Factors affecting alkali jarosite precipitation

Several factors affecting the precipitation of the alkali jarosites (sodium jarosite, potassium jarosite, rubidium jarosite, and ammonium jarosite) have been studied systematically using sodium jarosite as the model. The pH of the reacting solution exercises a major influence on the amount of jarosite formed, but has little effect on the composition of the washed product. Higher temperatures significantly increase the yield and slightly raise the alkali content of the jarosites. The yield and alkali content both increase greatly with the alkali concentration to about twice the stoichiometric requirement but, thereafter, remain nearly constant. At 97 °C, the amount of product increases with longer retention times to about 15 hours, but more prolonged reaction times are without significant effect on the amount or composition of the jarosite. Factors such as the presence of seed or ionic strength have little effect on the yield or jarosite composition. The amount of precipitate augments directly as the iron concentration of the solution increases, but the product composition is nearly independent of this variable. A significant degree of agitation is necessary to suspend the product and to prevent the jarosite from coating the apparatus with correspondingly small yields. Once the product is adequately suspended, however, further agitation is without significant effect. The partitioning of alkali ions during jarosite precipitation was ascertained for K:Na, Na:NH₄, K:NH₄, and K:Rb. Potassium jarosite is the most stable of the alkali jarosites and the stability falls systematically for lighter or heavier congeners; ammonium jarosite is slightly more stable than the sodium analogue. Complete solid solubility among the various alkali jarosite-type compounds was established.     

The behavior of thallium during jarosite precipitation

Jarosite precipitation provides an effective means of eliminating thallium from zinc processing circuits, and a systematic study of the extent and mechanism of thallium removal during the precipitation of ammonium, sodium, and potassium jarosites was carried out. Thallium (as Tl⁺) substitutes for the “alkali” ion in the jarosite structure. Nearly ideal jarosite solid solutions are formed with potassium, but thallium is preferentially precipitated relative to either ammonium or sodium. Approximately 80 pct of the dissolved thallium precipitates during the formation of ammonium jarosite; the extent of thallium removal is virtually independent of thallium concentrations in the 0 to 3000 mg/L Tl range and of the presence of 75 g/L of dissolved Zn. Although the deportment of thallium is nearly independent of (NH₄)₂SO₄ or Na₂SO₄ concentrations >0.1 M, the precipitates made from more dilute media are relatively enriched in thallium. Likewise, the precipitates made from dilute ferric ion media are also Tl-rich. Low solution pH values or low temperatures both significantly reduce the amount of jarosite formed, but the precipitates made under these conditions are enriched in thallium. Furthermore, because thallium jarosite is more stable than the ammonium or sodium analogues, the initially formed precipitates are consistently Tl rich. The presence of jarosite seed accelerates the precipitation reaction, but dilutes the thallium content of the product. The results suggest that most of the thallium in a hydrometallurgical zinc circuit could be selectively precipitated in a small amount of jarosite, by carrying out the precipitation reaction for a short time in the absence of seed and from solutions having low alkali concentrations.     

Rubidium jarosite and thallium jarosite — new synthetic jarosite-type compounds and their structures

Rubidium jarosite (RbFe₃(SO₄)₂(OH)₆) and thallium jarosite (TlFe₃(SO₄)₂(OH)₆) were synthesized as single phase products by precipitation from aqueous solution. Hydronium ion (H₃O⁺) substitutes for part of the “alkali” metal in these compounds. Both jarosites are hexagonal (R3m) and have similar unit cell dimensions. During heating rubidium jarosite undergoes two major decompositions; initially water is evolved and subsequently sulphur oxides are emitted. Thallium jarosite decomposes in three principal stages during programmed heating. The first two stages are similar to the decomposition of rubidium jarosite; the third decomposition involves the breakdown of thallium sulphate and the subsequent sublimation of thallous oxide.     

Behaviour of cesium and lithium during the precipitation of jarosite-type compounds

End-member cesium jarosite [CsFe₃(SO₄)₂(OH)₆] and end-member lithium jarosite [LiFe₃(SO₄)₂(OH)₆] do not exist. Cesium-bearing potassium, sodium and rubidium jarosites were synthesized, however, with cesium contents > 2 wt% being noted in potassium jarosite. The cesium content of the jarosites increases with increasing cesium concentration in solution, and the order of cesiumincorporation is potassium jarosite > sodium jarosite > rubidium jarosite. Only trace amounts of lithium are incorporated in jarosite; the maximum obtained was ∼ 0.2 wt% lithium in potassium jarosite. The lithium content increases with increasing lithium concentration in the synthesis solution, but is nearly independent of the potassium concentration or the solution pH. Although some of the lithium may be structurally incorporated in the jarosite, most seems to be physically entrained.     

Surface exsolution and local atomic structure of amorphous CeTiOx

We made precipitated nano-ceria (～5 nm) on the surface of the catalyst by heat treatment of Ce-supersaturated amorphous CeTiO_x to improve the oxygen storage properties of CeO₂. The catalysts were prepared by sol-gel methods and TiO₂ nanoparticles were preferentially generated as a core material to form selective Ce-supersaturated structure on the catalyst surface. Reaction temperature and amount of doping element are optimized to induce selective crystallization of CeO₂. CeCe (2nd shell) bond around 0.38 nm of Ce L₃-edge extended X-ray absorption fine structure is reduced and nanostructure of precipitated ceria on the surface is observed by HREM. The catalyst is present as amorphous with precipitated nano-CeO₂ on the surface. The de-NO_x efficiency of the catalyst, which has precipitated CeO₂, improves by ～50% owing to the simultaneous reactions of the nano CeO₂ and the amorphous CeTiO_x.     

Hematite formation from jarosite type compounds by hydrothermal conversion

Abstract

The hydrothermal conversion of K jarosite, Pb jarosite, Na jarosite, Na–Ag jarosite, AsO₄ containing Na jarosite and in situ formed K jarosite and Na jarosite to hematite was investigated. Potassium jarosite is the most stable jarosite species. Its conversion to hematite in the absence of Fe₂O₃ seed occurred only partially after 5 h reaction at >240°C. In the presence of Fe₂O₃ seed, the conversion to hematite was nearly complete within 2 h at 225°C and was complete at 240°C. The rate of K jarosite precipitation, in situ at 225°C in the presence of 50 g L^?1 Fe₂O₃ seed, is faster than its rate of hydrothermal conversion to hematite. In contrast, complete conversion of either Pb jarosite or Na–Pb jarosite to hematite and insoluble PbSO₄ occurs within 0·75 h at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed. Dissolved Fe(SO₄)_1·5 either inhibits the conversion of Pb jarosite or forms Pb jarosite from any PbSO₄ generated. The hydrothermal conversion of Na–Ag jarosite to hematite was complete within 0·75 h at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed. The Ag dissolved during hydrothermal conversion and reported to the final solution. However, the presence of sulphur or sulphide minerals caused the reprecipitation of the dissolved Ag. The conversion of AsO₄ containing Na jarosite at 225°C in the presence of 20 g L^?1 Fe₂O₃ seed was complete within 2 h, for H₂SO₄ concentrations <0·4M. Increasing AsO₄ contents in the Na jarosite resulted in a linear increase in the AsO₄ content of the hematite, and ～95% of the AsO₄ remained in the conversion product. Increasing temperatures and Fe₂O₃ seed additions significantly promote the hydrothermal conversion of in situ formed Na jarosite at 200–240°C. However, the conversion of previously synthesised Na jarosite seems to proceed to a greater degree than that of in situ formed Na jarosite.

On a examiné la conversion hydrothermale en hématite de la jarosite de K, de la jarosite de Pb, de la jarosite de Na, de la jarosite de Na-Ag, de la jarosite de Na contenant de l’AsO₄, et de la jarosite de K et de la jarosite de Na qui sont formées in situ. La jarosite de potassium est la plus stable des espèces de jarosite. Sa conversion en hématite ne se produisait que partiellement après 5 h de réaction à >240°C en l’absence d’amorce de Fe₂O₃. En présence d’amorce de Fe₂O₃, la conversion en hématite était presque complète à moins de 2 h à 225°C et était complète à 240°C. La vitesse de précipitation de la jarosite de K, in situ à 225°C en présence de 50 g L^?1 d’amorce de Fe₂O₃, est plus rapide que sa vitesse de conversion hydrothermale en hématite. Par contraste, la conversion complète soit de la jarosite de Pb ou de la jarosite de Na-Pb en hématite et en PbSO₄ insoluble se produit à moins de 0·75 h à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃. Le Fe(SO₄)_1·5 dissous soit inhibe la conversion de la jarosite de Pb ou forme de la jarosite de Pb à partir de tout PbSO₄ produit. La conversion hydrothermale de la jarosite de Na-Ag en hématite était complète à moins de 0·75 h à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃. L’Ag se dissolvait lors de la conversion hydrothermale et se rapportait dans la solution finale. Cependant, la présence de soufre ou de minéraux sulfurés avait pour résultat la re-précipitation de l’Ag dissous. La conversion de la jarosite de Na contenant de l’AsO₄ à 225°C en présence de 20 g L^?1 d’amorce de Fe₂O₃ était complète à moins de 2 h, avec des concentrations d’H₂SO₄ <0·4 M. L’augmentation de la teneur en AsO₄ de la jarosite de Na avait pour résultat une augmentation linéaire de la teneur en AsO₄ de l’hématite et ～95% de l’AsO₄ demeurait dans le produit de conversion. L’augmentation de la température et d’additions d’amorce de Fe₂O₃ favorisait significativement la conversion hydrothermale de la jarosite de Na qui est formée in situ à 220–240°C. Cependant, la conversion de la jarosite de Na synthétisée antérieurement semblait se produire à un plus grand degré que celle de la jarosite de Na qui est formée in situ.     

Removal of iron and formation of copper(i) from solutions containing iron(ii) and copper(ii) by addition of chloride ion or acetonitrile

Solutions of Cu(II) and Fe(II) establish the redox equilibrium

$\text{Cu(II) + Fe(II)}\text{?}\text{K}\text{Cu(I) + Fe(III)}$

which is displaced to the right by addition of either Cl^? or acetonitrile (AN). Log K varies from ?10.5 in water to about ?2.5 in 4 M NaCl or AN, allowing iron to be removed selectively from copper (II) solutions either by solvent extraction with Versatic acid or by precipitation as goethite or j jarosite. To establish the required conditions Eh-pH diagrams have been developed for the CuH₂OCl and CuH₂OANSO₄²-systems at 25°C and 90°C. It is demonstrated that the catalytic effect of Cu(II) on the oxidation of Fe(II) to Fe(III) by O₂ is dependent on the concentration of Cl^? or AN and on the position of this redox equilibrium. Applications to removing iron from hydrometallurgical solutions are discussed and tested.     

Double loop electrochemical potentiokinetic reactivation test optimization in checking of duplex stainless steel intergranular corrosion susceptibility

The double loop electrochemical potentiokinetic reactivation (DL-EPR) test using an electrolyte of 33 pct H₂SO₄ solution with 0.3 pct HCl, at room temperature and at a potential scan rate dE/dt of about 2.5 mV/s, was chosen to evaluate the sensitization of austeno-ferritic duplex stainless steels (DSS). Reproducible and optimal test responses and high test selectivity in detecting integranular corrosion (IGC) susceptibility were verified for four DSS differing in their method of fabrication (cast or wrought) and their ferrite phase content (44 to 57 pct). The test was successfully used to analyze the interactions between precipitation, chromium depletion, and IGC sensitization of the UNS S31250 steel, which was aged between 6 minutes and 120 hours at temperatures varying from 500 °C to 900 °C. The eutectoid decomposition of the ferrite, at different aging temperatures, was investigated using various techniques. The chromium depletion was analyzed qualitatively by X-ray mapping in a scanning transmission electronic microscope (STEM) and quantitatively by analytical calculation based on the chromium diffusion in the ferrite. It was shown that the chromium content in the ferrite can decrease from 30 to 7.5 pct by weight during aging before total decomposition occurs. The interactions between precipitation and IGC sensitization during DSS aging were clearly shown by superimposing the time-temperature-start of precipitation (TTP) and time-temperature-sensitization (TTS) diagrams obtained from the DL-EPR tests performed for various levels of sensitization.     

Tunable Curie temperature and magnetocaloric effect of FeCrMoCBYNi bulk metallic glass with different crystallized phases

The investigation on Curie temperature and magnetocaloric effect of the FeCrMoCBYNi bulk metallic glass(BMG) with different crystallized phases was carried out by XRD,TEM and PPMS. The experimental results show that the Curie temperature(T_c) of Fe_(45)Cr_(15)Mo_(14)C_(15)B_6 Y_2 Ni_3 BMG with different annealing condition reaches a highest value of 95 K. The value of magnetic entropy change △S_M(T) of Sample 3 reaches a maxima of 0.48 J/(kg·K) at Tc temperature, which result from the interaction among the precipitated phases of(Fe,Cr)_(23)(C,B)_6, Fe_3 Mo_3 C and residual amorphous phase. Based on the experiment results, it can be obtained that the Curie temperature, magnetocaloric effect can reach their optimal value at low temperature, when the content of amorphous phase and precipitated phases type run up to certain value. The magnetic properties of Sample 1 with full amorphous phase and Sample 4 with full crystalline phase will both decrease.     

The co-precipitation of copper and zinc with lead jarosite

The formation of lead jarosite, Pb_0.5Fe₃(SO₄)₂(OH)₆, in the presence of dissolved copper and/or zinc results in a significant substitution of these metals in the jarosite phase; the co-precipitation is most pronounced in sulphate media but also occurs, to a lesser degree, in chloride solutions. The copper and/or zinc substitute for iron, and under extreme conditions the product approaches beaverite, Pb(Cu,Zn)Fe₂(SO₄)₂(OH)₆, in structure and composition. The extent of co-precipitation increases sharply with increasing concentrations of dissolved CuSO₄ or ZnSO₄ and slightly with either an increasing stoichiometric ratio of PbSO₄/Fe³⁺ or increasing ionic strength. The co-precipitation of copper or zinc is not significantly affected by acid concentration although the yield of product declines with increasing concentration of H₂SO₄. The extent of reaction is relatively insensitive to reaction temperatures in the range 130–180°C and to reaction times in excess of 2 h. Copper is strongly co-precipitated in preference to zinc from solutions containing both metals. Other divalent base metals such as Co, Ni and Mn are also co-precipitated with lead jarosite although not to the same degree as copper or zinc.     

Synthesis, Thermodynamic, and Kinetics of Rubidium Jarosite Decomposition in Calcium Hydroxide Solutions

Rubidium jarosite was synthesized as a single phase by precipitation from aqueous solution. X-ray diffraction and scanning electron microscopy energy-dispersive spectrometry analysis showed that the synthetic product is a solid rubidium jarosite phase formed in spherical particles with an average particle size of about 35???m. The chemical analysis showed an approximate formula of Rb_0.9432Fe₃(SO₄)_2.1245(OH)₆. The decomposition of jarosite in terms of solution pH was thermodynamically modeled using FACTSage by constructing the potential pH diagram at 298?K (25?°C). The E-pH diagram showed that the decomposition of jarosite leads to a goethite compound (FeO·OH) together with Rb⁺ and $ {\text{SO}}_{4}^{2 - } $ ions. The experimental Rb-jarosite decomposition was carried out in alkaline solutions with five different Ca(OH)₂ concentrations. The decomposition process showed a so-called ??induction period?? followed by a progressive conversion period where Rb⁺ and $ {\text{SO}}_{4}^{2 - } $ ions formed in the aqueous solutions, whereas calcium was incorporated in the solid residue and iron gave way to goethite. The kinetic analysis showed that this process can be represented by the shrinking core chemically controlled model with a reaction order with respect to Ca(OH)₂ equals 0.4342 and the calculated activation energy is 98.70?kJ mol^?C1.     

EFFECT OF CERTAIN SURFACTANTS ON THE LEACHING AND PRECIPITATION PROCESSES IN ZINC FERRITE RESIDUE TREATMENT

The effect of a surfactant mixture of nonylphenolpolyethylene glycol (D1), dinaphthylmethane-4,4′-disulphonic acid (D2), and polyethylene glycol with molecular weight 400 (D3) on the dissolution of zinc and metal impurities present in zinc ferrite residue in dilute sulfuric acid (160 g L^?1 H₂SO₄) as well as on both jarosite and goethite precipitation was studied at 90°C. The following influences of the surfactant mixture (D1 + D2 + D3), determined by comparing the results obtained in the presence and absence of surfactants, were found. Adsorption of surfactants on zinc ferrite residue surface decreases the dissolution of zinc and metal impurities (Fe, Cu, Cd, As, Sb, and Co). Their extraction efficiencies at the end of the super hot leaching process carried out with surfactants are 4.85–6.29% lower than without them. The formation of a sulfur “sponge” layer on the surface of liquor during the dissolution of ZnS present in zinc ferrite residue is hindered by the surfactants due to their effect as wetting agents and sulfur dispersants. The presence of surfactants reduces the amount of zinc and metal impurities (Fe, Cu, Cd, and As) remaining in the solution after jarosite or goethite precipitation by 5.33–5.86% or 8.03–9.93%, respectively. The volume of jarosite and goethite precipitates increases in the presence of surfactants due to their effect as wetting and flocculation agents. On the other hand, D1 and D3 act as complexing agents. The abovementioned effects of surfactants improve the sorption capacity of both jarosite and goethite, thus ensuring better purification of zinc sulphate solutions, but hindering zinc leaching.     

Reaction Process of Chromium Slag Reduced Industrial Waste in Solid Phase

M, a particular industrial waste, was selected to detoxify chromium slag at a high temperature. The carbon remaining in M reduced Cr (VI) of Na₂CrO₄ borne in the chromium slag to Cr (III) in the solid phase reaction, and its thermodynamics and kinetics were studied. The reduction process of Na₂CrO₄ by carbon produced CO, which was endothermic. Under the experimental condition, the apparent activation energy was 4. 41 kJ · mol⁻¹, the apparent order of reaction for Na₂ CrO₄ was equal to one, and the partial pressure of CO was only 0.22 Pa at 1 330 °C.     

Characterization of the iron arsenate–sulphate compounds precipitated at elevated temperatures

To help clarify the nature of the iron arsenate–sulphate compounds produced during the autoclave treatment of refractory gold ores and concentrates, systematic synthesis studies were undertaken; in addition to scorodite and Fe(SO₄)(OH), two other compounds, designated as Phase 3 and Phase 4, were identified. Whereas Fe(SO₄)(OH) is predominantly an orthorhombic compound, Phase 3 can have the same composition but is predominantly the monoclinic polytype, the formation of which is promoted by the solid-solution uptake of As; substitution of As results in a corresponding decrease in the OH required to maintain the charge balance; e.g., Fe[(SO₄)_0.60(AsO₄)_0.40]_∑1.00[(OH)_0.6(H₂O)_0.4]_∑1.00. Phase 4 corresponds to Fe(AsO₄)·¾H₂O. In 0.4 M Fe(SO₄)_1.5 (22.3 g/L Fe), 0.41 M (40 g/L) H₂SO₄, 0.09 M (7 g/L) As(V) solutions, sulphate-containing scorodite was formed at 150–175 °C. Phase 3 precipitated at 175–210 °C, but mixtures of Phase 3 and Fe(SO₄)(OH) formed above 210–220 °C. The Fe content of Phase 3 is about 30 mass %, whereas the AsO₄ and SO₄ contents vary widely and in an inversely proportional manner, reflecting the extensive mutual structural substitution of these anions. At 205 or 215 °C, Fe(SO₄)(OH) was precipitated from 0.4 M Fe(SO₄)_1.5 (22.3 g/L Fe), 0.41 M (40 g/L) H₂SO₄ solutions containing < 0.03 M (2 g/L) As(V). Increasing As(V) concentrations enhance the precipitation of Phase 3, but only Phase 4 was precipitated from solutions containing > 0.33 M (25 g/L) As(V). The composition of Phase 4 is nearly constant and it contains < 1 mass % SO₄. Acid concentrations > 0.2 M H₂SO₄ had little effect on the composition of the precipitates. At 205 °C in 0.41 M (40 g/L) H₂SO₄, 0.09 M (7 g/L) As(V) media, mixtures of scorodite and Phase 4 precipitated from 0.0–0.1 M Fe(SO₄)_1.5 (0.0–5.6 g/L Fe) solutions; for Fe(SO₄)_1.5 concentrations > 0.1 M, only Phase 3 formed. To provide a preliminary indication of the solubility of Phase 3 and Phase 4 in tailings impoundments, the various precipitates were leached at room temperature for 40 h in water. The As concentrations dissolved from Phase 3 were consistently < 0.1 mg/L, which suggests that Phase 3 might be an acceptable medium for arsenic disposal. In contrast, the soluble As concentrations from Phase 4 were 1–3 mg/L.     

The mechanisms of improved creep strength in a new austenitic stainless steel

An addition of 40 ppm boron, 0.4 pct vanadium and 0.12 pct nitrogen to an austenitic stainless type steel AISI 316L (0.02 C, 18 Cr, 12 Ni, 2.7 Mo) has considerably improved the creep properties. The improved creep properties are due to a combination of the precipitation of fine stable vanadium nitrides on the dislocations and the precipitation of chromium carbides (M₂₃C₆) in the grain boundaries. The latter process is thought to be enhanced by the presence of boron and helps to improve the creep ductility. The precipitation of vanadium nitrides on the dislocations retard the creep rate. The nitrides retain their small size even after long creep testing times. A model is proposed to explain this behavior of the precipitated particles and their interactions with the dislocations. Formerly with Uddeholm AB, Hagfors, Sweden     

Some Thermodynamic Aspects of the Oxides of Chromium

To understand Cr emissions from slag melts to a vapor phase, an assessment of the stabilities of the chromium oxides at high temperatures has been carried out. The objective of the present study is to present a set of consistent data corresponding to the thermodynamic properties of the oxides of chromium, with special reference to the emission of hexavalent chromium from slags. In the current work, critical analysis of the experimental data available and a third analysis in the case of Cr₂O₃ have been carried out. Commercial databases, Fact Sage and ThermoCalc along with NIST-JANAF Thermochemical Tables, have been used for the analysis and comparisons of the results that are presented. The significant discrepancies in the available data have been pointed out. The data from NIST-JANAF Thermochemical Tables have been found to provide a set of consistent data for the various chromium oxides. An Ellingham diagram and the equations for the ΔG° (standard Gibbs free energy change) of formation of CrO _x have been proposed. The present analysis shows that CrO₃(g) is likely to be emitted from slag melts at high oxygen partial pressures.     

Factors affecting lead jarosite formation

The importance of lead jarosite in hydrometallurgical processing and the factors affecting its formation in both the slow addition and autoclave synthesis techniques are discussed. In the slow addition method the principal factors are the amount and rate of delivery of soluble lead to the hot ferric sulphate solution; high temperatures and good agitation are also essential to avoid the formation of PbSO₄. The key step in the autoclave synthesis process is the selective removal of residual PbSO⁴ from the reaction product and methods of accomplishing this are described. The major factors affecting the autoclave synthesis of lead jarosite are the ratio of $\text{PbSO}{}_4\text{Fe}{}^{\mathrm{3+}}$, acid concentration and the ionic strength of the solution. Time, temperature, degree of agitation and seeding all affect the reaction but to a lesser degree. The principal techniques identified to suppress lead jarosite formation were high acidity (> 0.3 M H₂SO₄ or the presence of substantial quantities (> 0.3 M) of other jarosite formers such as K₂SO₄. Lead jarosites containing more than 16% Pb were produced and X-ray diffraction data for such material are presented.     

Nanocrystalline films of soft magnetic iron-based alloys

The physicochemical and structural aspects of designing soft magnetic alloys Fe-MX (where M is a Group III–V metal of the periodic table and X = C, N, O) in the form of nanocrystalline films precipitation-hardened by refractory interstitial phases are discussed and developed. The results of studying the structure and magnetic properties of Fe₇₈Zr₁₀N₁₂ films are reported. The films in the amorphous state are produced by reactive magnetron sputtering. Upon annealing at 300–600°C, the amorphous films crystallize to form mainly a bcc α-Fe-based phase and the fcc ZrN phase. The grain size of the bcc phase is shown to increase from ～3 nm to ～30 nm as the annealing temperature increases; the grain size of the fcc phase does not exceed 2–3 nm. Films annealed at 400°C exhibit a record level of magnetic properties: H _c = 5–6 A/m and B _s = 1.7–1.8 T. The experimental results obtained confirm the validity of our scientific approach.     

Characterization and alkaline decomposition/cyanidation of beudantite–jarosite materials from Rio Tinto gossan ores

Jarosite-type minerals are the major silver carriers in the gossan ores from Rio Tinto (Spain). Two types of minerals were found: one corresponding to beudantite variable enriched in sulfate; the other is potassium jarosite containing various amounts of arsenate and lead. They are isostructural with cell parameters intermediate between those reported for end members. Silver is present in both jarosites as dilute solid solution (230 ppm Ag in average). The cyanidation of potassium jarosite in saturated Ca(OH)₂ at 70–100°C consists of two step in series: a slow step of alkaline decomposition followed by a fast step of Ag complexation from the decomposition solids. The alkaline decomposition is characterized by the simultaneous removal of sulfate and K ions and the formation of an amorphous hydroxy-arsenate of Fe, Pb and Ca. The kinetics are chemically controlled, with an activation energy of 86.5 kJ mol⁻¹. The nature of the alkaline decomposition of beudantite was similar but extremely slow at ≤100°C.