3.2.1. The VM goethite process

The iron is reduced from the ferric to the ferrous state with zinc sulphide (zinc concentrate) and subsequently re-oxidised with air in the precipitation tanks to maintain low concentrations of ferric iron.(2)2Fe3+ + ZnS ↔ 2Fe2+ + Zn2+ +S0(3)2FeSO4 + ½O2 + 3H2O ↔ 2FeO·OH + 2H2SO4

To compensate the generated acid, calcine is continuously added to achieve the preferred pH values between 3.5 and 4.0. The required precipitation temperature is around 95 °C (Sinclair, 2005).

3.2.2. The para-goethite process

Hereby the low ferric iron concentration which is necessary for the formation of goethite is achieved by controlling the solution flow to the precipitation tanks. In this case it is more difficult to maintain iron concentrations below 2 g/l. This results in the formation of an iron precipitate which is not pure goethite but contains hydrated basic sulphate. The para-goethite process involves the continuous addition of concentrated acidic Fe3+ liquor (85 °C, pH~1) into a large reactor to promote dilution. Precipitation is carried out by neutralization with calcine to maintain the pH at 3.5 (Loan et al., 2006).(4)Fe2(SO4)3 + 4 H2O ↔ 2FeO·OH + 3H2SO4

Although the para-goethite process bears advantages like the elimination of the iron reduction state there are significant disadvantages like increased amounts of produced iron residues and zinc losses (Cardarelli, 2018; Dyer et al., 2012; Loan et al., 2006; Sinclair, 2005).

4.1.1. Thermal decomposition of jarosite

It was found that in contrast to synthetic jarosite, natural jarosite shows lower decomposition temperatures. This is attributed to a reduced freezing point induced by impurities in the natural jarosite (Frost et al., 2005). For instance, jarosite with silver substituting the monovalent cation (AgFe3(SO4)2(OH)6) can be formed during the precipitation in small quantities related to the very low silver contents. (Dutrizac and Jambor, 2000).

Table 3 summarizes findings from experiments with different kinds of jarosite. Depending on heating rate, actual chemical composition and analytical setup, the results varied in different research projects. However, comparison of further literature showed that the temperatures, mass losses etc. fit the same ranges. A splitting of the OH-groups was investigated for sodium-jarosite at temperatures higher than 420 °C followed by SO3 at 600–800 °C with a total theoretical mass loss of 35.9 wt-% (Steinlechner and Antrekowitsch, 2018). Ammonium-jarosite was found to decompose under release of H2O and NH3 between 200 and 300 °C, the liberation of the sulphate groups took place at temperature ranges from 300 to 550 °C (Kerolli-Mustafa et al., 2016; Kunda and Veltman, 1979) found that the two reactions take place at 360 °C, respectively 600 °C. Spratt et al. detected the first step of decomposition of hydronium-jarosite at 256 °C and the final conversion to hematite at 500–650 °C, for ammonium jarosite the liberation of NH3 and H2O at 300–400 °C and the decay to hematite in a range of 500–600 °C (Spratt et al., 2014).

4.1.2. Thermal decomposition of goethite

The thermal decomposition of goethite consists of one single step. When studying the influence of the heating rate on the reaction to hematite under the liberation of gaseous water it was found, that, with increasing heating rate, the temperatures that mark the start and the end of the decomposition rise respectively (Ammasi, 2020). Taking heating rates from 5 to 50 °C/min into account, the temperature range for the decomposition is 210–350 °C. The beginning of the conversion was detected at slightly over 200 °C, at 400 °C the degree of decomposition was 96% (Naono et al., 1987).2FeOOH ↔ Fe2O3 + H2O

4.1.3. Thermal decomposition of hematite

Jarosite and goethite decompose to hematite during thermal treatment which can further be utilized in processing concepts. Therefore, hematite waste may need thermal treatment solely for the evaporation of volatiles which could influence the following recycling procedure but not for changing its structure. However, it was found that in an atmosphere of pure nitrogen hematite starts to decompose to magnetite at 1150 °C (Chen et al., 2020).

4.2.1. Utilization of jarosite for construction purposes

Asokan et al. found that the major mineral phases in jarosite waste are jarosite (KFe3(SO4)2(OH)6) and iron sulphate hydrate (2Fe2O3SO3·5H2O) of which the second is hydrophilic and solubilizes easily, bearing the risk of exposition of toxic elements to the environment. Studies were carried out whereby jarosite waste was mixed with clay and fly ash as an additive as well as with water in defined ratios. The mixture was air-dried and sintered in a muffle furnace at 960 °C. Mechanical properties to evaluate the potential as a building material including comprehensive leachate testing and determination of the radioactivity levels were tested. It was found that for the mixing ratio of jarosite and clay of 2:1 with 15% fly ash addition a satisfactory compressive strength, shrinkage behaviour and an elution of toxic elements below safe limits were achieved. The levels of radioactivity were below those of conventional building materials in India (Pappu Asokan et al., 2006). In later investigations the optimized ratio of jarosite and clay was numbered with 1:1, with an addition of 15% fly ash, whereby slightly higher values for the compressive strength and good elution properties are obtained (P. Asokan et al., 2010; Pappu et al., 2011). Furthermore, the ration of 3:1 with no additional fly ash was identified as a suitable mixture (Pappu et al., 2006). It was found, that the presence of jarosite in the brick material can cause corrosion due to the oxidation of pyrite but this is stated to be a very rare phenomena (Sharma, 2016).

An alternative way for the production of ceramic composite bricks (CCB) from waste materials is the mixing of clay soil, jarosite and marble processing rejects (MPRs). Around 200 million tons of MPRs are produced annually as a waste powder/sludge which can cause major environmental concerns. It was found that an addition of 15–30% of MPRs to a mixture of jarosite and clay with a ratio of 1:1 enables materials with satisfactory values for compressive strength, shrinkage and density. The toxicity leachate tests showed that the concentrations were below the US EPA TCLP (Toxicity Characteristic Leaching Procedure of the US Environmental Protection Agency) limits, meeting equivalent qualities to those of burned clay bricks (Pappu et al., 2019).

The possible utilization of jarosite for the substitution of materials in mixtures for construction purposes was developed. It was observed that the addition of jarosite as a partial sand replacement of up to 25% in concrete accompanied by the addition of fly ash, could be a feasible solution to use jarosite in the production of concrete or for road construction purposes in India (Mehra et al., 2016).

It was found that jarosite waste can be converted to a suitable construction material if it is mixed with a type of ferrous slag and another, not specified, industrial waste used as a liquid activator. The mechanical properties of the mixtures increased significantly with the addition of up to 4% of CaO or Portland Cement. Heavy metal leaching tests showed that the produced materials fulfilled the national environment standards of Spain, Russia and Brazil. This reveals the usability as bases or sub-bases for roads, airfield runways, brick production or industrial and municipal dumps (V. A. Mymrin et al., 2005; V. Mymrin and Vazquez Vaamonde, 1999).

Winters et al. proposed a method for the conversion of goethite waste to building materials which was developed in the late 1990s. Goethite was mixed with blast furnace and converter slag in pilot plant scale. Thereby the acidic goethite reacted with the basic components of the converter slag like CaO to form a hard, inert rock-like material with the brand name Graveliet® (Winters et al., 2000).

4.2.2. The Jarofix process

The Jarofix process is a recognized method for the treatment of jarosite and thus named in the document for best available techniques (BAT) of the European Union. The stabilization of jarosite and elimination of the need for its storage in basins, as well as the possibility of land reclamation are named as the achieved environmental benefits (Cusano et al., 2017). It is the best researched and most used technique to address the problem of hazardous jarosite iron precipitates. The residues are mixed with Portland cement, lime and water to generate a chemically and physically stable material. As an example, sodium jarosite undergoes a reaction with the alkaline phases contained in the cement. Sodium sulphate subsequently reacts to gypsum.(8)2NaFe3(SO4)2(OH)6 + 6 Ca(OH)2 + nH2O ↔ Na2SO4 + Ca6Fe2(SO4)3(OH)12·nH2O + 4Fe(OH)3Na2SO4 + Ca(OH)2 + 2H2O ↔ CaSO4·2H2O + 2Na(OH)

The stability of the produced jarofix material depends on the chemical composition of the jarosite residues as well as on the amount of Portland cement in the mixture. On the one hand the cement provides physical strength for the products but furthermore and more importantly it provides an alkaline environment which offers an extra degree of alkalinity for neutralizing acid phases. Jarofix has undergone years of laboratory pilot testing followed by its implementation in zinc plant circuits including proven environmental compliance (Chen and Dutrizac, 2000; Chen and Dutrizac, 2001).

As a pre-step for the development of the Jarofix process a US patent registered in 1980 which described the conversion of jarosite into an inert rock-like mass when mixed with Portland Cement powder and fly ash (Robinson-Todd, 1980).

The annual production of Jarofix in India in 2019 was numbered with 3 million tons resulting in an accumulated amount of approximately 26 million tons until today (Sinha et al., 2019). Due to the sealing of significant areas of land, research is carried out to develop strategies for utilization of Jarofix. Santhosh et al. investigated the influence of the replacement of fine aggregate with Jarofix to produce concrete. The study revealed that a substitution of the fine aggregate up to 10% in concrete constructions is suitable without affecting mechanical properties (Santhosh and K. S, 2018). Furthermore, it was found that Jarofix material can be utilized as an alternative embankment material, leading to the possibility of sustainable road construction (Santhosh & Beena K.S., 2019; Sinha et al., 2018; Sinha et al., 2019).

4.2.3. Alternative ways for immobilization

Montanaro et al. investigated the sintering behaviour of jarosite and goethite waste. It was observed that these materials can be sintered to a high final density at temperatures close to 1300 °C. The sinterability is ensured by the presence of impurities such as SiO2, PbO, CaO and alkaline oxides which lead to the formation of transitory liquid phases. Those assist the densification of the main crystalline phases like hematite, magnetite and zinc ferrite. The long-term objective is the processing of hazardous wastes to materials which combine good elution properties and the possibility of utilizing them for sound and heat insulation (Montanaro et al., 2001). A self-sealing isolating structure is achievable by mixing jarosite waste with alkaline coal fly ash (Ding et al., 2002).

Jarosite precipitation is also used for processing low-grade nickel ores. Nickel and cobalt are recovered from the acidic liquor which is produced during heap leaching of the ores. Iron, aluminium and chromium are furthermore precipitated. Research was carried out in Norway for co-treating sewage sludge and this certain jarosite/alunite residue to produce a material which could be used for construction purposes. Portland cement, jarosite/alunite residues and sewage sludge were mixed which leads to the formation of materials with relatively high strength and chemical stability which reveals the potential for its usability in construction of roads, foundations or parking areas. The precipitation residues are therefore used to replace Portland cement partly as a binder. A challenging aspect is the high cost of cement which needs to be added for the stabilization of the sewage sludge (Boura et al., 2005).

The possibility of a replacement of natural gypsum in cement production with jarosite/alunite from nickel ores was confirmed. A substitution of gypsum of up to 20% does not affect the properties of the produced cement. Higher amounts of jarosite/alunite were found to decrease the solubility of Cr6+ due to the presence of manganese in the jarosite/alunite. On the other hand the compressive strength is lowered with increased replacement (Katsioti et al., 2005).

As this research covered precipitation residues which are in some extents different to residues from the zinc metallurgy (high aluminium contents), it would have to be evaluated if the findings are as well applicable for common jarosite types.

4.3.1. Bioleaching of jarosite residues

Biohydrometallurgical processes are suitable methods to extract metals from residues with low energy and chemical consumption. Research was carried out using Acidithiobacillus ferrooxidans microbes for the recovery of valuable components from jarosite with a simultaneous reduction of the released metals during long-term storage. Both oxidative and reductive environments were investigated. It was shown that the liberation of iron is enhanced in reductive environments. In comparison the leachability of Cu and Zn was not significantly influenced. Furthermore, it was found that the leaching yields were higher in reactor experiments compared to flask trials. Extraction rates of 35% for Zn, 38% for Cu, 5–8% for In and Ga and 40% for Ge were achieved (Mäkinen et al., 2017). Castro et al. investigated the anaerobic bio reduction of synthetic jarosite using the strain Aeromonas hydrophila and found that, during the reduction, lactate is used as an electron donor which leads to the formation of a number of iron-containing minerals like siderite or magnetite. Furthermore, the influence of chelating agents such as citrate and EDTA, as well as humic substances (anthraquinone-2,6-disulfonate, AQDS) was evaluated. The chelants were found to increase the iron reduction whereby on the other hand humic substances showed no effect. The results showed the potential of bio reduction processes using Aeromonas hydrophila withlower energy inputs, no formation of mineral acids or emissions of noxious gases (Castro et al., 2016). Other studies evaluated the possibility of reductive leaching of jarosites using the strain Shewanella putrefaciens and revealed that the iron reduction is improved in presence of AQDS and citrate. Silver effects the cell growth and biological mineral transformations due to its toxicity to most microorganisms. It was revealed, that microbial catalysis of anaerobic iron leaching procedures have commercial potential, as the iron is obtained in concentrated Fe(II) solutions which should require less reducing power for the conversion to elemental iron in further steps of iron-production (Castro et al., 2013, 2015).

The strain Aspergillus terreus was used for the leaching of jarosite and the production of nanoparticles which can be used for agricultural applications. The biosynthesized particles were tested for their influence on seed emergence and it was shown that the growth of wheat plants was significantly enhanced (Bedi et al., 2018).

4.3.2. Conversion to magnetite and hematite

The production of magnetite or hematite from iron precipitation residues provides significant advantages as it has positive influence on the landfill situation in case of disposing the products. This is due to the significantly lower volumes of hematite e.g. compared to jarosite minerals. Furthermore, hematite could potentially be utilized in cement production or for colouring ceramic products and has potential for being used in great extents in iron production. In contrast, magnetite can be applicable in the pigment industry. Nevertheless, contained alkali-elements, zinc, sulphur, and others can influence the product quality of such pigments in negative ways.

Hydronium jarosite can be converted into magnetite with goethite formation as an intermediate step. A hydronium jarosite suspension in a FeSO4 solution was neutralized with NH3 followed by a thermal treatment at controlled pH (6.5) and temperature (90 °C). (Boháček et al., 1993).

Vu et al. carried out a similar procedure for the production of magnetite, but in reverse order. Ammonium and sodium jarosite suspensions were neutralized using aqueous solutions of either NaOH or respectively ammonia under the formation of amorphous ferric oxide. In addition, magnetite was produced with the aid of an FeSO4 addition. It was found, that magnetite particles produced from ammonium jarosite were uniform with a particle size of about 15 μm. In comparison, the product obtained when converting sodium jarosite differs in terms of particle dimensions. The contained impurities in the jarosite sample were not found to influence the magnetite product in significant manners (Vu et al., 2010).

A method for the conversion of jarosite to hematite was proposed by Dutrizac et al., in 1990. Sodium jarosite was converted in 0.2 M H2SO4 media at 230 °C in the presence of hematite seeds under release of sulphuric acid. It was observed that an increased initial H2SO4 concentration clearly promotes the dissolution of jarosite but hinders the Fe2O3 precipitation due to the formation of basic iron sulphate. The received hematite grains contained minor amounts of sulphur which was uniformly distributed. Iron sulphate phases were not observed (Dutrizac, 1990).

Studies on a similar process were also carried out for potassium jarosite, finding that the conversion to hematite is as well possible in presence of hematite seeds. In a short reaction time, Pb-jarosite and Na–Pb-jarosite were transformed to hematite and PbSO4 but it was found that the presence of dissolved Fe2(SO4)3 significantly inhibits the conversion (Dutrizac and Sunyer, 2012).

A different approach for the conversion of jarosite waste materials to hematite was proposed in an European patent in 1990. The jarosite residues are leached together with a zinc sulphide bearing concentrate with sulphuric acid (40–100 g/l H2SO4) at elevated temperatures (130–170 °C) and increased pressures (>1000 kPa). The leach liquor contains most of the iron and zinc and is separated from the solid residue. The remaining solution is fed into the leach circuit of a zinc electrolysis with hematite precipitation. Hematite has a higher iron content than goethite or jarosite and could be used in the cement or the iron and steel industry (Röpenack et al., 1990).

4.3.3. Alkaline leaching of jarosite and cyanidation of precious metals

The extraction of silver from jarosite using cyanidation (reaction with CN−) is a well-developed field of research where comprehensive investigations were carried out. Cruells et al. synthesized potassium argentojarosite which was subsequently decomposed in a hydrothermal treatment using NaOH or Ca(OH)2, accompanied by a silver extraction step using NaCN. It was observed that the cyanidation occurs with a start phase which decreases exponentially with rising temperature and concentration of OH−. The ensuing conversion period is characterized by the liberation of sulphate, potassium and silver ions which leave an amorphous iron hydroxide. The addition of potassium chloride can increase the reaction rate. The same effect was observed for the addition of NaCl or NaSO4 when treating sodium-argentojarosite (Cruells et al., 2000; Patiño et al., 1998). It was found that the decomposition is characterized by the removal of sulphate ions as well as a formation of a gel consisting of iron and silver hydroxides. In absence of cyanides the silver hydroxide would be converted to silver ferrite (Roca et al., 1993).

It was proven by González-Ibarra et al. that the temperature has a higher influence on the decomposition when using NaOH instead of Ca(OH)2. For pH values of 8–10, the decomposition rate is higher with Ca(OH)2, in case of pH 11, it is significantly increased by NaOH leading to very low reaction times (González-Ibarra et al., 2016).

The cyanidation rate is influenced by the level of impurities in the jarosite material. Roca et al. observed that an increasing substitution level in the jarosite lattice leads to a significant reduction of the cyanidation rate. The effect of decreased reactivity of jarosites with rising substitution in the lattice is also used for immobilization of arsenic in effluents by precipitation as arsenical jarosite (Alcobé et al., 2001; Roca et al., 2006).

Experiments showed by treating ammonium argento-jarosite in a lime medium that the reaction is happening in two phases, induction and conversion. The same mechanism was observed for the treatment of industrial ammonium argento-jarosite in NaOH media (Patiño et al., 2003; Salinas et al., 2001). It was stated furthermore, that the reaction rate of the cyanidation is high for synthetic potassium jarosite, similar for ammonium argento-jarosite and sodium argento-jarosite but 10 times lower for plumbo-jarosite (Patiño et al., 2003).

Patiño et al. focused on the cyanidation behaviour of silver when processing lead argentojarosite. It was observed, that at high pH values in Ca(OH)2 medium a saturation point is reached where the precipitation of CaCO3 takes place which tends to block the plumbojarosite aggregates. The reaction rate is significantly lower than for NaOH at same pH levels. On the other hand the cyanidation rates were not influenced by the NaCN concentration or the silver content (Patiño et al., 1994).

4.3.4. Combined treatment of sewage sludge and jarosite in an autoclave process

In the Netherlands comprehensive research was carried out by a consortium of institutes regarding the combined hydrometallurgical processing of jarosite and cellulose bearing residues like sewage sludge. The treatment of those two hazardous waste streams in an autoclave was found to be a suitable method for a zero-waste recycling process. Experiments proved that jarosite is reduced by the cellulose (C6H10O5) from sewage sludge. This leads to the release of significant amounts of acid, which can dissolve the contained metals (Hage, 1999).44MFe3(SO4)2(OH)6 + 2C6H10O6 ↔ 44M+ + 88SO42− + 44Fe3O4 + 132H+ + 12CO2 + 76H2O

Therefore, a neutralizing agent like NH4OH or MgO has to be added. The autoclave process is carried out at around 250 °C and 40 bar whereby heat is created due to the oxidation of the cellulose which maintains the heat balance. A subsequent solid-liquid separation splits the material in two fractions. The effluent which consists mainly of ammonium sulphate is treated with Ca(OH)2 which leads to the precipitation of gypsum. Due to the high pH values the ammonium can be stripped as NH3 and thus be recycled. The solid residue, consisting of iron and other metal oxides can be used as a construction material if the metals are immobilized. This was proven by comparative elution tests for treated and untreated jarosite (Hage and Schuiling, 2000). Otherwise, the residue can be further processed at temperatures of 1500 °C, leading to an evaporation of Zn, Ag and Pb which can subsequently be condensed and thus extracted. The remaining residue could be cast into stones as an alternative of basaltic rocks (Hage, 1999).

It was found in trials that the production of magnetite during the processing of jarosite and sewage sludge is possible at higher pH while hematite is formed at lower values. The magnetite formation is furthermore influenced by the content of both the reducing and neutralizing agent. It was also observed that the jarosite can be transformed to goethite which is then recrystallized to magnetite ( Hage et al., 1999).

4.3.5. Roasting and flotation

Han H. et al. studied the recovery of anglesite and silver from jarosite waste using a roasting process followed by a flotation with addition of Na2S to increase the hydrophobicity. The roasting procedure at 600–700 °C leads to a decomposition of jarosite, releasing anglesite, silver sulphate, zinc sulphate and hematite. Zinc sulphate was extracted by washing the roasted material and leaving the other materials in a solid residue. Na2S was used in the flotation procedure consisting of three subsequent steps whereby lead and silver were recovered in extents of 67% and 82% respectively. The product contained 44% of lead and 1.3 kg/ton of silver and can be used as a feed material for further processing (Han et al., 2014).

The flotation behaviour of anglesite from zinc leach residues with 6.9% lead was studied by Rashchi et al. showing, that the highest recovery rate of lead was achieved at a pH of 9.6 when using Na2S as a source of sulphur (Rashchi et al., 2005). Zinc leach residue has different chemical and morphological properties compared to jarosite – nevertheless a deeper understanding of the flotation behaviour possibly can enable conclusions for the treatment of jarosite residues.

4.3.6. Alternative hydrometallurgical strategies

Another possibility for the treatment of jarosite waste material is a leaching process using NH4Cl. After the thermal decomposition of a sample of potassium jarosite, a leaching with NH4Cl released valuable metals like Pb, Cu, Cd, Ag which are then reduced with zinc powder. In the optimum case the extraction was higher than 95%, while Fe remained in the residue. The leach residue reacts with a NaOH solution to extract Si and furthermore As which leads to significant detoxification of the final waste material (Ju et al., 2011).

In other research activities the leachability was investigated using the software OLI systems. The findings were similar to the previous study from Ju et al. it was observed that the leachability of Zn, Cu, Ag and Pb, when processing decomposed jarosite in an NH4Cl system, is high while the solubility of Fe2O3 is low (Ju et al., 2013; S. 1181).

Valuable metals can also possibly be recovered using resins for extraction from the leach liquor. In a study on the selective separation of Al, Co, Cu, Fe, Ni, Mn and Zn different kinds of resins were investigated. Experiments were carried out with a synthetic solution produced by dissolving their respective sulphates in sulphuric acid. The proposed method for processing actual industrial jarosite waste materials consists of crushing, sulphatising roasting and leaching with water to produce a metal-rich leach solution. It was found that there is no single resin which is suitable for the extraction of all valuable metals from the solution when used exclusively. A combination of base and acid resins showed promising results. The proposed process is to remove particulate and colloidal material using a sand filter followed by an extraction of Fe using the strong acid functional resin Purolite S957. Cu2+, Co2+, Ni2+ and Zn2+ can be removed by the weak base resin Dowex M4195 followed by a treatment with Dowex M31SA, a strong acid resin to remove Al3+ and Mn2+ (Riley et al., 2018).

Rodriguez et al. studied the utilization of deep-eutectic solvents for the extraction of zinc from goethite waste. Solvometallurgy is a new approach in the field of extractive metallurgy which uses non-aqueous solvents like ionic liquids, molecular organic solvents or deep-eutectic solvents (DESs). DESs generally describe a mixture of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The temperature of the eutectic point is hereby lower than the one of an ideal mixture. In the quoted study, levulinic acid-choline chloride was identified as the solvent to achieve high zinc leaching efficiency and good selectivity against iron. Furthermore glycol-choline chloride shows good results and was even more selective against iron. It was observed that both the temperature and the water content of the solvents are affecting the leaching efficiency. The treatment time has strong influence on the selectivity against iron. It is stated nevertheless that the industrial implementation seems unattractive due to the high prices of DESs compared to the low concentration of cheap base metals in the residues (Rodriguez et al., 2020).

During the selective recovery of lead and zinc from jarosite residues by leaching with ionic liquids the iron was found to remain in the residue. Ionic liquids which were equilibrated with HCl leached the highest amount of lead and zinc. In case of higher HCl concentration, the leaching efficiency increased while the selectivity declined. The metals were stripped from the leach liquor using aqueous NH3 solutions (Palden et al., 2019).

The selective recovery of indium from a solution of dissolved goethite using the Aliquat336 iodide supported ionic liquid phase (SILP) was investigated. Indium was recovered with an indium-over-iron mass ratio of 7.9 and a selectivity factor equal to 5400 (van Roosendael et al., 2019).

A further possible way for the extraction of silver from jarosite waste is the leaching with thiourea. Two methods were studied at a temperature of 90 °C, the simultaneous jarosite decomposition and silver leaching with thiourea as well as the decomposition followed by a leaching step. For the first method it was found that the silver recovery is higher with lower pH, 70% silver recovery were achieved for pH = 1. Low pH values can have negative effects and can lead to the release of Cu2+ ions which can increase the redox potential and therefore the oxidation of thiourea. When jarosite was decomposed prior to thiourea, the cupric ions were eliminated which enabled silver recoveries of up to 90%. The decomposition was carried out at pH = 0.5 followed by the thiourea leaching step at pH = 1, both at 90 °C. It is striking that lead remained in the final residue while zinc and copper were dissolved entirely (Calla-Choque et al., 2016).

Liu et al. carried out a microwave leaching treatment to recover a number of valuable elements from jarosite. The waste material was mixed with H2SO4 in different proportions and roasted in a microwave reactor at temperatures ranging from 150 to 450 °C. After cooling, the samples were mixed with distilled water and kept at temperature of 50–90 °C for 1–4 h. After a solid/liquid separation step, the iron was removed from the leach liquor using goethite precipitation. Compared to roasting experiments in a conventional muffle furnace it was found that the extraction of the valuable elements was 10% higher when applying a microwave roasting treatment. The achieved extraction of Fe, Zn, In, Cu, Ag and Cd was 89.4, 80.7, 85.1, 90.7, 61.3 and 48.8%, respectively (Liu et al., 2017).

Furthermore, a possibility to convert jarosite into goethite after dissolution was proposed by Weber et al. whereby the contained valuable metals were precipitated and extracted in a subsequent step. The procedure was based on the fact, that goethite is formed at significantly lower pH values than the hydroxides of valuables which allows a selective separation. It was found, that the ratios of zinc, cadmium, copper and lead to iron show low values in a pH range of 4–6 (Weber and Schuller, 1988).

A patent was registered in 1973, proposing a way to treat jarosite for the production of a fertilizer. Ammonium and potassium jarosite residues are dissolved in sulphuric acid followed by a fractional crystallization. Subsequent to an evaporation step superphosphate and potash can be added to produce an enriched fertilizer (Jackson, 1975).

4.4.1. Treatment of iron precipitation residues in pyrometallurgical industrial-scale facilities

According to Creedy et al. only pyrometallurgical methods are implemented on an industrial scale for the recovery of metals during processing of precipitation residues from the zinc industry. Besides the Ausmelt TSL technology in South Korea and Australia, the waelz process (Brazil, China, Italy, Bulgaria) and lead smelters plus fumers (Australia, Japan, Canada) are used. Advantages are the ability to process a wide range of secondary materials like jarosite and goethite, primary leach residues, steel mill dust and imperial smelting furnace slag. Another benefit is the high recovery of Pb, Zn, Ag and other valuables which is stated for the usage of the Ausmelt technology. The slag which is produced in course of the process is non-hazardous and can be discarded or used for construction purpose. Furthermore, the energy can be recovered from the off-gas. A number of converters with capacities of up to 120,000 tons per year were in operation, mainly in South Korea with two more under construction in 2012 (Creedy et al., 2013).

It was reported that factories which adapted rotary kilns to fume valuables from jarosite waste achieved recoveries of 75% Zn, 68% Pb and 80% Ge. However, several aspects such as high fixed investments, high operation costs and substantial air pollution during the fuming process are noteworthy disadvantages (Ju et al., 2011).

4.4.2. Processing in plasma furnaces

Studies were carried out to evaluate the potential of treating residues from the Boliden Kokkola zinc plant in Finland using an ArcFume plasma process. Salminen et al. investigated the fuming behaviour of valuable elements when treating jarosite and a sulphur residue in a two stage (smelting and reduction) treatment. Plasma generators heat compressed air to temperatures of about 3000–5000 °C which is injected into the slag bath. In the first reaction step the residues are treated in an oxidative environment leading to the release of SO2, H2O, Hg, Se and an metal oxide dust consisting of Zn, Pb, Ag, In, Ge, As, Sb, Cd, F and Cl. The produced slag is further treated in a reductive environment leading to the generation of a clean slag for granulation and again a metal fume which is subsequently collected as a metal oxide fraction. This fraction can be treated hydrometallurgically with e.g. halide washing and selective leaching stages. The achieved recoveries were stated with >90% for Zn, Pb > 99%, Ag 75–85%, In 60–70% and Ge 80–90%. The leaching test of the residues were promising with only Sb showing levels slightly above landfill criteria. The ArcFume process is suitable for processing feed materials with high amounts of sulphur whereby the processing of only jarosite is easier because less off-gas handling is required. It was furthermore found that the processing of combined waste is possible in a single oxidative stage (Salminen et al., 2020).

The treatment of jarosite sludges with plasma technology was also investigated in Italy whereby an Arc Transferred Plasma (ATP) reactor was used. Jarosite was reduced under formation of cast iron and a glassy slag that could easily be landfilled or even processed to alternative stone materials. Jarosite was calcined at temperatures up to 1000 °C in a rotary kiln to remove crystal water and to reduce the total sulphur content. The calcine was reduced in an ATP reactor at 1600–1700 °C by the addition of metallurgical coke as a reducing agent. A CaO/SiO2 ratio of 0.3 was adjusted by addition of limestone. About 6% of the jarosite input mass was extracted as a metal dust consisting of Zn, Pb and Ag compounds. The produced slag consisted mainly of Al2O3, SiO2 and Fe2O3, leaching tests point to non-hazardous properties and it could therefore be landfilled as an inert waste or even utilized in civil applications. The produced cast iron contained noteworthy amounts of P, S and Cu and could therefore not be used unless diluted with clean Cu-free raw materials (Mombelli et al., 2018).

4.4.3. Production of glass-ceramic materials

The conversion of goethite to glass-ceramics is a comparatively well investigated field. Pelino et al. showed that goethite can be converted to ceramic glass when melting and quenching certain mixtures. The goethite waste was dried and mixed with sand, feldspar, limestone, MgO and Al2O3 in various proportions. Small amounts of TiO2 were added to promote heterogeneous nucleation. The produced crystalline phases and the crystalline/amorphous phase ratio were similar to those of glass-ceramics obtained from fused rocks. One of the major problems related to glass production from hydrometallurgical waste materials of the zinc industry is the high sulphur content which can account for corrosion of the refractory during the melting operation. Furthermore, the sulphur contents released to the atmosphere need to be controlled. The hazardous elements like Pb, Zn and Cd were stabilized in the crystalline phase of the glass ceramic. The mechanical properties were found to be comparable to or even better than those of commercial glass ceramics. Beyond that, it was found that the chemical stability is higher with increasing percentage of crystallinity (Mario Pelino et al., 1996; M. Pelino, Cantalini, Boattini, et al., 1994; M. Pelino, Cantalini, Veglio' and Plescia, 1994). Further research showed that the combined processing of goethite waste with granite scraps and glass cullet is a promising alternative for the production of glass-ceramics (M. Pelino et al., 1997).

The manufacturing of glass and glass ceramics was also investigated for jarosite waste. Jarosite was mixed with granite scraps and mud which was generated when sawing and cutting granite blocks as well as glass cullet. The material was molten at temperatures ranging from 1400 to 1450 °C and subsequently quenched. The produced glassy phase is suitable for the building industry and can be utilized as paving tiles, wall covering panels or glass fibres for insulation and ceramic pigments (M. Pelino, 2000; Ramachandra Rao, 2006).

4.4.4. Reduction and magnetic separation

Reduced residues can be subjected to magnetic separation to achieve two products, a magnetic fraction rich in iron and a non-magnetic fraction rich in zinc. Piga et al. used para-goethite in a process consisting of a roasting and a reducing step followed by magnetic separation. The roasting was carried out at 1200 °C for 3 h, subsequently the reactor was cooled to 700 °C and the furnace was flushed with N2 followed by a stream of reducing gas (N2 and H2). The produced gas was treated with NaOH and burned, the solid material was furthermore magnetically separated in four steps. The produced magnetic fraction contained 81% of Fe2O3 as well as 13% of Zn and 8% of lead. The non-magnetic product showed a concentration of 20% for Zn which indicated a recovery of 49% (Piga et al., 1995).

A similar approach for the treatment of iron precipitation residues is the mixing with coal and CaO to produce pellets which are treated in a reduction roasting process. Zn and Pb are fumed leaving a product which is subsequently milled and magnetically separated. Ammonium-plumbo-jarosite was used for the trials. The coal dosage showed significant influence on the iron metallization rate whereby the extraction of lead and zinc was not affected. The optimum conditions were found to be an addition of 25 wt% of coal and a roasting temperature of 1250 °C. The volatilization was high for Pb (97.0%) and Zn (99.9%) while the metallization rate of iron was 92%. After magnetic separation the magnetic concentrate contained 90.6% of iron, indicating a recovery rate of 50.9% (Wang