The reduction of Tokyo and Sinter 75 nickel oxides with hydrogen

Industrial Tokyo and Sinter 75 nickel oxides were reduced with 35 and 80% volume hydrogen-argon mixtures between 350 and 1000 °C in a TGA apparatus. Several abnormalities were observed. At low temperature and in 35% H₂, an incubation period was observed. As the temperature was increased the rate of reduction increased up to some temperature, following which a decrease in the reaction rate was observed, which again was followed by an increase in the rates as the temperature exceeded 900 °C. It was found that reduction kinetics of these oxides strongly depend on the morphological features of the oxides. It became clear that the shrinking core model was not applicable and instead the kinetic parameters were assessed by the grain model.     

High temperature oxidation mechanism of dilute copper aluminium alloys

Dilute copper-aluminium alloys were oxidized in air from 700 to 1000 °C. Two distinctive behaviours were observed: alloys with at least 3 wt% aluminium showed excellent oxidation resistance in the whole temperature range. Alloys with 2 wt% or less aluminium exhibited good oxidation resistance up to 800 °C; but as the temperature was further increased, the oxidation rate of these alloys increased and became comparable to that of pure copper. A kinetic model was developed to explain the oxidation behaviour and indirectly determine the amount of dissolved oxygen in the alloys tested. It was found that the oxygen dissolved in alloys with up to 2 wt% Al exceeded its solubility limit in copper, whereas the dissolved oxygen in alloys with higher aluminium contents was below the solubility limit. This difference may account for the significantly different oxidation resistance.     

An analysis of slag stratification in nickel laterite smelting furnaces due to composition and temperature gradients

The density of FeO-MgO-SiO₂ and FeO-Fe₂O₃-SiO₂ based slags has been analyzed in terms of smelting of lateritic ores for the production of ferronickel. The density of these slags decreases with increasing MgO, SiO₂, and Fe₂O₃ contents as well as with increasing temperature. During the electric furnace smelting of calcined and prereduced garnieritic ores, the slag temperature decreases from the upper layer down toward the slag/metal interface. Together with precipitation of either olivine or silica, this leads to the formation of a dense and stagnant slag layer at the slag/metal interface. For limonitic ores, the use of deep electrode immersion and high currents leads to slag reduction and increased slag temperatures toward the bottom part of the slag layer. The reduction of Fe₂O₃ to FeO increases the slag density. In this manner, it may be possible to maintain a hot slag layer in the region of the slag/metal interface, without buoyancy-induced flow.     

Removal of Boron and Phosphorus from Silicon Using CaO-SiO2-Na2O-Al2O3 Flux

A combination of solvent refining and flux treatment was employed to remove boron and phosphorus from crude silicon to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper, and the alloy was subjected to refining by liquid CaO-SiO₂-Na₂O-Al₂O₃ slags at 1773 K (1500 °C). The distribution of B and P between the slags and the alloy was examined under a range of slag compositions, varying in CaO:SiO₂ and SiO₂:Al₂O₃ ratios and the amount of Na₂O. The results showed that both basicity and oxygen potential have a strong influence on the distributions of B and P. With silica affecting both parameters in these slags, a critical $ P_{{{\text{O}}_{2} }} $ could be identified that yields the highest impurity pick-up. The addition of Na₂O to the slag system was found to increase the distributions of boron and phosphorus. A thermodynamic evaluation of the system showed that alloying copper with MG-Si leads to substantial increase of boron distribution coefficient. The highest boron and phosphorus distribution coefficients are 47 and 1.1, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.3 times and 17 times mass of alloy would be required, respectively.     

Hydrogen reduction of oxidized nickel concentrates

Thermal gravimetric analysis was used to investigate the weight change of Ni/Cu/Co calcines upon heating in an inert as well as hydrogen atmosphere. The two calcines investigated contained approximately 50 wt pct combined of hematite and magnetite in addition to sulfides of Ni, Cu, Co, and Fe. Mass spectrometry was used to analyze the gas species evolved during heating and reduction. The calcine samples are 100 pct less than 100 μm with hematite/magnetite rims around a central sulfide core. When heating the calcines at 10 °C/min in hydrogen, reduction starts at around 400 °C and is nearly complete at about 700 °C with all the reducible oxygen removed. Isothermal reduction tests show that at temperatures from 650 °C to 800 °C, half the oxygen is removed in less than 4 min. The TGA results combined with microscopic analysis show that the reduction followed a uniform internal reduction model. The reduced calcines will quickly get re-oxidized if they are allowed to contact air while they remain hot.     

Roasting of Nickel Concentrates

The oxidation of three nickel concentrates from two Canadian smelters was studied by thermogravimetric analysis. Concentrate samples were heated to 1223 K (950 °C) in inert or oxidizing atmospheres to determine the reaction behavior. By recording the mass change as well as the SO₂ content in the outlet gas, the oxidation behaviors were quantified. Isothermal roasting tests were carried out on the concentrates over the temperature range of 673 K (400 °C) to 1123 K (850 °C). When heated in air, the samples gain mass as a result of sulfate formation at temperatures up to approximately 873 K (600 °C) to 973 K (700 °C), whereas at higher temperatures, the samples exhibit a large mass loss attributed to sulfate decomposition as well as direct SO₂ formation by oxidation. In a 4 pct O₂ gas atmosphere, significantly less sulfates were formed. Mixed reactions take place, in which some lead to mass loss and SO₂ generation, and others lead to mass gain and SO₂ consumption. The relative importance of the various reactions depends on the experimental conditions.     

Impurity removal and overall rate constant during low pressure treatment of liquid silicon

The research presented analyzes the effect of low pressure on the amount and reduction of impurity elements in upgraded metallurgical grade silicon. The achieved pressure was 5 kPa in the commercial electro-resistance furnace in the magnesia and mullite refractory material. The chemical composition was determined by ICP-MS method. Elements such as Al, Fe, Mn, Cu or Zn had the highest evaporation rates where higher evaporation was achieved at higher melt temperatures. The overall rate constant was deduced for four melt temperatures indicating high values even for low melt temperatures. The interfacial boundary between Si and mullite refractory showed no dissolution of Al into the liquid Si.     

Deoxidation of Liquid Copper with Reducing O<Subscript>2</Subscript>/CH<Subscript>4</Subscript> Flames

The rate of deoxidation of molten copper during top blowing with various reducing gases has been investigated using thermogravimetry. It was observed that the rate of deoxidation increases with an increasing flow rate of H₂ or CO and that H₂ is a more effective reducing reagent than CO. The rate of deoxidation using methane was measured for O₂/CH₄ ratios from 1.5 to 2.0. As expected, the deoxidation rate decreased with an increasing O₂/CH₄ feed ratio because the flame became less reducing. For all tests, initially there is a linear decrease in mass as oxygen is removed. However, for some experiments, after some time, a sudden acceleration in the rate of mass loss occurs. Using video and X-ray imaging, it was found that this pattern corresponded to gas evolution from within the molten copper. This finding can be explained by the sudden water vapor evolution because the hydrogen dissolved in the copper reacts with the remaining oxygen, and “boiling” takes place, leading to an enhanced stirring of the copper.     

Copper Removal from Hypereutectic Cu-Si Alloys by Heavy Liquid Media Separation

A high-capacity method for silicon refining is investigated. The biggest obstacle in Si refining using the alloying technique is the amount of solute element. In the current work, Cu is removed from two Cu-Si hypereutectic alloys by the heavy liquid media separation technique. The results indicated 86.0 pct silicon recovered, which is close to the theoretical limit. The chemical analysis showed a Cu concentration of 0.68 wt pct in the 50 Cu-Si alloy for a 75-μm average particle size after heavy liquid media separation. The optimal particle size was found in the range of 75 to 125 μm. Heavy liquid media separation is an efficient technique in the process of liberating Si dendrites that can be used as feedstock in solar cell applications.     

The oxidation resistance of copper-aluminum alloys at temperatures up to 1,000°C

In this study, the high-temperature oxidation resistance of copper and copper-aluminum alloys in air was investigated using thermo-gravimetric analysis. Upon heating in air, copper starts to noticeably oxidize at temperatures above 400°C. It was found that as the temperature increased, more aluminum was required in order to protect the copper. Alloying with 4 wt.% aluminum leads to a remarkable improvement in oxidation resistance. The atmosphere used to heat the samples to the required test temperatures had a noticeable impact on the subsequent oxidation rates. When heated in argon before being oxidized, copper alloys with 3 wt.% and 4 wt.% aluminum showed excellent oxidation resistance with rates 1,000 times less than that of pure copper at 1,000°C. However, when these alloys were heated in air, they were much less effective at providing oxidation resistance. For more information, contact Gabriel Plascencia, University of Toronto, Materials Science & Engineering Department, 184 College St., Toronto, ON, Canada, M5S 3E4; (416) 978-0912; fax (416) 978-4155; e-mail g.plascencia@utoronto.ca.