The mechanism of ferrite formation from iron sulfides during zinc roasting

The formation of zinc ferrite (ZnFe₂O₄) during the fluidized-bed roasting of zinc concentrates presents subsequent processing difficulties both for zinc recovery and for iron separation and disposal. A major source of iron in these concentrates is from the iron sulfides — pyrite and pyrrhotite. This study examined the changes undergone by these iron minerals when roasted together with sphalerite at 1223 K in a fluidizing gas mixture of 3 pct oxygen and 97 pct nitrogen. Optical microscopy and electron microprobe analysis were employed to identify the three stages that lead to ferrite formation and to examine the processes that occur within each stage. The first stage is oxidation of the sulfides to highly vesicular, amorphous magnetite particles containing small amounts of zinc. The second stage involves both densification of these particles by sintering and counterdiffusion of iron and zinc cations to form a continuous phase of homogeneous zinc-rich spinel and a precipitate of hematite. In the third stage, continuation of cation diffusion and increasingPo ₂ results in the formation of stoichiometric zinc ferrite. These observations have been interpreted by reference to the established phase relationships that occur in the Zn-Fe-O system, and a detailed, solid state reaction mechanism for the formation of zinc ferrite has been proposed.     

The mechanism of ferrite formation from iron sulfides during zinc roasting

The formation of zinc ferrite (ZnFe₂O₄) during the fluidized-bed roasting of zinc concentrates presents subsequent processing difficulties both for zinc recovery and for iron separation and disposal. A major source of iron in these concentrates is from the iron sulfides — pyrite and pyrrhotite. This study examined the changes undergone by these iron minerals when roasted together with sphalerite at 1223 K in a fluidizing gas mixture of 3 pct oxygen and 97 pct nitrogen. Optical microscopy and electron microprobe analysis were employed to identify the three stages that lead to ferrite formation and to examine the processes that occur within each stage. The first stage is oxidation of the sulfides to highly vesicular, amorphous magnetite particles containing small amounts of zinc. The second stage involves both densification of these particles by sintering and counterdiffusion of iron and zinc cations to form a continuous phase of homogeneous zinc-rich spinel and a precipitate of hematite. In the third stage, continuation of cation diffusion and increasingPo ₂ results in the formation of stoichiometric zinc ferrite. These observations have been interpreted by reference to the established phase relationships that occur in the Zn-Fe-O system, and a detailed, solid state reaction mechanism for the formation of zinc ferrite has been proposed.     

Sintering and ferrite formation during high temperature roasting of sulfide concentrates

The newly developed toroidal fluidized bed reactor has potential for improving sulfide roasting efficiency due to its unique and good mass/heat transfer characteristics. To achieve effective roasting at high temperature using the toroidal fluidized bed reactor, engineering issues associated with sintering and ferrite formation are investigated in this study by roasting industrial sulfide concentrates of zinc and copper. Laboratory tests are conducted at 800–1100°C by using an electric tube furnace under controlled roasting conditions. The test program is based on statistical design, and the products are characterized by XRD, SEM and EDX. It is revealed that the oxygen concentration plays an important role in promoting roasting. Roasting temperatures higher than 950°C do not favor roasting conversion due to the sintering of sulfide particles which inhibits oxygen diffusion into the particle core. Close contact of the oxidized particles favors the atomic interdiffusion between particles, leading to the formation of undesired zinc ferrite. Roasting in toroidal fluidized bed reactor results in zinc calcines with increased surface area and reduced zinc ferrite formation, thus improving zinc recovery.     

锌窑渣氧化气固相反应过程分析

基于锌窑渣氧化气固相氧化反应的脱硫曲线,分析了氧化脱硫过程有明显差异的温度范围。对锌窑渣中Cu、Zn及Fe的物相组成在不同温度范围内的变化进行了研究,并据此讨论了氧化过程中Cu、Zn及Fe的行为和反应机制。结果表明：大部分的硫化锌及硫化亚铁在398K-1073K的温度范围内将被氧化,而较多的硫化亚铜是在1073K-1373K的温度范围内被氧化。铜、锌氧化产物均易与铁氧化产物化合为铁酸盐。氧化过程中形成的铁酸锌不利于锌的挥发,仅当温度大于1073K时锌才会挥发,此时铁酸锌含量已达到高温分解与化合平衡含量。     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.     

Sintering and Ferrite Formation During High Temperature Roasting of Sulfide Concentrates

Abstract

The newly developed toroidal fluidized bed reactor has potential for improving sulfide roasting efficiency due to its unique and good mass/heat transfer characteristics. To achieve effective roasting at high temperature using the toroidal fluidized bed reactor, engineering issues associated with sintering and ferrite formation are investigated in this study by roasting industrial sulfide concentrates of zinc and copper. Laboratory tests are conducted at 800–1100°C by using an electric tube furnace under controlled roasting conditions. The test program is based on statistical design, and the products are characterized by XRD, SEM and EDX. It is revealed that the oxygen concentration plays an important role in promoting roasting. Roasting temperatures higher than 950°C do not favor roasting conversion due to the sintering of sulfide particles which inhibits oxygen diffusion into the particle core. Close contact of the oxidized particles favors the atomic interdiffusion between particles, leading to the formation of undesired zinc ferrite. Roasting in toroidal fluidized bed reactor results in zinc calcines with increased surface area and reduced zinc ferrite formation, thus improving zinc recovery. © 2000 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Mining and Metallurgy. All rights reserved.

Résumé

Le réacteur toroïdal à lit fluidisé, développé récemment, a le potentiel d'améliorer l'efficacité du grillage du sulfure à cause de ses caractefristiques uniques de bon transfert de masse/chaleur. Afin d'obtenir un grillage efficace à haute température avec le reacteur toroïdal à lit fluidisé, on a examiné,dans cette étude, des questions d'ingénierie associées avec le frittage et la formation de ferrite, en grillant des concentrés industriels de sulfure de zinc et du cuivre. On a effectué des épreuves de laboratoire entre 800–1100°C avec une fournaise électrique tubulaire sous conditions controlées de grillage. Le plan d'évaluation est basé sur la conception statistique. On a caratérisé les produits par XRD, SEM et EDX. On revele que la concentration d'oxygene joue un role important dans l'activation du grillage. Les temperatures de grillage de plus de 950°C ne favorisent pas la transformation du grillage à cause du frittage des particules de sulfure, ce qui inhibe la diffusion d'oxygene au coeur de la particule. Le contact etroit entre les particules oxydees favorise l'inter-diffusion atomique entre les particules, conduisant à la formation de ferrite de zinc indesirable. Le grillage dans le reacteur toroidal à lit fluidise resulte en zinc de calcination ayant une plus grande superficie et en une réduction de la formation de ferrite de zinc, améliorant ainsi la récuperation du zinc. © 2000 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Mining and Metallurgy. All rights reserved.     

Oxidative roasting of covellite with minimal retardation from the CuO⋅CuSO4 film

Copper (II) sulfide can be efficiently converted to the oxide at lower temperatures than normally required in aerobic roasting by a new method involving programed environment roasting (PER). When heating was conducted in absence of oxygen up to about 650°C, and then nitrogen was replaced by air, the sulfide was easily converted to oxide without need for further increase in temperature. Traditional oxidative roasting of chalcocite required a temperature range of 800° to 850°C for conversion to tenorite. Unlike the situation with conventional roasting, CuSO₄ was not detected in the X-ray diffractogram of the product obtained with the PER method above 625°C. Also, the amount of CuO ⋅ CuSO₄ significantly decreased as the halt temperature in the PER process increased from 600° to 700°C. Apparently the shell of copper oxysulfate is impervious to oxygen and/or sulfur dioxide and delays the formation of tenorite until the sulfate and oxysulfate are decomposed. If the oxysulfate stage were bypassed with an inert atmosphere, then, even if small amounts of this salt were formed upon introducing the oxidant, it would decompose at an appreciable rate and the impedance of its thin film to gaseous transport would be considerably diminished. By contrast, the accelerating effect of externally added iron on the oxidative roasting of covellite was confined to the low temperature reactions and hence iron promoted the extent of sulfate formation. Iron did not, however, lower the thermal requirement for complete oxidation of CuS or Cu₂S because it had virtually no effect on the thermal decomposition of CuO ⋅CuSO₄.     

Caustic roasting and leaching of electric arc furnace dust

Electric arc furnace (EAF) dust is produced when iron and steel scrap is remelted in an electric arc furnace. There are still significant problems associated with the pyrometallurgical and/or hydrometallurgical processes for the treatment of this dust. In the present research, the dust was roasted with caustic soda at low temperatures. It was found that the zinc ferrite (ZnFe₂O₄) in the dust was converted into sodium zincate (Na₂ZnO₂) and iron oxide (Fe₂O₃). In the subsequent dilute caustic leaching process both the zincite (ZnO) and the sodium zincate were soluble and the hematite was relatively insoluble. After roasting and leaching, the zinc recovery was found to be about 95%, while the majority of the iron oxide remained in the leach residue. The lead, cadmium and chromium recoveries were approximately 85, 89 and 37%, respectively, for roasting tests in which moisture was added. Without a moisture addition in roasting, the lead recovery decreased to 63% while the chromium recovery increased to 81%. Based on the experimental results, a hybrid low temperature roasting and dilute caustic leaching process followed by zinc cementation and electrowinning is proposed and discussed. This proposed hybrid EAF dust treatment process provides some potential advantages in comparison to the other hybrid processes which have been described in the literature.     

A Visual Insight into the Oxidation of Sulfide Minerals During Bioleaching and Chemical Leaching of a Complex Ore

A scanning electron microscope (SEM) study was performed to provide a visual insight into the oxidation patterns of sulfide minerals during chemical and bacterial leaching of a complex ore for 3 days. The mineral grains were studied under SEM before and after bacterial and chemical leaching with or without the addition of ferrous iron to generate ferric iron in situ by bacteria or chemical oxidant (MnO₂). Both mesophilic and moderately thermophilic cultures of bacteria were used in bioleaching tests. A limited oxidation of sphalerite and pyrite, similar to those in acid leaching (control), was observed to occur when no ferrous iron was added. However, the initial addition of ferrous iron into bioleaching media was shown to significantly improve the oxidation of sphalerite and pyrite. Galena was readily oxidized in the presence or absence of bacteria. Sphalerite was oxidized more extensively/selectively than chalcopyrite and pyrite, consistent with their respective nobility/electrochemical activity. Provided that chemical/biological oxidation of sphalerite was intensive, a sulfur-rich layer appeared to form on mineral surface. But, no such layer on pyrite surfaces was discernable. Supplementary bioleaching data were also provided to support SEM observations and to further elucidate the bioleaching characteristics of these sulfide phases. It can be inferred from this study that the oxidation of sulfides proceeds most discernibly via “indirect mechanism” and the generation of ferric iron by bacteria in sufficient quantity is essential for the effective oxidation of sulfide minerals.     

Dissolution of zinc ferrite samples in acids

The dissolution of rotating discs of synthetic zinc ferrite — the principal constituent of the ‘Moore Cake’ residue in zinc extraction plants — was studied in mineral acids, particularly in 1–5 N H₂SO₄ at 70–99°C. This dissolution was found to be directly proportional to the surface area, and the order of the zinc ferrite-sulphuric acid reaction with respect to proton activity, [H⁺], to be 0.6. The apparent energy of activation was established as 15 kcal/mole, and the chemical reaction on the solid surface as the rate-controlling step.What appeared to be ‘non-stoichiometric’ or preferential dissolution of zinc (over iron) from zinc ferrite was observed during the initial stages of reaction. This was attributed to the existence of trace amounts (undetectable by X-ray methods) of unreacted zinc oxide grains in the zinc ferrite matrix. This is, to our knowledge, the first time that electron microprobe analysis has been used to identify and analyse these grains. Prolonged sintering at 1200°C for 48 hours eliminated the ZnO phase.Dissolution of zinc ferrite in acid is stoichiometric. A typical dissolution rate is ～ 10^?8 mol cm^?2 sec^?1, which corresponds to almost complete extraction of zinc from ‘Moore Cake’ particles in 2–5 N H₂SO₄ solution at 95°C in 1–2 hours.     

锌焙砂的选择性还原焙烧硫酸浸出工艺研究

研究了选择性还原焙烧-硫酸浸出两段工艺处理高铁锌焙砂的方法.首先在CO还原气氛下将锌焙砂中的铁酸锌选择性转化为氧化锌和磁铁矿,然后采用硫酸浸出使可溶锌溶出而铁存留于渣中,实现铁锌有效分离.主要考察了还原焙烧以及硫酸浸出的工艺条件对铁锌分离效果的影响,并采用化学分析法及XRD、SEM-EDS的检测手段对焙烧样品进行分析.以可溶性锌和亚铁的含量作为焙烧评价指标,得出最佳焙烧条件为:焙烧温度750℃,焙烧时间60 min,CO浓度8%,CO/(CO+CO2)气氛比例20%,此条件下可溶锌率由原焙砂中的79.64%提高到91.75%;以铁锌浸出率为考察指标,得出最佳浸出条件为∶常温浸出,浸出时间30 min,浸出酸度90 g/L,液固比10∶1,此条件下锌铁浸出率分别为91.8%和7.17%.     

The mineralogical deportment of germanium in the Clarksville Electrolytic Zinc Plant of Savage Zinc Inc.

A mineralogical study was carried out on the neutral leach residue and weak acid leach residue generated from Gordonsville zinc concentrate at the Clarksville Electrolytic Zinc Plant of Savage Zinc Inc. The intent was to characterize the mineral forms and associations of germanium. The Gordonsville zinc concentrate consists mostly of sphalerite which has a solid solution Ge content of ~400 ppm; the sphalerite is the dominant, if not only, Ge carrier in the concentrate. The neutral leach residue consists principally of iron gel-silica gel, ZnO, and basic zinc sulfate, (Zn,Cu)₄(SO₄)(OH)₆·4H₂O, together with minor amounts of ZnFe₂O₄, sphalerite, Zn₂SiO₄, Zn-Fe-Pb silicate, and PbSO₄, as well as traces of quartz, silicates, Pb-K jarosite solid solution, Fe₂O₃, and FeO·OH. The major Ge carrier is the iron gel-silica gel phase, but modest amounts of Ge are present in the ZnO, ZnFe₂O₄, sphalerite, and Zn-Fe-Pb silicate phases. The weak acid leach residue consists mostly of iron gel-silica gel, ZnFe₂O₄, PbSO₄, Pb-K jarosite, Zn-Fe-Pb silicate, and quartz. The major Ge carrier is the iron gel-silica gel phase which contains up to 1.7 pct Ge and accounts for ~70 pct of the total Ge content of this residue. The remaining Ge is carried by the Zn-Fe-Pb silicate, ZnFe₂O₄, and some of the rare Mn-Pb-Fe oxide phases.     

Fluidized Bed Selective Oxidation-Sulfation Roasting of Nickel Sulfide Concentrate: Part II. Sulfation Roasting

The fluidized bed sulfation roasting process followed by water leaching was investigated as an alternative process to treat nickel sulfide concentrate for nickel production. The effects of several roasting parameters, such as the sulfation gas flow rate, roasting temperature, the addition of Na₂SO₄, and the roasting time, were studied. 79 pct Ni, 91 pct Cu, and 95 pct Co could be recovered with minimal dissolution of Fe of 4 pct by water leaching after two-stage oxidation-sulfation roasting under optimized conditions. The sulfation roasting mechanism was investigated, showing that the outermost layer of sulfate melt and the porous iron oxide layer create a favorable sulfation environment with high partial pressure of SO₃. Sulfation of the sulfide core was accompanied by the conversion of the sulfide from Ni_1?x S to Ni₇S₆ as well as inward diffusion of the sulfation gas.     

Thermal treatment of complex sulfide ores in N2 and H2 Atmospheres: A new approach for the extraction of their valuable elements

The thermal treatment of natural sulfidic minerals such as sphalerite, galena, chalcopyrite, and pyrite in an N₂ or H₂ atmosphere was studied to examine the nature of reactions taking place. Such treatments have the potential of avoiding sulfur dioxide production which is associated with the roasting of complex sulfide ores (CSOs). The thermal treatment of CSO concentrates at temperatures less than 1000 °C in a nitrogen atmosphere leads to the decomposition of the pyritic matrix to pyrrhotite and the volatilization of sulfur, galena, and some of the CSOs’ trace elements. Treating the CSO in a reducing atmosphere converted sphalerite to zinc and produced a solid containing Cu^o, Fe^o, and silicoaluminates. Selective dissolution of copper may be achieved by a hydrometallurgical process. Hydrogen sulfide could be reacted with pyrrhotite to form pyrite and hydrogen. A flow sheet is proposed. M.-CH. Meyer-Joly formerly Researcher with Mineral Processing and Environmental Engineering, Institut National Polytechnique de Lorraine K. MALAU formerly Researcher with Mineral Processing and Environmental Engineering, Institut National Polytechnique de Lorraine     

A kinetic study on the pressure leaching of sphalerite

The dissolution of sphalerite (ZnS) in sulfuric acid solution under oxygen pressure was investigated. Effects of temperature, percent solids, agitation, sample size, oxygen partial pressure and foreign ions were evaluated. The effect of hydrogen pretreatment on sphalerite leaching rate was also examined. Leaching of sphalerite at 90°C and 150 psi oxygen pressure was found to occur at a constant rate. This rate was determined from the experimental data observed under the different leaching conditions mentioned above. The constant leaching rate was attributed to the chemical reaction occurring on the surface of the flat-plate type sphalerite sample. The rate-controlling step of the reaction was determined to be the oxidation of hydrogen sulfide to elemental sulfur. Oxidation of hydrogen sulfide was studied through the addition of iron and through the observation of the change in iron concentration during leaching. The oxidation was concluded to be by reaction with ferric ion rather than by direct oxygen oxidation. Leaching tests run with samples pretreated with hydrogen do not show any increase in the rate of zinc extraction. M. T. HEPWORTH, formerly with University of Denver.     

Oxidation of mixed copper-iron sulfide

. A rectangular plate of mixed copper-iron sulfide composed of bornite (Cu₅FeS₄) and troilite (FeS) was oxidized in an O₂-Ar mixed gas stream at 1023 to 1123 K. At the start of the oxidation, iron was preferentially oxidized with the rapid formation of a dense Fe₃O₄ layer of about 10 μm thickness on the sample surface, without the evolution of SO₂ gas. Following this reaction, layers of both Fe₃O₄ and Fe₂O₃ grew on the sulfide surface in accordance with the parabolic rate law. The diffusion of iron through the oxide layers was assumed to control the oxidation rate during this stage. The effect of oxygen partial pressure on the parabolic rate constants was minor and an apparent activation energy of 126 kJ/mol was obtained. During the later stages of the reaction, when the sulfur activity in the inner sulfide core increased, the oxidation proceeded irregularly to the interior of the remaining sulfide with the formation of a porous oxide and the evolution of gaseous SO2. The remaining sulfide core was found to be a mixture of bornite (Cu₅FeS₄) and djurleite (Cu_1.96S). H. TSUKADA, former Graduate Student at Kyoto University     

电炉粉尘锌元素回收利用基础分析

通过对电炉粉尘进行化学成分、物相结构、粒度组成、颗粒形貌等基础物性进行分析,可知电炉粉尘中锌元素主要以铁酸锌(ZnFe2O4)形式存在,相较于ZnO而言ZnFe2O4不易还原,给电炉粉尘中锌的回收造成了困难。采用ZnO和Fe2O3化学纯试剂制备纯ZnFe2O4,通过XRD、DSC和SEM- EDS等方法对ZnFe2O4的碳热还原机理进行分析,探讨了温度和还原剂种类对ZnFe2O4碳热还原的影响。结果表明,ZnFe2O4的碳热还原分为3个阶段,即ZnFe2O4分解段、氧化锌还原段和铁氧化物还原段,第1阶段主要为ZnFe2O4的分解反应,第2阶段主要为ZnO的还原反应,并伴随少量铁氧化物的还原反应,第3阶段主要为铁氧化物还原和碳气化反应;在反应初期,煤粉转化率大于石墨,而在反应后期则相反。     

The Dissolution of Sphalerite in Ferric Chloride Solutions

The kinetics of dissolution of both sintered sphalerite disks and untreated sphalerite particles in ferric chloride-hydrochloric acid solutions have been investigated. Over the temperature interval 25 to 100°C, the dissolution occurred according to a linear rate law and with an associated apparent activation energy of about 10 kcal/mole. Most of the oxidized sulfide ion reported as elemental sulfur in the leach residues. The leaching rate was independent of the disk rotation speed and this fact, together with various hydrodynamic calculations, indicated that the reaction was chemically controlled. The dissolution rate increased as the 0.36 power of the ferric chloride concentration and it also increased substantially in the presence of dissolved CuCl₂. The accumulation of the ferrous chloride reaction product severely retarded the leaching reaction, but the presence of dissolved zinc chloride only slightly impeded it. The leaching rate was relatively insensitive to low levels of HC1 (>1 M), but increased dramatically at higher acid concentrations because of direct acid attack of the ZnS.     

铁矾渣熔化-烟化过程中铅元素的挥发性质研究

采用火法中的熔化-烟化法回收铁钒渣中的有色金属铅，并通过热力学计算在还原焙烧过程中金属铅所需的反应条件。探究回收铅过程中不同工艺参数对铅回收率的影响，并确定回收的较优工艺条件。用扫描电子显微镜、能谱仪和X射线衍射技术对收集到烟尘的形貌、成分和结构进行表征。结果表明:铁钒渣中的铅物相在熔化-烟化过程中，当温度高于1 000 ℃时，PbS最先开始挥发进入烟尘中；烟尘中回收的铅主要以PbO和PbS形式富集，其中PbO是单质铅挥发后被氧化的产物，呈四棱柱形状；在较优反应温度1 242 ℃下铅的回收率达99.70%。