates (e.g., copper). The effluent (3) is partially dewatered in a thickener (Thick. 1). The underflow of the thickener (4) is partly returned as seed material in the mixer (MT1). The remaining material is further dewatered in a solid-matter separator (S / Ll). The recovered solid (5) has a low moisture content of about 40 wt% and contains a certain metal sulfide (e.g. copper sulfide) of high purity. The washing liquid (6) from (S / Ll) is sent back to the thickener (Thick. 1). The next metal can be selectively removed from the overflow (8) of the thickener (Thick. 1) by a second precipitation step. If necessary, the pH is monitored in the precipitation steps.

Process control'

In order to selectively precipitate the metal sulfide, the absolute H 2 S feed must be matched to the metal load to the precipitator. This is controlled by controlling the absolute H2S supply to the redox potential in the liquid.

A second possibility to minimize the local supersaturation in a precipitation step, such that high selectivities and well-dewaterable crystalline precipitates are created, is to produce the H2S in a slow and uniform manner in the liquid. In this case, for example, the bioreactor can be combined with the precipitation unit. A concentrated metal-containing stream can be fed to a very well mixed bioreactor, the metal in the bioreactor, in which the metal sulfide concentration is kept high by applying solids retention, precipitates as crystalline metal sulfide due to the production of sulfide in the bioreactor. This is shown in Figure 2. This possibility is especially useful for separating pollutant metals such as magnesium which are not or badly precipitated as sulfide, whereby such metal is discharged from the production unit as side stream and the metal produced, such as zinc, is precipitated and recycled. to the production unit.

The metal (sulphate) -containing water (1) is fed to a well-mixed bioreactor, preferably a gas lift run reactor (R1), in which mixing is effected by recycling gas. Optionally, a gaseous electron donor for the biological reduction step, in the form of H2 (7), can be supplied or a small amount of inert gas, such as N2 (7), which can be used to remove the biologically produced CO2 through the gas bleed . (8). Furthermore, inoculum (4) 1013590 5 of the thickener is recycled. The effluent (2) from the reactor is dewatered using a thickener (Thick.3). The overflow (3) contains sulphate and metal purified water. The underflow (4) is partially recycled. The remaining underflow is further dewatered using a solid-liquid separator (S / L3). The product (5) is metal sulfide with about 40% moisture. The wash water (6) from the solid-liquid separator (S / L 3) is sent back to the thickener (Thick.3). The bioreactor can also be run with an internal settler. In this case, a smaller flow from the bioreactor is sent to the thickener.

10 Process control

In this case too, the H2S production can be controlled on the basis of the redox signal in the liquid in the bioreactor. Another possibility is an online sulphide measurement in the liquid or gas phase.

15 EXAMPLES Example 1:

A 1L stirred tank reactor with a synthetic electrolyte solution is fed through a glass pipe with an H2S containing (25% by volume) gas mixture. The effluent gas is passed through a caustic soda bottle to collect excess H2S. No excess H2S is measured during the precipitation. The reaction temperature is 60 ° C and the pressure is 1 bar. The electrolyte solution contains 10.47 g / L copper, 6.75 g / L arsenic, 200 g / L sulfuric acid and 60 g / L copper sulfide as the seed material. Table 1 shows that copper and arsenic precipitate simultaneously. The selectivity for copper is only 50 to 60%. The filtration rate of the effluent is 313 kg dry matter per m2 and µm. X-ray diffraction shows that the copper sulfide is crystalline.

Analogously, a 1L stirred tank reactor containing a synthetic electrolyte solution is fed through a glass pipe with an H2S containing (15% by volume) gas mixture. The effluent gas is passed through a caustic soda bottle to collect excess H2S. No excess H2S is measured during the precipitation. The reaction temperature is 60 ° C and the pressure is 1 bar. The electrolyte solution contains 10.34 g / L copper, 6.85 g / L arsenic, 200 g / L sulfuric acid and 60 g / L copper sulfide as the seed material. Table 2 shows that more copper precipitates than arsenic. The selectivity for copper is about 60 -70%. The filtration rate of the effluent is 142.5 kg dry matter per m2 and hour. X-ray diffraction shows that the copper sulfide is crystalline.

Table 1: Change of metal concentration over time 101 3590 6

Time Cu-extraction As-extraction Selectivity [min] [wt-%] [wt-%] [%] Ö ÖfiÖ iöö 15 4.59 2.71 62.91 30 13.94 6.09 69.58 45 16, 90 17.03 49.80 60 28.66 25.91 52.53 90 48.36 40.24 54.58 120 67.41 55.14 55.01 180 97.45 88.20 52.49 240 100, 00 87.95 53.21

Table 2: Change of metal concentration over time Time Cu-extraction As-extraction Selectivity [min] [wt-%] [wt-%] [%] öfiö ÓiÖÖ 0 2.32 0.00 100.00 15 6.11 3.39 64.33 30 14.09 7.23 66.08 45 22.72 10.68 68.02 60 34.78 15.09 69.74 90 46.26 24.65 65.24 120 70.99 36.35 66.14 180 86.06 42.88 66.75 210 92.28 56.70 61.94 240 99.70 70.26 58.66 5 In a similar manner, a 1L stirred tank reactor containing a synthetic electrolyte solution through a glass pipe fed with an H2S-containing (5% by volume) gas mixture. The effluent gas is passed through a caustic soda bottle to collect excess H2S. No excess H2S is measured during the precipitation. The reaction temperature is 60 ° C and the pressure is 1 bar. The electrolyte solution contains 9.78 10 g / L copper, 6.09 g / L arsenic, 200 g / L sulfuric acid and 60 g / L copper sulfide as the seed material. Table 3 shows that only copper precipitates. Very high selectivity for copper is achieved (> 95%). The filtration rate of the effluent is 173.15 kg of dry matter per m2 and hour. X-ray diffraction shows that the copper sulfide is crystalline.

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Table 3: Change of metal concentration with time Time Cu-extraction As-extraction Selectivity [min] [wt-%] [wt-%] [%] 0 0.00 0.00 60 5.26 0.00 100, 00 120 10.75 0.00 100.00 180 22.24 0.00 100.00 240 28.71 2.20 92.89 300 37.70 0.61 98.41 360 52.97 0.00 100, 00

Figure 3 shows the results of the three tests again. It can be seen that at lower H2S concentrations in the gas the selectivity for copper precipitation increases sharply.

Example 2:

Treatment of magnesium bleed from the zinc electrolysis process.

Magnesium bleed containing 150 g / L zinc, 16 g / L magnesium and 280 g / L sulfate was fed directly to a sulfate-reducing bioreactor. The bioreactor consisted of a 5 m3 gas lift loop reactor with a hydrogen gas recycle for mixing and transferring hydrogen gas to the bacteria for the sulphate reduction process. In the bioreactor, up to 150 g / l ZnS accumulated and the effluent from the bioreactor contained 50 mg / l sulfide, <0.1 mg / l zinc and <100 mg / l sulfate. The ZnS produced was crystalline and could be easily dehydrated into a paste with 20 to 40% moisture.

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Claims (9)

Method for precipitating metals as metal sulfide from a concentrated solution using sulfide in a precipitator, characterized in that (a) the sulfide is added to the solution in the precipitator from a dilute H2S gas stream containing a maximum of 25vol% H2S or causes the sulfide to form in the precipitator by biological reduction of sulfate, (b) adding seed material of the metal sulfide to be precipitated at a concentration of at least 5 g / L, and (c) mixing the precipitator well. 10 Process according to claim 1, characterized in that (b) seed material of the metal sulfide to be precipitated is added in a concentration of at least 10 g / l. 3. Process according to claim 2, characterized in that (a) the sulfide is added from a dilute H2S gas steam containing 1-15% H2S. Process according to any one of claims 1-3, characterized in that (a) the sulfide is added such that the concentration of dissolved sulfide remains below 10 mg / l. 20 Method according to any one of claims 1-4, characterized in that the metal solution results from an electrowinning process. 6. Process according to claim 5, characterized in that copper sulfide is selectively precipitated. Process according to claim 1, characterized in that (a) the sulfide is produced by biological reduction of sulfate and the metal solution is added with a metal concentration of at least 2 g / l, preferably more than 5 g / l, in the especially 30 more than 10 g / l. 8. Process according to any one of claims 1-7, characterized in that the pH is adjusted so that the metal sulphide to be precipitated does not precipitate, but a metal sulphide which has a higher solubility at the same pH. 35 9. Process according to any one of claims 1-8, characterized in that the metal sulfide is precipitated in crystalline form. 1 01 3590