Closed-Loop Process of Extracting and Separating Zinc Impurities from Industrial Cobalt Products—Pilot Test Study

Abstract

The cobalt-containing products of an enterprise were selected as the object of this study. The separation effect of Cyanex 272 on Zn and Co was studied through a pilot test. The results showed that Cyanex 272 had a high extraction rate for Zn at pH 3–3.4, up to 99.8%. The extracted Zn²⁺ was enriched in the organic phase, and the Zn²⁺ in the organic phase was extracted back into the aqueous phase in the stripping stage by adding strong acid. Addition amounts of strong acid of 50 g·L⁻¹, 80 g·L⁻¹, and 100 g·L⁻¹ were set. The results showed that the addition amounts of 80 g·L⁻¹ and 100 g·L⁻¹ could significantly reduce the pH of the back-extraction solution and effectively enrich Zn²⁺ in the solution. There was a large amount of Zn²⁺ in the back-extraction solution, which could be used twice to prepare zinc carbonate products through neutralization and precipitation. By comparing the extraction effect, economic cost, and resource loss under different strong acid addition amounts, it was found that the 80 g·L⁻¹ strong acid addition amount was more suitable for the actual production process. This study provides data support and practical evidence for the selection of industrial extraction process parameters for Zn²⁺ separation in actual cobalt products.

1. Introduction

Tailings are a mixture of waste and process fluids produced during the extraction of minerals and metals from ores, and their mineral composition depends mainly on the original ore body [1,2]. It is estimated that 5–7 billion tons of tailings are produced each year [3]. Studies have reported that these tailings usually contain a large number of metals, such as copper (Cu), cobalt (Co), zinc (Zn), calcium (Ca), nickel (Ni), etc., piled up beside the river, which may cause a waste of resources and the pollution of soil and groundwater [4,5]. With the growth of the world’s population, the demand for industrial minerals will continue to increase, and the resource utilization of tailings and the reproduction of valuable metals are also of increasing concern. In recent years, the global demand for cobalt resources has further increased with the rapid development of the new energy industry [6,7,8], and the resource utilization of Co in tailings is urgent. Mostly, Co in tailings comes from by-products produced in the production process of Ni systems, Cu systems, and Zn systems. Impurities such as Zn, Cu, and Cd often exist in tailings containing Co, which seriously affects the quality of cobalt products. Therefore, it is important to separate Co and Zn impurities efficiently using a proper method.

The extraction method is one of the most effective technologies for the selective separation of valuable metals from an aqueous solution, and is widely used in rare earth element production, metallurgy, petroleum production, and other industrial production [9,10,11,12,13]. At present, the common acidic extractants are P204 (di (2-ethylhexyl) phosphate), P507 (2-ethylhexyl phosphate mono (2-ethylhexyl) ester), and Cyanex272 [14]. Jin Fengfeng [15] conducted a detailed study on the application of P204 and P507 extractants in industry, and found that under weak acidic conditions, P204 and P507 will extract Zn²⁺ in the solution while also extracting Ca²⁺, which leads to the use of sulfuric acid or acid sulfate in the washing and stripping stage, and induces CaSO₄ precipitation. This has adverse effects on the follow-up production. In addition, under the condition of weak acid, P507’s extraction of Zn and Co is difficult to separate, and is not suitable for the separation of Zn and Co containing impurities such as Ca²⁺ and Mg²⁺.

Cyanex 272 is one of the most frequently studied acidic extractants. Cyanex272 has almost no extraction effect on Ca²⁺ under pH < 4.0. Tsakiridis et al. [16] used 20%Cyanex301 + 5%TBP + d-80 diaromatic solvent oil to extract cobalt and nickel efficiently. Cyanex272 can extract Ni and Co effectively, which was also confirmed by Nayl et al. [17] However, at present, there are few studies on the extraction separation of Co and Zn by Cyanex272. In addition, most of the research uses a separation funnel for separation, but the feasibility of continuous operation and large-scale production in industrial production must be considered. Therefore, it is necessary to further develop an industrial extraction process suitable for Zn²⁺ separation in cobalt-containing products by simulating a continuous extraction process on an industrial scale.

Zn is a non-ferrous heavy metal, widely used in non-ferrous the metallurgy, chemical industrial, electrical, military, and medical fields [18,19]. In the actual production process, the Zn²⁺ impurities are enriched in the organic phase after extraction. Ali et al. [20] simulated and compared HCl, H₂SO₄, and HNO₃, and found that H₂SO₄ was more likely to reverse Zn²⁺ extraction due to its easy performance of electrodeposition. The solution after stripping contains a large amount of Zn²⁺, and if properly treated, zinc can be recycled and used as a secondary resource, which has considerable economic benefits and environmental significance [21,22]. However, in the actual industrial production process, the selection of process parameters is affected by a variety of factors, such as economic cost, income, extraction effect, etc. Therefore, this study intends to select the actual cobalt products as the research object and aims to, (1) through a pilot test, verify the extraction separation effect of Zn and Co by Cyanex272, and (2) select the optimal process parameters by comprehensively analyzing the extraction effect, economic benefit, and resource loss in the whole process (from extraction to the production of a by-product). This study will help to provide data support and practical evidence for the selection of industrial extraction process parameters for zinc ion separation in actual cobalt products.

2. Materials and Methods

2.1. Material Preparation

The cobalt hydroxide leaching solution used in this study was prepared by the studied company, which produces Co. The components of the configured cobalt hydroxide leaching solution are shown in

Table 1. Main components of cobalt hydroxide leaching solution.

Items	Co/g·L−1	Zn/g·L−1	Mn/g·L−1	Mg/g·L−1	Ca/g·L−1	Cd/mg·L−1	Fe/mg·L−1	Al/mg·L−1	Cu/mg·L−1	pH
Content	3.05	1.07	0.60	2.04	0.49	19.21	<0.1	0.20	5.08	5.01

.

The chemical agents used in the extraction system of this study include Cyanex-272 extractants (analytically pure, Shanghai Lyas Chemical Co., Ltd., Shanghai, China 90%); 260# sulfonated kerosene (industrial pure, Henan Mengfei Industrial Co., Ltd., Luoyang, China, 99%); sulfuric acid (analytically pure, Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China, 98%); and sodium hydroxide (analytically pure, Sinopharm Group Chemical Reagent Co., Ltd., 96%). The chemical properties of the extractants are shown in Table S1.

The chemical agent used in the neutralization process of this study is 20%w Ca(OH)₂ solution, and the chemical agent used in the precipitation process is Na₂CO₃ solution.

2.2. Extraction Experiment

The Zn extraction, washing of Co, and stripping of Zn were carried out in an extraction mixing and clarifying tank with a baffle plate. It was divided into three processes, including four stages, namely two Zn extraction stages, one cobalt washing stage, and one Zn stripping stage. The test lasted for 130 h. The leaching solution of cobalt hydroxide and the organic phase were injected at the same time, and the NaOH solution flowed into the extraction mixing chamber (the primary extraction mixing chamber was E1, and the secondary extraction mixing chamber was E2) to adjust the pH value. The leach solution of cobalt hydroxide first entered E1 and was evenly mixed with the organic phase returned by the secondary extraction, and then entered the primary extraction and clarification chamber. After clarification, the aqueous phase entered E2 and was again evenly mixed with the organic phase, and the raffinate was collected and used after being clarified by the secondary extraction. The organic phase entered the washing chamber (Sc1) and was mixed with weak acid for cobalt washing. After the washing of cobalt, the organic phase entered the back-extraction stage (S1), the zinc in the organic phase was back-extracted, the organic phase was recycled, and the spent washing liquid was returned to the feed liquid (Figure 1).

The organic phase of the extraction test was a mixture of 260# sulfonated kerosene and Cyanex-272 extractor (5%). The organic phase flow rate was 448.33 mL·min⁻¹ and the aqueous phase flow rate was 358.33 mL·min⁻¹. The ratio of organic phase to aqueous phase (O/A) was 1.25, and the mixer was 1.10. The concentration of NaOH was 40 g·L⁻¹, and the pHs of the solution in the primary extraction mixing tank and the secondary extraction mixing tank were adjusted to 3.0 and 3.4, respectively. In cobalt washing, O/A ratio was 50, the mixer was 1.25, and the concentration of sulfuric acid was 10 g·L⁻¹. In the re-extraction stage, the O/A ratio was 50, the mixer was 1.25, and the concentration of sulfuric acid was set to three gradients: 50 g·L⁻¹, 80 g·L⁻¹, and 100 g·L⁻¹.

2.3. Neutralization and Precipitation Experiment

Neutralization and precipitation tests were carried out. The neutralization test was performed by adding 20%w of Ca(OH)₂ solution to the back-extract and stirring to test the pH value of the solution. The neutralization process was completed when the pH was adjusted to 5.0 and the pH of the solution remained unchanged for one hour (Figure S1). The precipitation test was similar to the neutralization test. Na₂CO₃ solution was added to the neutralized solution, and the pH was adjusted to 9.0. The test cycle duration was 6 h. The precipitation was left to stand for stratification and filtration, the solid was dried for use, and the filtrate was retained for testing.

2.4. Calculation

The composition of the solution was analyzed by inductively coupled plasma emission spectrometry (ICP-OES). The zinc (Zn) and cobalt (Co) contents were determined using an inductively coupled plasma emission spectrometer (ICP-OES, Optima 5300 DV, Perkin Elmer, Shelton, CT, USA). The cadmium (Cd) content was determined using an inductively coupled plasma mass spectrometer (ICP-MS, Elan DRC-e, Perkin Elmer, USA).

In this study, the extraction rate of valuable metals was determined according to the solution metal concentrations of E1, E2, Sc1, and S1, as shown in Equation (1) [19,23,24]. The calculation formula includes a feed ratio of raw liquid and weak acid of 40:1 in the return eluent (Sc1) extraction tank.

$$f_{Total}={\displaystyle \frac{c_{Feed}\times 40+c_{Sc1}\times 1-c_{E2}\times (40+1)}{c_{Feed}\times 40+c_{Sc1}\times 1}}\times 100\mathrm{\%}$$

(1)

In the above formula, f_total represent the extraction rates of the total extraction, while C_Feed, C_SC1, C_E1, and C_E2 correspond to the ion concentrations in the feed, Sc1, E1, and E2, respectively.

The recovery rate is another important index to evaluate the extraction effect.

$$f_{Recovery}=1-{\displaystyle \frac{c_{S1}\times 1}{c_{Feed}\times 40}}\times 100\mathrm{\%}$$

(2)

where C_Feed, C_S1 are the ion concentrations of the feed and S1, respectively.

According to Meng et al. [25] and Liu et al. [26], whether the Zn content in the raffinate meets the standard, the economic cost, the income of new products, and the loss of valuable metals are comprehensively analyzed by assigning weights to indicators, including the extraction efficiency (Supplementary Materials). The reagent addition amount most suitable for actual production is analyzed so as to screen out the best extraction method for the Zn removal, neutralization, and precipitation of zinc products with a comprehensive effect.

$$S={\sum }_{k=0}^n[({\displaystyle \frac{H}{H_{max}}})\times W]\times 100$$

(3)

where S is the score, H is the input value of each indicator, H_max is the highest input value of each indicator, and W is the weight of each indicator.

2.5. Data Analysis

The variance and significance were analyzed using ANOVA with SPSS Statistics 19. The significance of the differences among the mean values was compared using least significant difference (LSD) tests at a 5% level of probability, and the values were plotted using Origin 2017.

3. Results and Discussion

3.1. Effects of Different Sulfuric Acid Addition Amounts on Zn Removal in Extraction Experiment

3.1.1. Effects of Different Sulfuric Acid Addition Amounts on pH

pH is an important index that affects the extraction effect of extraction systems [14,27,28]. Studies have shown that the Cyanex-272 extractant can extract different valuable metals by adjusting the pH [29,30]. Ahmadipour et al. [31] found that the pH of Cyanex-272 extractant for the effective extraction of Zn was about 1–3.5, and the closer the pH was to 3–3.5, the better the extraction effect was. The results of this study showed that during the operation of the extraction system, the pH of the E1 and E2 solutions in the extraction stage remained at 3–3.5, but in the washing and stripping stages, the pH of the extraction system changed greatly with an increase in time under the condition of strong acid feed. At the 14th hour of the reaction, the pH of the stripping stage increased to 2.52 under the condition of 50 g·L⁻¹ strong acid feed. This may have caused most Zn²⁺ to be adsorbed in the organic phase, reducing the effect of Zn²⁺ back-extraction. The addition of 80 g·L⁻¹ and 100 g·L⁻¹ strong acids kept the pH of the S1 solution stable at around 0.7 and 1, respectively (Figure 2). When the pH of the solution is <1, the organic phase Cyanex-272 has almost no extraction effect on Zn. Therefore, most of the Zn²⁺ in the organic phase will be desorbed to the aqueous phase. However, in actual production, the choice of an 80 g·L⁻¹ or a 100 g·L⁻¹ strong acid addition amount for back-extraction still needs to be combined with the economic effect, extraction efficiency, and other considerations.

Kazak et al. [32] found that the effective extraction pH of Cyanex-272 extractants for Co²⁺ was 3–5.9, and the closer the pH was to 5.0–5.9, the better the extraction effect of Co was. In the washing stage, the pH of 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition can reach 2.5, which can effectively elute Co²⁺, while under the condition of 50 g·L⁻¹ strong acid addition, the pH of the washing stage slowly drops from 3.3 to less than 3 in the first 5 h. This may have a certain effect on the elution of Co²⁺ and increase the loss of Co resources.

3.1.2. Effects of Different Sulfuric Acid Addition Amounts on Zn²⁺ and Co²⁺

The concentrations of Zn²⁺ and Co²⁺ in the aqueous solution changed significantly after treatment with the extraction system (Figure 3). Comparing the concentration of Zn²⁺ and Co²⁺ in the liquid phase at different stages, it was found that the concentration of Zn²⁺ in the liquid phase in E1 and E2 was significantly reduced compared with the concentration of Zn²⁺ in the feed, especially in E2 (after E2 was separated by the organic phase and liquid phase, the liquid phase was raffinate). The concentration of Zn²⁺ was less than 6 mg·L⁻¹, which meets the standards for producing cobalt products. In addition, the extraction efficiencies for Zn²⁺ with the addition of strong acid at 80 g·L⁻¹ and 100 g·L⁻¹ were both higher than 99.3% and could be up to 99.80% (Figure S2), indicating that Zn²⁺ can be effectively extracted under the conditions of this test, which is consistent with the results of many studies and in line with the characteristics of acidic extractants [18,19,31,33].

Cyanex-272 exists in the form of bis (2,4,4-trimethylpentyl) phosphinic acid, and when NaOH is added, the strong base reacts with the ester through saponification (Formula (4)) [33]. Part of the saponified extractant reacts further with Zn²⁺ to extract Zn²⁺ into the organic phase [34,35] (Formula (5)). In contrast to the change in Zn²⁺ concentration, the Co²⁺ content in the E1 and E2 liquid solutions decreased slightly compared with the Co²⁺ content in the feed, and the difference was not large. The Co²⁺ extraction efficiency also shows that the extraction efficiency of the extraction system in this study was lower than 6.5%. The extraction effect of the extraction system on Co²⁺ was low, which enabled us to effectively achieve the separation of Zn²⁺ and Co²⁺. Subsequently, the extracted organic phase entered the washing stage, and the Co²⁺ in the organic phase was washed back into the liquid phase to reduce the loss of Co²⁺.

The content of Zn²⁺ in the liquid phase increased significantly after back-extraction. Under the conditions of 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition, Zn in the liquid phase was enriched, and the content of Zn reached 37.78~43.75 g·L⁻¹ and 41.52~42.98 g·L⁻¹, respectively. The removal of Zn²⁺ from the organic phase occurred easily, and the organic phase reacted with H⁺ to form bis (2,4,4-trimethylpentyl) phosphinic acid (Formula (6)) under acidic conditions, and Zn²⁺ was stripped back into the liquid phase [18]. Some studies have shown that it is easier to electrodeposit zinc from sulfate solution at 0.01M H₂SO₄ than 1M H₂SO₄ at the reverse extraction stage. However, in this study, the efficiencies of 80 g·L⁻¹ and 100 g·L⁻¹ sulfuric acid are similar, so we still need to determine the best choice for combining Zn²⁺ in the reverse extraction solution at the reverse extraction stage, and the economic effect. In contrast to Zn²⁺, Co²⁺ strips the Co²⁺ in the organic phase back into the solution during the washing stage, so the Co²⁺ content in the solution is close to 0 during the stripping stage.

(4)

2R₂P(O)ONa_(org) + Zn²⁺ ⟺ [R₂P(O)O]₂Zn_(org) + 2Na⁺_(aq)

(5)

[R₂P(O)O]₂Zn_(org) + 2H⁺_(aq) ⟺ 2R₂P(O)OH_(org) + Zn²⁺

(6)

3.1.3. Effects of Different Sulfuric Acid Addition Amounts on Recovery Rate of Each Element

The results of this study found that under the conditions of three different strong acid addition levels, the Co²⁺ and Cd²⁺ recovery rates were the highest at the addition level of 50 g·L⁻¹, at which the Co loss rate was 0.039%, 2.05 and 2.44 times that of 80 g·L⁻¹ and 100 g·L⁻¹, respectively (

Table 2. Recovery rate of Co.

	50 g·L−1	80 g·L−1	100 g·L−1
Co recovery rate/%	0.961 ± 0.034 AB	0.981 ± 0.001 A	0.984 ± 0.001 B

Note: The differences between the recovery rates of metal elements caused by different amounts of strong acids for the same metal element are indicated by capital letters (p < 0.05).

). In addition, under the condition of the 80 g·L⁻¹ strong acid supplementation level, the Co loss rate was significantly higher than that of the 100 g·L⁻¹ strong acid supplementation level (p < 0.05), which was an increase of 18.75% compared with the 100 g·L⁻¹ strong acid supplemental. However, due to the relatively low amount of loss, it is still necessary to further combine these results with an economic cost analysis to determine the optimal amount of strong acid addition. For Cd²⁺, under the condition of 50 g·L⁻¹ strong acid addition, the loss rate of Cd was higher, while under the condition of a high concentration of strong acid addition, there was almost no loss of Cd²⁺.

3.2. Application of Simulated Deep Processing to Produce Zinc Products

The back-extraction solution produced after the back-extraction stage of an extraction system contains a large amount of Zn²⁺. In order to avoid resource loss, the Zn-containing products can be further recovered through neutralization and precipitation. The composition of the back-extraction solution used in this study is shown in Table S2. The pH of the back-stripping solution was adjusted to pH 5.40 by using 20% w/w Ca(OH)₂. Due to the different amounts of H₂SO₄ added in the back-stripping stage, the amounts of Ca(OH)₂ consumed in the neutralization process were also different. The reverse extract with the 50 g·L⁻¹, 80 g·L⁻¹, and 100 g·L⁻¹ strong acid additions to regulate the pH to 5.40 consumed 1.83, 16.67, and 26.29 g of 20% w Ca(OH)₂, respectively (Table S3). During the neutralization process, it was found that precipitation was generated during the neutralization process in the reverse extract solution with 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition, and the precipitation amounts were 34.31 g and 55.46 g, respectively. Meanwhile, the precipitation amount of the reverse extract solution with 50 g·L⁻¹ strong acid addition was small, at 0.092 g (

Table 3. Characterization of precipitates resulting from neutralization of feed solutions.

Items	Units	Zn LSL 1
		50 g·L−1	80 g·L−1	100 g·L−1
Ca	%		24.7	22.95
Cd	%		<0.005	<0.005
Co	%		<0.005	<0.005
Na	%		-	-
Zn	%		1.19	1.15
Mass of precipitates	g·L−1 feed solution	0.092	34.31	55.46

¹ Zn LSL means loaded strip liquor solution.

).

During the neutralization process, it was found that precipitation was generated during the neutralization process in the reverse extract solution with 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition, and the precipitation amounts were 34.31 g and 55.46 g, respectively, while the precipitation amount of the reverse extract solution with 50 g·L⁻¹ strong acid addition was small, at 0.092 g (Table 3). This may be due to the possible reaction of SO₄²⁻ with Ca²⁺ in part of the neutralization process, resulting in CaSO₄ precipitation. The excessive Ca content in the analysis of the physico-chemical properties of the precipitation in this study also confirms this hypothesis. The contents of Zn in the neutralized back-extract were 13.36 g·L⁻¹, 39.38 g·L⁻¹, and 38.64 g·L⁻¹, respectively (Table S4).

In order to obtain zinc carbonate, a sodium carbonate solution was added to the neutralized solution so that the Zn²⁺ in the solution reacted with CO₃²⁻ to form ZnCO₃ products. During the precipitation process, the pH was controlled near 9.0, which was maintained after 6 h to ensure the complete precipitation of ZnCO₃. The results showed (Table S3) that the consumption values of Na₂CO₃ during the precipitation process of the back-extract with 50 g·L⁻¹, 80 g·L⁻¹, and 100 g·L⁻¹ strong acid added per liter were 37.04 g, 116.46 g, and 120.89 g, respectively. The precipitation amounts were 23.99 g, 87.46 g, and 78.31 g (

Table 4. Characterization of ZnCO₃ products.

Items	Units	Zn LSL 1
		50 g·L−1	80 g·L−1	100 g·L−1
Ca	%	3.96	0.69	0.8
Cd	%	<0.005	<0.005	<0.0005
Co	%	0.012	0.033	0.029
Na	%	0.18	5.28	1.98
Zn	%	53.7	46.74	50.91
Mass of precipitates	g/L feed solution	23.99	87.46	78.31
Recovery of Zn	%	108.52	100.29	96.09

¹ Zn LSL means loaded strip liquor solution.

). The analysis of the ZnCO₃ products produced showed that the reverse extract with 80 g·L⁻¹ and 100 g·L⁻¹ strong acid content contained higher SO₄²⁻, so a stronger reaction occurred with Ca²⁺ during the neutralization process, forming CaSO₄ precipitation and consuming a large amount of Ca. Therefore, the main impurity in the precipitation generated during the precipitation process was Na. In contrast to the reverse extraction liquid phase with 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition, the reverse extraction solution with 50 g·L⁻¹ strong acid addition contained relatively less SO₄²⁻ and less precipitation during the neutralization process, so the main impurity in the precipitation substance during the precipitation process was Ca.

3.3. Economic Cost and Benefit Analysis

The extraction system effect, economy, including cost and earnings; and resource loss were calculated to evaluate the effect of Zn impurity separations and Zn production. The results showed that in terms of the Zn extraction system, there was almost no difference in the back-extraction effect of Zn²⁺ under the conditions of 80 g·L⁻¹ and 100 g·L⁻¹ strong acid addition, both of which were significantly better than the condition of 50 g·L⁻¹ strong acid addition (

Table 5. Scores for extraction effect.

	Input		Weight		Output
	Zn Extraction Rate/(%)	Zn Content in Raffinate/(mg·L−1)	Zn Extraction Rate	Zn Content in Raffinate	Zn Extraction Rate	Zn Content in Raffinate
50 g·L−1	88.394	0.131	0.5	0.5	44.31	1.53
80 g·L−1	99.562	0.004	0.5	0.5	49.90	50.00
100 g·L−1	99.755	0.004	0.5	0.5	50.00	50.00

). In terms of economy, the costs and benefits were analyzed and compared, and the results showed that the costs of Zn²⁺ extraction were not much different under the conditions of the three strong acid addition quantities. In terms of benefits, Zn products generated relatively low benefits due to the low extraction efficiency and poor effect in the back-extraction stage under the condition of a strong acid addition quantity of 50 g·L⁻¹, which is significantly lower than under the 80 g·L⁻¹ and 100 g·L⁻¹ strong acid supplementation conditions (

Table 6. Scores for economy.

	Input		Weight		Output
	Extraction and Production Costs/(103 RMB)	Zn Earnings (103 RMB)	Extraction and Production Costs	Zn Earnings	Extraction and Production Costs	Zn Earnings
50 g·L−1	6856	107	0.5	0.5	49.99	13.72
80 g·L−1	6855	391	0.5	0.5	50.00	50.00
100 g·L−1	6854	350	0.5	0.5	50.00	44.77

). By comparing the costs and benefits, the 80 g·L⁻¹ strong acid extraction condition and the production of Zn products had higher economic benefits. Compared with the extraction effect and economic benefits, the loss rate and amount of Co²⁺ and Zn²⁺ were relatively small. Under the condition of 50 g·L⁻¹ strong acid, the loss rate of Co was greater. On the contrary, the addition amounts of 80 g·L⁻¹ and 100 g·L⁻¹ strong acid led to a greater loss of Zn during the precipitation process (

Table 7. Scores for resource recovery rate.

	Input		Weight		Output
	Co Loss Rate in Extraction	Zn Loss Rate in Precipitation/(%)	Co Loss Rate in Extraction	Zn Loss Rate in Precipitation	Co Loss Rate in Extraction	Zn Loss Rate in Precipitation
50 g·L−1	0.961	0.052%	0.5	0.5	20.51	50
80 g·L−1	0.981	1.022%	0.5	0.5	42.11	2.54
100 g·L−1	0.984	1.566%	0.5	0.5	50.00	1.66

). After a comprehensive comparison of the extraction effect, economic cost, and resource loss, our research team believes that the 80 g·L⁻¹ strong acid addition condition is more suitable for the actual extraction field of zinc removal due to its higher economic benefits and better extraction effect, followed by the 100 g·L⁻¹ strong acid addition condition.

4. Conclusions

In this study, the removal of Zn impurities from the leaching solution of a cobalt mine and the methods of resource recovery and utilization of Zn were explored through pilot experiments. The results showed that Cyanex272 extractants could effectively extract Zn²⁺. Under the conditions of 50 g·L⁻¹, 80 g·L⁻¹, and 100 g·L⁻¹ sulfuric acid, the extraction efficiencies of Zn were 88.394%, 99.562%, and 99.76%, respectively. The content of Zn in raffinate with 80 g·L⁻¹ and 100 g·L⁻¹ sulfuric acid was lower than 30 mg·L⁻¹, which meets the requirement of Co production. Our results showed that during the process of neutralization and precipitation, the addition of 80 g·L⁻¹ and 100 g·L⁻¹ sulfuric acid to the back-extract would consume more alkaline reagents, but at the same time, produce more Zn products. Our comprehensive analysis of extraction efficiency, cost-effectiveness, and resource loss showed that under the condition of sulfuric acid addition of 80 g·L⁻¹, the zinc product produced by the extraction of zinc had the highest yield, and the extraction effect was not much different from that under the condition of 100 g·L⁻¹ strong acid reflux, although the Co loss rate was slightly higher than that under the condition of 100 g·L⁻¹ strong acid reflux. However, because the amount of Co loss was small, the difference between them can be considered the final factor. In this study, it is concluded that Zn removal and the recovery zinc products are more sustainable under the condition of 80 g·L⁻¹ sulfuric acid addition, and this is more suitable for industrial production.