C.A. Pickles

Caustic roasting and leaching of electric arc furnace dust

Abstract 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.

Improved grindability and gold liberation by microwave pretreatment of a free-milling gold ore

Abstract The gravity concentration of gold is very efficient if the metal is fully liberated and the particle size of the gold is relatively coarse. Liberation is usually achieved by comminution, but due to the association of the gold with the other minerals in the ore, overgrinding occurs in conventional comminution circuits and slime generation leads to inefficient recovery. The liberation of minerals can be improved by adding grinding aids, which modify the mechanical properties of the ore and allow breakage at lower stress levels. In this research, microwave pretreatment was used to augment the grinding of a free-milling gold ore containing quartz, silicates and iron oxides. Under microwave irradiation, selective heating of the different mineral components resulted in thermal stresses that caused cracking. These intergranular and transgranular fractures were confirmed by scanning electron microscopy. After microwave processing, the grindability of the ore was improved and the crushing strength and the Bond Work Index were reduced by 31˙2% and 18˙5%, respectively. In addition to the enhanced grindability, gold was released from the matrix of the host minerals at a coarser size, resulting in a significant increase in free gold recovery by gravity concentration. For a gold ore with a head grade of 6˙4 gt−1, the gold recovery improved from about 28% to 40% after microwave pretreatment.

Kinetics of zinc ferrite leaching in caustic media in the deceleratory period

Abstract In this research, the role of zinc ferrite in the hydrometallurgical recovery of zinc in both primary zinc production and zinc-containing waste treatment processes is discussed. In the present work, experiments were performed on the caustic leaching of pure zinc ferrite. The effects of leaching time, caustic concentration and temperature were investigated in the deceleratory period. Atomic Absorption Spectroscopy (AAS) and X-Ray Diffraction (XRD) analyses were employed to analyze both the leach solutions and the residues. It was found that the percentage of decomposed zinc ferrite increased linearly with increasing leaching time. Although the decomposition rate increased with the caustic concentration, it was limited by the high viscosity of the concentrated leach solutions. The maximum percentage of decomposed zinc ferrite was only about 9% under the experimental conditions. Based on the experimental results, it was postulated that the dissolution rate of zinc ferrite in the deceleratory period, was controlled by the diffusion of zinc ions in the imperfect zinc ferrite lattice. The dissolution reaction could be represented as follows: ZnFe 2 O 4 +2[OH − ]=ZnO 2 2− +Fe 2 O 3 +H 2 O

Drying kinetics of nickeliferous limonitic laterite ores

Abstract The drying kinetics of a nickeliferous limonitic laterite ore were investigated in the temperature range of 44–228 °C, using a thermogravimetric technique, and the results were characteristic of a non-hygroscopic porous solid. For drying temperatures below 100 °C, the drying rate curves exhibited three stages: a relatively short incubation or set-up period, a constant rate drying period and a decelerating rate drying period. At temperatures above the boiling point of water, there was no constant rate drying period and only the incubation and the decelerating rate stages were observed. The graphical critical moisture fraction, X Cr , which is the moisture content at which the transition from the constant rate period to the decelerating rate period occurs, was found to be 0.064 and was independent of the drying temperature and thus the drying rate. Using the concepts of the relative drying rate (f) and the characteristic moisture content (Φ), a characteristic drying curve was determined for the ore. The drying kinetics, when the sample had attained the equilibrium temperature, exhibited non-Arrhenian behavior and the drying process was approximated using two linear relationships. The activation energies over the two temperature ranges were determined to be as follows: 25.5 kJ / mole 44 ° C ⩽T⩽100 ° C 12.9 kJ / mole 100 ° C ⩽T⩽228 ° C The activation energies were both relatively low and this indicates that the process was diffusion controlled in both temperature regimes. It is postulated that at temperatures below 100 °C, the rate-determining step was attributed to boundary layer control while at higher temperatures the rate-determining step was internal diffusion control. Effective diffusion coefficients (De) were calculated from the drying rate data and they varied with both concentration and temperature. At low moisture contents, a maximum was observed in the effective diffusion coefficient and this was explained in terms of a maximum in the vapour phase diffusivity at low moisture contents. At temperatures below the boiling point of water, the plateau at the maximum was extended and the diffusion coefficient was relatively constant over the concentration range X =0.02 –0.055. The maximum in the diffusion coefficient was found to vary with temperature as Tn, where n=1.8.

The soda ash smelting of lead-acid battery residue

Abstract A pyrometallurgical process for the recovery of lead from battery residue was investigated. The process involved pelletizing the residue with sodium carbonate follwed by a carbothermic smelting operation. The overall in the process is as follows: PbSO4(s) + Na2CO3(s) + C(s) = Pb(1) + CO(g) + CO2(g) + Na2 SO4 The sulphur dioxide emissions are minimal since the sulphur is fixed in the slag as sodium sulphate. Lead emissions are considerably reduced due to the use of pellets and a lower smelting temperature. The optimum conditions for the maximum recovery of lead were determined. These were: a smelting temperature of 1223 K, a charcoal addition of 3 mass% and a sodium carbonate addition of 20 mass percent. A moisture addition of 10 mass% during pelletizing was found to be sufficient to complete the sulphate-carbonate exchange reaction and to produce a strong green pellet. Sintering of the green pellets prior to reduction was found to have no effect on the lead recovery. However, a minimum in the strength and abrasion resistance of the pellets occurred at 773K. Lead recoveries of 96–98% were achieved and the lead content of the slag was in the range of 1–2 mass%.

Reduction of electric arc furnace dust with solid iron powder

Abstract Electric arc furnace (EAF) dust contains, on average, about 19mass% zinc and 3mass% lead. Due to the presence of this lead, as well as small amounts of cadmium and hexavalent chromium, EAF dust has been classified as a hazardous waste by various government regulatory agencies. Currently, most dust treatment processes employ carbon as a reducing agent for the zinc oxide in the EAF dust. The zinc vapour that is produced can be condensed. In the present work, the reaction of EAF dust with iron was studied in an argon atmosphere using a thermogravimetric technique. First, a thermodynamic study was performed using the F*A*C*T computational system. Then, the effects of briquetting pressure, argon flowrate, briquette aspect ratio (l/ d ), iron particle size, amount of iron added, as well as additions such as lime, sodium chloride and zirconium oxide on the kinetics of the reduction reaction were investigated. It was found that, initially, the reaction was chemically controlled with an activation energy of 159 kJ/mol. Then, after a product layer had formed on the outer surface of the briquette, the reaction was controlled by the transport of the zinc vapour away from the reaction interface.

A study of reduction and magnetic separation of iron from high iron bauxite ore

Abstract The removal of iron as magnetite from a high iron bauxite ore was investigated by carbothermal reduction and magnetic separation. Thermodynamic calculations using HSC Chemistry 6·1 were utilised to determine the equilibrium compositions of the reaction products as a function of temperature and carbon additions. Under equilibrium conditions, the formation of hercynite (FeAl2O4) restricted the conversion of haematite to magnetite and the equilibrium calculations were adjusted so that the amount of hercynite was limited. Also, thermogravometric analysis and differential thermal analysis with evolved gas analysis tests were performed in order to elucidate the reduction reactions. Furthermore, high temperature reduction experiments were carried out in order to quantify the effects of processing time, temperature and carbon additions on the amount of the magnetic fraction. The degree of removal of the iron and the alumina recovery were determined. For a given set of conditions, the maximum iron removal as magnetite could only be achieved by quenching the reacted samples. The results demonstrated that at a reduction temperature of ∼1073 K, ∼58% of the iron could be removed in the magnetic fraction with an alumina recovery of ∼85% in the non-magnetic fraction. On a examiné l’enlèvement du fer sous forme de magnétite d’un minerai de bauxite à haute teneur en fer, au moyen de la réduction carbothermique et séparation magnétique. On a utilisé des calculs de thermodynamique avec ″HSC Chemistry 6·1″ pour déterminer les compositions d’équilibre des produits de la réaction en fonction de la température et des additions de carbone. Sous les conditions d’équilibre, la formation d’hercynite (FeAl2O4) restreignait la conversion d’hématite en magnétite et l’on a ajusté les calculs d’équilibre afin de limiter la quantité d’hercynite. On a également effectué des essais de TGA-DTA avec EGA afin d’élucider les réactions de réduction. De plus, on a exécuté des expériences de réduction à haute température afin de quantifier l’effet de la durée de traitement, de la température et des additions de carbone sur la quantité de fraction magnétique. On a déterminé le degré d’enlèvement du fer et de la récupération d’oxyde d’aluminium. Pour un ensemble donné de conditions, on peut obtenir l’enlèvement maximal de fer sous forme de magnétite seulement par trempe de refroidissement des échantillons réagis. Les résultats ont démontré qu’à une température de réduction d’environ 1073 K, on pouvait enlever environ 58% du fer dans la fraction magnétique, avec une récupération de l’oxyde d’aluminium d’environ 85% dans la fraction non magnétique.

Carbide and hardness development in the heat-affected zone of tempered and postweld heat-treated 2.25Cr-1Mo steel weldments

The temper-bead and the conventional weaving, multipass welding procedures have been developed to eliminate postweld heat treatment (PWHT) of creep resistant 2.25Cr-1Mo steel. Both procedures aim to refine and temper the heat-affected zone (HAZ). The temper-bead procedure resulted in a martensitic HAZ and a homogeneous fine-grain size. Hardness was not decreased by high heat input weaving fillout passes. Upper bainite developed in the conventional weaving HAZ, although grain refinement was inhomogeneous and some martensite was present. However, the conventional weaving procedure appears to produce a lower as-welded hardness than the temper-bead procedure.The carbides within the temper-bead HAZ aged more rapidly than those in the conventional weaving HAZ as a result of the initial martensitic temper-bead HAZ microstructure. Previous work indicated that the temper-bead HAZ toughness decreased after 1000 h of tempering at 538°C. This correlated with the coarsening and the agglomeration of the carbides.The maximum hardness occurred within the as-welded coarse-grained HAZ. The PWHT resulted in the greatest decrease in hardness and also reduced hardness variability throughout the HAZ. The hardness decreased to postweld heat-treated values after approximately 1000 h at 538°C and softening was associated with the precipitation and coarsening of acicular carbides and the development of coarse grain-boundary carbides.

Separation of HfCl4 from ZrCl4 by reaction with solid and liquid alkali chlorides under non-equilibrium conditions

Abstract The separation of hafnium tetrachloride (HfCl4) from zirconium tetrachloride (ZrCl4) by preferential reaction of the hafnium tetrachloride vapours with either solid or molten alkali chlorides was investigated in a flowing system. Thermodynamic models for the separation processes for both the solid and the liquid alkali chloride reagents are discussed. Experiments were performed under non-equilibrium conditions by passing the Zr(Hf)Cl4 vapours, in an inert carrier gas, over both the solid and the liquid alkali chlorides. The effects of gas flow rate, reaction temperature, ratio of Zr(Hf)Cl4 to alkali chloride, type of alkali chloride and, in the case of solids, the particle size of the alkali chloride were studied. The separation efficiency increased with decreasing argon flow rate, decreasing ratio of Zr(Hf)Cl4 to alkali chloride, decreasing particle size and increasing temperature. In the case of the liquid salt reactor, the separation efficiency was higher than for the solid system for the same recovery of the Zr(Hf)Cl4 vapours.

MICROWAVE DRYING OF NICKELIFEROUS LIMONITIC LATERITE ORES

Abstract Nickeliferous laterite ores contain large amounts of both free and combined water which are removed during processing for metal extraction. In this study, the application of microwaves for the drying of a briquetted nickeliferous limonitic laterite ore, was investigated. Microwave drying tests were performed at a frequency of 2460 MHz and the sample mass was continuously monitored in order to determine the drying rate. The variables investigated were sample mass, microwave power, briquette aspect ratio, specific surface area and compaction pressure. For comparison purposes, conventional drying tests were performed in the temperature range of 44 to 228°C. It was found that the microwave drying rates were two to three times higher than the conventional drying rates. This was attributed to the inverse temperature gradient with microwave heating which promoted enhanced mass transfer of the water. Les minerais nickélifères de latérite contiennent de grandes quantités d’eau tant à l’état libre que combinées qui sont enlevées lors du traitement d’extraction du métal. Dans cette étude, on a investigué l’utilisation des micro-ondes pour le séchage d’un minerai nickélifère briqueté de latérite limonitique. On a effectué des essais de séchage par micro-ondes à une fréquence de 2460 MHz alors que la masse de l’échantillon était continuellement surveillée afin de déterminer le taux de séchage. Les variables étudiées incluaient la masse de l’échantillon, la puissance du micro-onde, le rapport longueur/diamètre de la briquette, la surface spécifique et la pression de compaction. Pour le but de comparaison, on a effectué des essais de séchage conventionnel dans la gamme de température de 44 à 228°C. On a trouvé que les taux de séchage par micro-ondes étaient deux ou trois fois plus élevés que les taux de séchage conventionnel. Ceci était attribué au gradient inverse de température du chauffage par micro-ondes qui favorisait un transfert amélioré de la masse d’eau.