Discussion on arsenic removal in arsenic-containing copper ...

1.1 Copper and copper minerals

The average content of copper in the earth's crust is about 0.01%. Due to different ore types and mining processes, the mining industrial grade is also different, sulfide ore (pit mining) 0.4% ~ 0.5%, sulfide ore (open mining), oxidized ore 0.5%. With the progress of flotation technology, about 0.3% of the ore also has mining value. Most of the copper in the earth's crust is in the form of compounds [1].

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The world is rich in copper ore resources [2, 3], with the largest copper resource reserves in North and South America, China and its surrounding regions, Central Asia, Russia, Africa and Oceania.

According to USGS data, the global copper reserves in are 1 billion tons, and the countries with rich copper resources mainly include Chile 190 million tons, Peru 120 million tons, Australia 10 0 million tons, Russia 80 million tons, Mexico 53 million tons and the United States 50 million tons. In terms of output, the global mineral copper production in was 22 million tons, and the main copper producers are Chile 5 million tons, Peru 2.6 million tons, China 1.7 million tons, the United States 1.1 million tons, etc.

More than 70% of the world's refined copper comes from the traditional concentrate fire smelting and electric refining production [4]. The general process of copper ore mining includes exploration, mining, crushing, mineral processing, etc. The flotation copper concentrate goes through a step of copper smelting process to obtain refined copper products. The direct smelting method is highly efficient, but the defect is that there are difficulties in handling some copper concentrates containing the following elements (As, Sb and Bi). Due to the economic punishment of smelters, the copper concentrate with high harmful impurities has no market prospect.

Secondly, for low-grade complex ore, copper oxide ore and copper-containing waste ore, we generly adopt heap leaching, trough leaching or in-situ leaching method, with large treatment scale and low cost, has been applied in many mines.

1.2 Arsenic and arsenic minerals

1.2.1 Overview

Arsenic is a non-metallic element, the element symbol As, atomic number 33, is the VA group non-metallic element. The content of arsenic in the earth's crust is about 2 ~ 5 mg /kg, and the toxicity of arsenic is closely related to its chemical properties and valence states. Arsenic has four valence states such as As3−, As0, As3+ and As5+. Except for elemental arsenic is not toxic, other forms of arsenic are toxic, trivalent arsenic is the most common and most toxic, and pentavalent arsenic is the most stable [5].

Arsenic exists mainly in the form of arsenide, sulfide, oxides, arsenate and arsenite. Most of these minerals are closely related to metals such as iron, copper, cobalt, nickel, cadmium, lead, silver and gold [6]. More than 200 kinds of arsenic-containing minerals have been identified in nature, the most abundant in the form of sulfide. The common arsenic minerals in the ore are arsenopyrite, tennantite, realgar, orpiment, enargite, scorodite. Among them, realgar and orpiment can be independently enriched for mineralization, and occasionally arsenate deposits have been found. Most arsenic is mostly produced by co-associated element minerals, which is difficult to recycle, and brings major environmental problems [7]. The arsenic minerals related to copper minerals mainly include: arsenopyrite, cobaltine, enargite, tennantite Table 1.

The common arsenic-containing minerals in copper deposits are arsenopyrite (FeAsS), followed by tennantite (Cu12As4S13) and enargite (Cu3AsS4), a small amount of scorodite, arsenolite, etc., are also produced in pyrite, marcasite, cerusite and dolomite by the isomorphism. Previous studies show that in [8] arsenic and metal association in copper deposits are not correlation by As and Cu, and As and Sb have good negative correlation.

Arsenopyrite is a highly toxic inorganic pollutant, often associated with sulfide ore, and has similar surface chemical properties and floatability with chalcopyrite, chalcocite and covelline. Therefore, using conventional flotation agent and process flotation of arsenic-containing sulfide copper ore, arsenopyrite is easily selected into copper concentrate, so that the arsenic content in copper concentrate seriously exceeds the standard. High arsenic copper concentrate not only increases the smelting cost, but also seriously affects the electrical conductivity and ductility of copper products. Inorganic arsenate and arsenic-containing organics produced by the oxidation of arsenic-containing minerals can pollute the environment and affect human health through large geological circulation and small biological circulation. Therefore, it is necessary to separate arsenic from copper concentrate prior to smelting [9].

1.2.2 Hazards of arsenic-containing copper minerals

Increasing content of impurities (such as Se, As, Te) due to decreasing copper grade. As is one of the harmful elements in copper ore, ore containing arsenic has seriously affected the world copper production, arsenic-containing copper concentrate into smelting, cause volatile As2O3 or As4O6 into the air, a serious threat to human health, such as endemic arsenic poisoning, malignant tumor, liver disease, lung disease and so on [10]. Arsenide removal in copper concentrate has become a major focus and challenge for many companies and researchers.

The arsenic removal process mainly includes three aspects: mineral processing, hydrometallurgical process, and pyrometallurgical process.

Many studies have been conducted on the separation process of copper and arsenic in copper ore worldwide [7]. The main sources of arsenic in copper concentrate are as follows: (1) arsenic exists in copper minerals in the form of isomorphism, which cannot be separated by flotation. Due to the low content, the arsenic content of copper concentrate is usually not greatly affected. (2) enargite and tennantite have a great influence on the quality of copper concentrate, which are not easy to be separated from other copper minerals. (3) the mixing of arsenic-containing minerals (mainly arsenopyrite).

Enargite (Cu3AsS4) is the most common in copper deposits. Because its surface properties and washability are similar to the associated copper sulfide (covelline, chalcocite, chalcopyrite, etc.), in the conventional flotation process, it inevitably enters the concentrate, affecting the quality of copper concentrate [11]. To solve the problems of arsenic-containing copper ore, we should consider the following two aspects: that is, change the surface properties of enargite and change the washability of copper mineral, or realize the separation of arsenic and copper in the final copper concentrate by using the wet leaching process.

2.1 Mineral processing

Under the usual mixed flotation conditions, the surface characteristics and flotation properties of enargite and chalcopyrite are almost the same by xanthate as a collector, and the conventional inhibitors such as lime, cyanide, sulfide and potassium permanganate cannot effectively realize the separation of enargite and copper sulfide ore [12,13,14,15].

Existing separation methods are based on the selective oxidation of arsenic-containing minerals. The electrochemical properties of different copper sulfide minerals are differ. Low and moderate degree of oxidation favors the adsorption of xanthate collectors, but at high oxidation, the mineral surface is easy to produce oxidation products, thus forming a layer of physical barrier, further hindering the adsorption of on the mineral surface [16].

Tajadod and Yen [14] reported in the literature that the arsenic content of in copper concentrate was reduced by MAA (Magnesium ammonia mixture: 0.5 mol of magnesium chloride hexahydrate, 2.0 mol of ammonium chloride and 1.5 mol of ammonium hydroxide). Castro SH and Honores S studied the surface properties and floability of enargite by measuring the Zeta potential, electrostatic potential and Harry Monte tube test of the enargite. It is considered that enargite is a sulfonyl mineral susceptible to flotation by xanthate [17].

Gong [18] in order to find out the optimal flotation potential of enargite, studied the potential and wetting property of enargite in pentyl methyl xanthate (PAX), and studied the differences in oxidation properties and flotation characteristics of enargite and chalcopyrite in mineral surface. The test of separating enargite from chalcopyrite under different pulp potentials was conducted to explore the feasibility of removing enargite from chalcopyrite mixed concentrate. The test results showed that chalcopyrite began to oxidize rapidly at a much lower potential than enargite, which showed good flotation at a potential above + 0.2 V, while chalcopyrite was completely inhibited. It can be seen that the enargite can be successfully removed from chalcopyrite concentrate by controlling the slurry potential [12].

Fornasiero, et al. [15] studied the selective oxidation-dissolution separation method of enargite, tennantite and copper sulfide minerals without arsenic, and showed that it was difficult to separate copper sulfide minerals (chalcocite, covelline and chalcopyrite) from the enargite and tennantite under ordinary oxidation conditions. After selective oxidation with H2O2, selective removal of the surface oxidation products followed by addition of EDTA at pH 5.0 or with H2O2 at pH 11.0. The XPS analysis shows that the good separation of these minerals with H2O2 is due to the stronger oxidation of arsenic-containing minerals than arsenic-free minerals. At pH 5.0, the oxide of arsenic is more stable than that of copper, but in the presence of pH 11.0 and EDTA, the oxide of arsenic is more soluble than that of copper.

Graham Long, et al. [7] reviewed arsenic removal from arsenic-containing copper concentrates through descriptions of arsenic sulfide mineralogy, removal options, chemical depression, collector type, pulp potential and oxidizing conditions, methods of potential control, and four flow sheets to separate copper-arsenic minerals from copper minerals. It is pointed out that dithiophosphate collectors provide lower copper recoveries but better selectivities than xanthates, so they may be preferred for a given mineral. Differential flotation with control of pulp potential represents an option for reducing the arsenic content of copper concentrates.

2.2 Hydrometallurgical process

2.2.1 Leaching of arsenic-containing copper minerals

Common leaching methods of arsenic-containing copper minerals include biological leaching, acid leaching, thermal pressure leaching, alkaline leaching, ammonia leaching and oxidation leaching.

1. (1)

   Bacterial leaching

   During the biological leaching of enargite, a metastable secondary mineral will be formed [19]. At the initial stage, the dissolution concentration of arsenic is low and inhibited; In the second stage, the dissolution rate is gradually released and accelerated. Professor Ruan Renman, Professor Wen Jiankang, et al. [20] carried out microbial wet leaching experiments for an arsenic-containing floating concentrate in China. Through the selection and domestication of submerged bacteria, the excellent bacteria could efficiently leach copper in a copper concentrate, and the copper leaching rate reached 85.52%.

   José Antonio Díaz et al. [21] summarized the research progress of biological leaching of enargite, and concluded that the leaching mechanism of clarite is as follows:

   $${\text{Cu}}_{{3}} {\text{AsS}}_{{4}} + {\text{ 9Fe}}^{{{3} + }} + {\text{ 2H}}_{{2}} {\text{O }} \to {\text{ AsO}}^{{{2} - }} + {\text{ 3Cu}}^{{{2} + }} + {\text{ 9Fe}}^{{{2} + }} + {\text{ 4S}}^{0} + {\text{ 4H}}^{ + }$$$${\text{Cu}}_{{3}} {\text{AsS}}_{{4}} + { 2}.{\text{25O}}_{{2}} + {\text{ 5H}}^{ + } \to {\text{ AsO}}^{{{2} - }} + {\text{ 3Cu}}^{{{2} + }} + {\text{ 4S}}^{0} + { 2}.{\text{5H}}_{{2}} {\text{O}}$$
2. (2)

   Acid leaching

   Sadegh Safarzadeh et al. [22] reported that the dissolution rate of copper sulfide minerals in acid iron sulfate solution medium is different. At 35 ℃ to 50 ℃, the dissolution order of copper sulfide minerals is from fast to slow, in sequence chalcocite, bornite, covelline, chalcopyrite and enargite. It can be seen that in iron sulfate solution medium, enargite is the most difficult to leach. The test of sulfuric acid roasting-water leaching of enargite was carried out. At 200 °C for 7 h, 90% of copper and 61% of arsenic could be leached and transferred to the solution, and less than 1% of arsenic was transferred into the gas.

   Hernández et al. [23] investigated the behavior of enargite and chalcopyrite in ClO−/OH− leaching with the aim of establishing favorable conditions for the selective leaching of these minerals. Given the rise in enargite content in Chilean copper concentrates and the challenges associated with treating this sulfarsenide using traditional pyrometallurgical methods, a study was conducted on a selective arsenic cleaning process. The research included a comparative evaluation of the behavior of pure samples of enargite and chalcopyrite in a ClO−/OH− medium. The conditions resulting in the highest selectivity were then applied to both a synthetic concentrate and an industrial concentrate. The findings indicated that the ClO−/OH− medium is an effective method for removing arsenic from copper concentrates containing enargite. In this comparative study, natural crystalline samples of enargite and chalcopyrite were used, which were reduced in size by grinding and were classified under 100 µm. The experimental procedure is: leaching of pure compounds, synthetic concentrate leaching in a ClO−/OH−medium and industrial concentrate leaching. The impact of ClO− concentration, pH, temperature, and particle size was quantified. Both chalcopyrite and enargite react in the evaluated media, and the highest selectivity between these mineral species is achieved in short times ( < 30 min), with low ClO− concentrations (0.1–0.3 M), pH of 12–12.5, temperature of 25 °C, and fine particle sizes (less than 15 µm). The selectivity of enargite can be up to five times greater than that of chalcopyrite. The leaching of the concentrate or mineral mixtures in a sodium hypochlorite media containing enargite dissolves arsenic into the solution, while copper and iron remain in the solid phase as CuO and Fe(OH)3, respectively. Tests conducted with a mineral mixture or synthetic concentrates yield similar results in terms of selectivity, even when increasing the solid/liquid ratio. Acopper concentrate with low arsenic content and a stable precipitated arsenic compound was achieved, enabling safe removal. The process should capitalize on the rapid leaching of enargite at low concentrations of 0.13–0.2 M ClO− at moderately low temperatures of 25–40 °C, and a pH of 11.5–12.0. These conditions demonstrate a low leaching rate for chalcopyrite and bornite species compared to sulfided Cu-As species.

   In order to speed up the leaching speed of enargite in acid medium, Hajime Miki et al. [24] found and studied the catalysis of the silver in the process of arsenic copper leaching. The results show that the leaching system add silver ions or silver sulfide greatly improve the dissolution rate of enargite, anion speed up the reduction of enargite to chalcocite. The thermodynamic calculation results show that adding silver and control the potential, can improve the dissolution rate of copper from the enargite.

3. (3)

   Hot pressure leaching

   Padilla et al. [25] studied a new method to produce copper by using a mixture of chalcopyrite and enargite as raw material, through vulcanization (adding sulfur heating with concentrate) and then pressurized leaching of copper. The mixed concentrate is mixed with elemental sulfur, at 350 °C to 400 °C, chalcopyrite produces CuS and FeS2, while the enargite not react, After that, the pressurized leaching under sulfuric acid-oxygen system has the greatest effect on copper leaching from 125 to 180 °C. The effect of oxygen within 507 kPa to  kPa is little, and the leaching rate of copper can reach 90% within 30 min. However, the copper leaching rate of enargite concentrate without vulcanized treatment is 10%.

4. (4)

   Alkaline leaching

   Curreli et al. [26] reported the study of arsenic removal in gold-containing enargite concentrate under alkaline sodium sulfide system. The results showed that at 110 ℃ and normal pressure conditions, arsenic could be leached, and part of gold was also leached. William Tongamp et al. [27] conducted leaching test on copper ore and copper concentrate under alkaline conditions. At 80℃, the raw ore leached for 1 ~ 3 h, and arsenic was reduced from 1 ~ 4% to less than 0.5%.

5. (5)

   Ammonia leaching

   Gajam et al. [28] conducted detailed study on the dissolution dynamics of natural enargite in ammonia solution. Different from other copper sulfide minerals, the dissolution rate of copper in enargite is very slow, only 60% copper in the optimal pH10 for 24 h. The dissolution rate increases with the increase of temperature and oxygen pressure, and the decrease of particle size; high temperature and high pressure are conducive to the dissolution speed.

6. (6)

   Oxidative leaching

   Pierfranco Lattanzi, Stefania Da Pelo et al. [29, 30]summarized the oxidation rate of enargite in different environments: ① the oxidation of enargite in air is very slow process; ② mechanical activation can greatly improve the reactivity of enargite; ③ under acidic to neutral conditions, oxidation and dissolution of enargite are slow; ④, the reactivity of enargite is significantly increased under alkaline conditions; ⑤ the electrochemical oxidation of enargite is usually low current density, and the oxidation process is slow.

   Ivan Mihajlovic et al. [31] studied the oxidation of copper concentrate under the conditions of alkaline and using sodium hypochlorite oxidation, which can make the enargite in copper concentrate oxidized to CuO and dissolve arsenic to form AsO43−ions, thus realizing the separation of copper and arsenic.

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   Vinals et al. [32] studied the oxidation of copper concentrate under the conditions of alkaline and using sodium hypochlorite oxidation, at 60 ℃, [OH−] greater than 0.05 M, which can transform the oxidation of enargite into CuO, thus realizing the separation of copper and arsenic.

2.2.2 Combination process of "flotation + wet leaching"

Zhihua Huang [1] using "selective oxidation-flotation separation-high arsenic copper concentrate selective leaching arsenic-arsenic ion precipitation curing" process combination, generated the process of the system, the raw copper concentrate arsenic content of 1.02% to the comprehensive arsenic grade 0.29% (calculation) of copper concentrate (part of high arsenic middles enter to the high arsenic copper concentrate), leaching arsenic in the form of magnesium arsenate/magnesium ammonium arsenate precipitation curing. The process achieved the successful separation of copper and arsenic, and the precipitation and curing of the isolated arsenic, reduced the impact of arsenic on the environment, and achieved the expected goal. The process flow is simple and environmentally friendly, and front-end control, which it is a worthy process line for selective arsenic removal of arsenic-containing copper ore.

2.3 Pyrometallurgical processes

2.3.1 Pyrometallurgical processes and disposal or burial of arsenic-containing minerals

Taylor, Putra. [33] reviewed the various pyrometallurgical methods for treating copper concentrates that contain appreciable arsenic. Various aspects of the engineering of these treatment methods are discussed. The methods discussed include: complete oxidation with arsenic fixation; selective volatilization of arsenic; acid baking, soda ash roasting, and others methods. Methods for the ultimate disposal, or marketing, of the arsenic are discussed.

Smelters generally accept copper concentrates with low arsenic levels (less than 0.2% to 0.5%) due to environmental and product purity concerns. Treating the waste stream from high arsenic copper concentrates requires significant investment. The separation and collection of arsenic as arsenic trioxide is problematic, primarily due to its toxicity and lack of market value.

Pyrometallurgical processing has several concerns: strict environmental regulations on arsenic release, complexity of gas/dust capture, separation facilities and the stabilization of the final arsenic compound. The application in the industry to treat high arsenic concentrate (enargite) has occurred at CENA in Belgium. It employed reductive roasting-smelting process for the treatment of enargite concentrates. Despite the perceived shortcomings for roasting to selectively volatilize arsenic, the process could an option under specific conditions, provided that arsenic precipitation is possible. Roasting is the only process that has been operated commercially for enargite concentrates; therefore the level of project risk is lower, i.e. based on past commercial experience. Another advantage of roasting is it is less sensitive to feed mineralogy than hydrometallurgical options and arsenic disposal as a compact crystalline compound with low solubility would appear possible.

Adham et al. [34] have studied the thermal method of arsenic removal from copper concentrate, including direct heating of concentrate and indirect heating of concentrate. Indirect drying of copper concentrate is commercially proven and can be used to minimize the heating requirement in the roaster. Capital cost of such an indirect calciner can be less than a typical fluid bed roaster, but the operating cost of fuel is higher, if no excess or waste energy is available. Direct heating of copper concentrate includes the roasting of multi-hearth, rotary kiln and fluid bed furnaces.The arsenic removal from off-gas, wet arsenic stabilization and vitrification drying stable arsenic are discussed. Arsenic removal from off-gas includes gas cooling, solids removal and ultimate treatment, desulphurization. Directly heated reducing roast for arsenic removal has been commercially proven, with the most recent applications mainly using fluid bed reactors due to their advantages in temperature and emission controls, relative to rotary kilns and multi-hearth roasters. Indirectly heated roasting for arsenic removal is potentially attractive. Ferrihydrite and scorodite precipitation processes have been applied effectively. The vitrification process, can provide promising cost-effective avenues to unlock values of the abundant arsenic-bearing concentrates.

2.3.2 Pyrometallurgical processes for the pretreatment of concentrates with high arsenic content by means of fluidized bed reactor roasting.

Henao et al. [35] describes the experimental results of removing arsenic from the dust collected in electrostatic precipitators of a fluidized bed roasting furnace (RP dust). The fluidized bed roasting process generates 600 kilotons of copper concentrate per year with 3–6 wt% of concentration of arsenic, producing a roasted product with a low content of arsenic below 0.3 wt%. The process generates 27 kilotons of RP dust per year with a concentration of arsenic of the order of 5 wt% and copper concentration of around 20 wt%. Subsequently, the dust collected in the electrostatic precipitators is treated by hydrometallurgical methods allowing the recovery of copper, and the disposition of arsenic as scorodite. This work proposes to use a pyrometallurgy process to the volatilization of arsenic from RP dust. The obtained material can be recirculated in copper smelting furnaces allowing the recovery of valuable metals. The set of experiments carried out in the roasting of the mixture of copper concentrate/RP dust and sulfur/RP dust used different ratios of mixtures, temperatures and roasting times. By different techniques, the characterization of the RP dust determined its size distribution, morphology, and chemical and mineralogical composition. RP dust is a composite material of small particles (< 5 μm) in 50 μm agglomerates, mostly amorphous, with a complex chemical composition of sulfoxides. The results of the roasting experiments indicated that for a 75/25 weight ratio of the mixture of the copper concentrate/PR dust under 700 ℃, 15 min of roasting time with injection of air, the volatilization of arsenic reached 96% by weight. The arsenic concentration after the roasting process is less than 0.3% by weight. For a 5/95 mixture of sulfur/RP dust, at 650 ℃, the volatilization of arsenic reached a promissory result of 67%. Even that this study was carried out for a particular operation, the results have the potential to be extended to dust produced in the roasting of concentrates of nickel, lead–zinc, and gold.

Arsenic is often associated with copper, nickel and gold in sulphide deposits. Adham et al. [36] describes a two-stage high-temperature fluid bed reactor for the removal and fixation of As from ores and concentrates. The first (bottom) stage of reactor removes the As through a neutral roast, by volatilization as mostly sulphide species. The second (top) stage of reactor captures the As from the gas phase through oxidative fixation, as a stable iron arsenate, by reaction with an appropriate iron source (sulphide or oxide). The second stage is placed on top of the first, so that their combined function can be achieved in a simple two-stage fluid bed arrangement, with the bottom stage off-gas providing the top stage fluidization.

Copper and arsenic separation has always been a difficult problem in the field of mineral processing, so it is particularly important to develop an economically feasible and environmentally friendly arsenic removal process.

3.1 Flotation agent

3.1.1 Selective collector

Fuxing et al. [37] used cupric sulfate as an activator, with the combination of butyl xanthate + highly selective auxiliary collector PZO (concurrently foaming agent) as copper ore collector, experimental study on flotation arsenic removal in a copper arsenic mine in Yunnan with 3.61% arsenic content. Linhai et al. [38] used the new 303 collector to conduct arsenic removal test study on an arsenic tin mine; Yan [39] uses OL-IIA independently developed by Central South University as a copper ore collector, uses lime + CTP to suppress arsenopyrite, experimental study on optimizing the mineral processing process. They all achieved a good arsenic removal effect.

3.1.2 High-efficiency inhibitors

Compared with arsenopyrite, copper sulfide ore is naturally floating and easy to be activated by activators, while arsenopyrite is relatively easy to act with inhibitors. Therefore, under the conventional flotation system of copper sulfide ore collector, the development of efficient aesenopyrite inhibitor can also realize the separation of copper and arsenic sulfide minerals.

3.1.2.1 Lime-based combined inhibitors

The use of single lime as arsenopyrite inhibitor, arsenic removal effect is not good, has been gradually eliminated. Lime was combined with bleaching powder, hypochlorite, ammonium salt and sodium sulfite into a lime-based arsenopyrite inhibitor, with significant arsenic inhibition effect. On the other hand, lime is simple to make, cheap, and has no pollution to the environment. Therefore, the combination of inhibitors mainly based on lime has been widely studied and used.

Hemiao et al. [40] conducted flotation and arsenic removal test on a high-arsenic copper mine; Jun et al. [41] research on arsenic reduction of a low-grade copper sulfur, arsenic and tin polymetallic copper concentrate; Ying et al. [42] research on arsenic reduction of a copper concentrate; Jingyu et al. [43] research on arsenic reduction recovery process of a associated copper and cobalt mine in Xinjiang, and they all used a combined inhibitor based on lime, and achieved good separation effect of copper and arsenic.

3.1.2.2 Combined inhibitors of sulfur-oxygen compounds

The sulfur oxides that inhibit the arsenopyrite mainly include sulfate and thiosulfate, such as sodium sulfide, sodium pentasulphate, sodium thiosulfate, sodium sulfite and nox reagent (P2S5 + NaOH).

Wei [44] used the combination of bleaching powder + sodium sulphite as arsenpyrite inhibitor for arsenic reduction test of a high arsenic sulfide copper mine; Guofu et al. [45] for the study on the arsenic reduction process of a yunxi copper concentrate; Li et al. [46] on the separation of copper and arsenic in a copper sulfide mine in Yunnan province, which they all used the combination inhibitor of oxygen sulfur-containing compounds, and achieved good separation effect of copper and arsenic.

3.1.2.3 Organic inhibitors

According to the relative molecular weight of organic inhibitors, they can be divided into large molecule inhibitors and small molecule inhibitors. Large molecule inhibitors have strong inhibitory ability, while small molecules show the characteristics of high selectivity. Dextrins, tannic acid, sodium lignosulfonate, steptic acid salt, fulvic acid and polyacrylamide are common organic inhibitors in copper and arsenic separation.

When separating arsenopyrite from copper sulfide ore, poor inhibition by using a single organic inhibitor, and the combination of several organic agents or organic and inorganic agents is often used as arsenopyrite inhibitors; new inhibitors can also be developed by changing the molecular structure of the agent. In recent years, there are few studies on the organic inhibitors of arsenopyrite. In the research and development of arsenopyrite inhibitors, we should not only consider the inhibitory effect of arsenopyrite, but also pay attention to the economic feasibility and environmental friendliness. Organic inhibitors have good inhibitory effect, low price and non-toxic, which is expected to achieve good separation of copper and arsenic.

3.2 Flotation process

3.2.1 Electrochemical regulation

The principle of electrochemical regulation is to change the redox potential by applying an electric field or the addition of oxidant to promote the arsenopyrite surface oxidation.

3.2.2 Applied electric field

On the basis of electrochemical kinetics, López Valdivieso et al. [46,47,48] studied the mechanism of inhibiting flotation of the oxidation products on the surface of the arsenopyrite and the adsorption of the arsenopyrite and the salinate, and found that the redox potential of the pulp could be controlled by applying the electric field and so that the arsenopyrite removed.

Mikhlin et al. [49] investigated the effect of electrochemical regulation on the floatability of arsenopyrite. The oxidation of arsenopyrite in air can occur when the potential is 0.6 V, and the external electric field can promote the oxidation of arsenopyrite. XPS results showed that 22.29% of arsenic in alkaline mineral slurry was in the oxidized state, and the hydroxide of hydrophilic iron oxidized on the arsenopyrite surface accounted for 66.74% of its surface area. The existence of hydrophilic film can block the interaction between the arsenopyrite and the collector, reduce the adsorption of the arsenopyrite to the collector, using a small amount of collector can achieve good flotation effect and improve the recovery of useful minerals.

3.2.3 Add the oxidant

Bleach powder, potassium permanganate, manganese dioxide, ammonium persulfate, hydrogen peroxide, potassium hypochlorite and potassium dichromate are the common oxidizers to regulate the pulp.

Mark et al. [50] showed that the bleaching powder or hydrogen peroxide can inhibit the arsenopyrite, because in the pH value = 8 ~ 10 alkaline medium, the arsenopyrite surface is easily oxidized to the hydrophilic iron hydroxide, the layer of hydrophilic film wrapped on the surface of the arsenopyrite can reduce the floatability of the arsenopyrite. In addition, potassium permanganate is also commonly used as arsenopyrite inhibitor. Potassium permanganate is the main oxidant in industrial production, which can oxidize the surface of arsenopyrite to form H2AsO3, reduce potassium permanganate to manganese dioxide under alkaline conditions, which can form hydrophilic Mn (OH) film with H2AsO3, reduce the floatability of arsenopyrite, and is conducive to flotation and removal of arsenic.

Guangming et al. [51] uses the mixture of sodium carbonate and strong oxidation of bleaching powder as the arsenopyrite inhibitor, which can strengthen the inhibition of the arsenopyrite. Bleaching powder can selectively oxidize the surface of arsenopyrite to generate hydrophilic SO42−and AsO43−. Sodium carbonate selectively reduces the adsorption amount of collector on the surface of arsenopyrite, and obtains sulfur concentrate with low arsenic.

In addition to adding external electric field and adding oxidant, it can also be oxidized by filling oxygen, extending stirring time, heating pulp and other methods. Strengthening the oxidation of arsenopyrite is conducive to the separation of copper and arsenic. The external electric field needs to install the electrode in the flotation tank, and the electrode cannot uniformly regulate the slurry Eh, the arsenic removal effect is not good, and it is difficult to be used in industrial production. The addition of oxidant to adjust the pulp redox potential has the advantages of simple operation, low cost and uniform regulation, so the addition of oxidant can better adapt to the industrial development.

3.2.4 Regrinding of coarse concentrate

The regrinding of coarse concentrate can not only fully dissociate copper sulfide minerals and arsenopyrite, but also scrub the minerals surface through grinding and expose the fresh surface, which is conducive to the pharmaceutical action. At the same time, the particle size of grinding ore should be controlled. Over-crushing will not only increase energy consumption, but also increase the difficulty of copper and arsenic separation.

Xiaohua [52] research on arsenic removal of arsenic-containing copper minerals by grinding fineness shows that when the grinding fineness increases from − 0.074 mm 80% to 95%, the arsenic content in the copper concentrate decreases from 0.450 to 0.201%; if the crude concentrate is added three times concentrations, the arsenic content in the copper concentrate decreases from 0.541 to 0.193%.

A high arsenic tin stone copper sulfide mine is mainly fine particles, and dense symbiosis with arsenopyrite and pyrite, Chengxiu et al. [53] adopts the process of "coarse grinding-mixed flotation-crude concentrate regrinding-copper arsenic separation", and obtained the grades of copper and arsenic in the copper concentrate were 23.58% and 0.19% respectively, with copper recovery as high as 91.17% and yield of 9.38%.

Xiangwen et al. [54] studied the effect of coarse concentrate regrinding on the separation of arsenopyrite, raw ore contains 2.03% arsenic, mainly by arsenopyrite. When the grinding fineness is − 0.074 mm 65%, inhibited arsenopyrite by the novel composite EM-421,and using butyl xanthate as a collector, the copper grade of copper concentrate products obtained by flotation is 7.61%, arsenic-containing 0.77%; if the coarse concentrate grinding is up to − 0.074 mm 95%, adjust the pH value = 10 to 11, ceteris paribus, the copper grade of copper concentrate was significantly increased by 16.17 percentage points, the recovery was 87.69%, the arsenic content was only 0.14%.

3.3 Separation arsenopyrite (FeAsS) from chalcopyrite

3.3.1 Study on floatability of arsenopyrite

Arsenopyrite (FeAsS) is the most important arsenic-containing mineral in copper sulfide ore. It is tin white with a density of 5.9 ~ 6.3 g/cm3. It is a monoclinic or trioblique crystal system, and its theoretical arsenic content is as high as 46.01%. In the strong acid medium with pH value = 3 ~ 4, the redox reaction occurs on the surface of the arsenopyrite to form elemental S, Fe2+, Fe3+, Fe (OH)+ and Fe (OH) 2+, which can strengthen the adsorption of the collector to the arsenopyrite, so the arsenopyrite has good floatability in the strong acid medium. Under neutral or strong alkaline conditions, the oxidant can promote the formation of hydrophilic Fe (AsO4)·H2O film on the surface of the arsenopyrite, prevent or reduce the adsorption of the collector to the arsenopyrite, reduce its floatability, and effectively inhibit the arsenopyrite. And the floatability of the arsenopyrite in pH = 6 ~ 11decreases with the increase of pH, and when pH > 11, the arsenopyrite is completely inhibited. Therefore, the addition of oxidant under medium and alkaline conditions can effectively suppress the arsenopyrite [9, 17].

3.3.2 Study on the separation of arsenopyrite and chalcopyrite

In order to reduce the arsenic content of copper concentrate due to the ores with higher arsenic content caused by the mixing of arsenopyrite into copper concentrate. The separation of the arsenopyrite from the chalcopyrite must be studied.

To make the effective separation of arsenopyrite and chalcopyrite, we must fully understand the flotability of them and increase the flotability difference between them. In practice, more to improve the PH and add sodium sulfite, activated carbon, water glass, soft manganese ore and other inhibitors of the arsenopyrite, so that the arsenopyrite and chalcopyrite effective separation.Another method is to carry out strong air mixing, while removing the drug membrane in the mixed flotation operation, the arsenic was inhibited. However, because there have been agents adsorbed on the surface of mineral particles, so it is difficult to separate [55].

To effectively separate arsenopyrite and chalcopyrite, methods such as selective copper collection, the use of lime and sodium sulphite to inhibit arsenopyrite, increasing the amount of fine processing, and increasing the grinding of coarse copper concentrate to further address copper and arsenic intergrowth and scrub the ore surface can be used to effectively separate arsenopyrite and chalcopyrite [56].

3.4 Separation of tennantite from Copper sulfide minerals

3.4.1 Separation of tennantite from chalcopyrite and chalcocite

Some copper minerals have high arsenic content, which complicates efforts to reduce the arsenic content of copper concentrate: this is likely to cause a strong decrease in the copper recovery rate and is not conducive to the comprehensive utilization of mineral resources if arsenic removal is the only objective; moreover, copper concentrate with high arsenic content cannot meet smelting requirements without undergoing arsenic removal. It is impossible to remove arsenic with the current mechanical mineral processing methods without affecting the copper recovery rate for this kind of copper mineral.

The most reasonable way to solve the problem of high arsenic content in such ore is the so-called "two products" solution: to effectively recover the copper mineral and to meet the requirements of copper fire-smelting in the presence of arsenic, tennantite is separated from chalcopyrite and chalcocite by dressing methods to obtain a low-arsenic copper concentrate that meets the requirements for smelting and a high-arsenic copper concentrate dominated by tennantite. The problems associated with high-arsenic copper concentrate and the new smelting and beneficiation methods have been studied. This approach is both economically and technically feasible [57].

There is a difference in the flotability of tennantite originating from chalcopyrite and chalcocite, and it is not easy to separate these minerals effectively. At present, there are very few studies on the separation of tennantite and other minerals. Research on the flotability of tennantite: Tennantite, like chalcopyrite, has the best flotability when the pH is increased from 11 to 11.6 in soda media. When the amount of free CaO is increased to more than 400 g/m3, the amount of tennantite separated is low, while the recovery of chalcopyrite is not affected. The flotability of tennantite can be improved, and the oxidized surface of the mineral can be vulcanized when the amount of Na2S added reaches 100 g/t. Tennantite separation can be inhibited completely when the amount of Na2S is increased [55].

3.4.2 Separation of tennantite and secondary copper minerals such as chalcocite

For copper minerals with high arsenic content, in addition to the main copper mineral chalcopyrite, can generate secondary copper minerals due to secondary enrichment, and these secondary copper minerals mainly consist of chalcocite. To treat this kind of ore, measures can be taken to separate tennantite; secondary copper minerals with better flotability can be preferentially selected to realize the separation of secondary copper minerals and tennantite; and two copper concentrates with high and low arsenic content can be obtained. Sometimes, the flotability of tennantite significantly improves due to the influence of soluble copper salts, making it difficult to separate the secondary copper minerals from tennantite. Under these conditions, lime and sulphurous acid can be used to separate copper and sulphur. A coarse copper concentrate is obtained first; then, the coarse concentrate is regenerated, potassium ferrocyanide is used to separate the secondary copper minerals, and tennantite is selected by the sulfate ammonium ester so that a better separation can be achieved [56, 57].

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