A two-dimensional finite difference model has been developed to describe the reduction of kilogram quantities of dehydrated phosphogypsum. The model's scope has been limited to focus on the heat transfer and reactions that occur within a mass of material contained in a vessel inside a furnace rather than also including the effects of heat transfer to the vessel. Changes in the heat transfer properties (k, p, and Cp) are incorporated as the composition of the mass changes as the chemical reactions progress. The model is validated against experimental data, with samples heated to iooo°C at 3°C min-1 while purging with nitrogen gas. A sensitivity analysis of model predictions to the pre-exponential factor of the reaction rate constant of the main chemical reaction and the thermal conductivity of the powder bed indicated that, at the envisaged process conditions, the behaviour of the system depends much more on the rate of heat transfer than on the rate of the chemical reaction. The model demonstrated a significant increase in accuracy when the thermal conductivity was modelled to increase linearly with temperature compared to assuming a constant value.

Abstract The obtaining of a flux for hardfacing by Submerged Arc Welding (SAW), using ferrochrome-manganese and slag obtained from the simultaneous carbothermal reduction of chromite and pyrolusite is addressed. The ferrochrome-manganese and the slag were obtained, conceiving that both products satisfy the requirements of the components (alloy system and matrix) of an agglomerated flux for hardfacing. The fusion-reduction process to obtain the alloy and the slag was carried out in a direct current electric arc furnace. The pouring was carried out into water to facilitate the separation and grinding of the cast products. An experimental flux was manufactured, using the obtained alloy and slag. Deposits were obtained by SAW, which were characterized in terms of: chemical composition, microstructure and hardness. It was concluded that the flux obtained from ferrochrome-manganese and slag from the simultaneous carbothermal reduction of chromite and pyrolusite, allows to deposit an appropriate metal for work under abrasion conditions, characterized by significant carbon and chromium contents and a martensitic microstructure predominantly, with hardness of 63 HRc.

Resumen Se aborda la obtención de un fundente para recargue duro por Soldadura con Arco Sumergido (SAW), empleando ferrocromo-manganeso y escoria obtenidos de la reducción carbotérmica simultánea de cromita y pirolusita. El ferrocromo-manganeso y la escoria fueron obtenidos, concibiendo que ambos productos satisfagan los requisitos de los componentes (sistema de aleación y matriz) de un fundente aglomerado para el recargue. El proceso de fusión-reducción para la obtención de la aleación y la escoria fue realizado en horno de arco eléctrico de corriente continua. El vertido fue realizado en agua para facilitar la separación y molienda de los productos de colada. Fue elaborado un fundente experimental, empleando la aleación y escoria obtenida. Fueron obtenidos depósitos por SAW, los cuales fueron caracterizados en cuanto a: composición química, microestructura y dureza. Se concluyó que el fundente obtenido a partir de ferrocromo-manganeso y escoria de la reducción carbotérmica simultánea de cromita y pirolusita, permite depositar un metal apropiado para el trabajo en condiciones de abrasión, que se caracteriza por significativos contenidos de carbono y cromo y una microestructura predominantemente martensítica, con dureza de 63 HRc.

ABSTRACT Rhenium is a metal that has different obtainment methods. The most used comes from molybdenite roasting. Traditional pyrometallurgical methods like oxidizing roasting are applied, but there are also innovative pyrometallurgical methods as is roasting with carbothermic reduction. This paper presents the behavior of rhenium onto two different roasting processes: oxidizing roasting and toasting with carbothermic reduction. The purpose is to compare both treatments discussing rhenium recovery values. Oxidizing roasting generates several SO2 emissions towards the environment when it is applied to metal sulphides. The carbothermal reduction is a process that uses CaO as sulfur capturer and C as a reductant agent, and eliminates the atmosphere pollution that would have been caused by SO2. Thus the optimum molar ratio of CaO with respect to Re and C, required to accelerate the reduction rate and control S emission to the atmosphere was studied in order to determinate metal sulfide dependence to operational factors of reduction temperature, molar concentrations, roasting times. Furthermore, the oxidizing roasting was worked out with the operating variables of temperature and roasting time for certain ambient conditions. Based on the current results for both roasting processes, the optimal operating conditions for the greatest rhenium recover were obtained.

RESUMEN El renio es un metal que presenta distintos métodos de obtención.El más utilizado es el que se lleva a cabo por la tostación de la molibdenita. En él se aplican métodos pirometalúrgicos tradicionales como es tostación oxidante, pero también existen métodos pirometalúrgicos innovadores como la tostación con reducción carbotérmica. En este trabajo se estudia el comportamiento que presenta el renio frente al uso de diferentes tostaciones, ellas son tostación oxidante y tostación con reducción carbotérmica. El objetivo es comparar ambos tratamientos discutiendo los valores de recuperación de renio. La tostación oxidante es un método que genera importantes emanaciones de SO2 al medio ambiente, cuando se aplica a sulfuros metálicos. En cambio, la reducción carbotérmica es un proceso que utiliza el CaO como agente captador de azufre y elC como reductor, junto al sulfuro metálico, eliminando de esta manera la contaminación de la atmósfera que había sido producida por el SO2. Con tal fin, se estudió la relación molar de CaO óptima con respecto al Re y C, requerida para acelerar la velocidad de reducción y controlar la emisión de S a la atmosfera. De esta manera se determinan la dependencia del sulfuro metálico con los factores operacionales de temperatura de tostación, concentraciones molares y tiempos de tostación. Por otro lado, en la tostación oxidante se trabajó con las variables operativas de temperatura y tiempos de tostación para determinadas condiciones atmosféricas. En base a los resultados obtenidos de ambas tostaciones se determinaron las condiciones óptimas de las operaciones para la mayor recuperación de Re.

ZrC-SiC nanopowders were prepared by carbothermal reduction of ZrO2 and SiO2 with carbon black in argon gas atmosphere, using microwave energy as an alternative heating source. The samples were characterized by XRD, SEM and BET surface area analysis. The results indicate that highly crystalline ZrC-SiC composites with a mean particle size of about 80 nm can be synthesized at a relatively low temperature (~1420 °C). Microwave-assisted heating to obtain the ZrC-SiC nanopowders significantly reduces the synthesis time to only 40 min when compared with the long reaction times involved in conventional carbothermal reduction, which requires about 8 hours to obtain ZrC and 2 hours to obtain SiC.

El proceso de reducción carbotérmica es una reducción directa de sulfuros. Permite realizar una tostación en donde se obtienen concentraciones mayores de metales como molibdeno, empleando un agente reductor (en este caso, el carbón C) y un captador de azufre como el CaO, evitando la contaminación del aire con gases tóxicos como el SO2. En este trabajo se analizan concentrados sulfurados ricos en cobre, hierro y molibdeno, obtenidos por flotación rougher y flotaciones diferenciales, dando origen a diferentes leyes. Los concentrados se sometieron al proceso de reducción carbotérmica en donde se estudiaron los parámetros de trabajo. Se ajustaron los valores del agente captador y reductor, para retener las mayores concentraciones de azufre. Analizando los resultados obtenidos se fijó la temperatura, el tiempo óptimo de trabajo y optimas relación molar del CaO y C para los cuales se alcanzó la mayor recuperación de metales como el molibdeno.

The carbothermic reduction process is a direct reduction of sulfides. It allows a roasting in which higher concentrations of metals such as molybdenum are obtained by using both a reducing agent (in this case, carbon C) and a sulfur scavenger, such as CaO so as to prevent air toxic gases pollution such as SO2. In this paper, we analyze sulfur concentrates rich in copper, iron, and molybdenum that are obtained through rougher flotation and differential floats, which give rise to different laws. The concentrates were subjected to a carbothermic reduction process in which the working parameters were studied. The values of both the capturing agent and the reducing agent were adjusted to retain higher concentrations of sulfur. Analyzing the results obtained, the temperature was set and we reached the optimal working time and optimum molar ratio of CaO and C for the greatest recovery of metals such as molybdenum.

This study investigates the recovery of water and the selective removal of valuable metals from acid mine drainage (AMD) using sulphidation media (CaS) derived from waste gypsum. AMD systems containing Fe(II), Ni, Co, Zn, and Pb were investigated using CaS produced from the carbothermal reduction of Anglo Coal waste gypsum at 1025°C to precipitate metals as insoluble metal sulphides. The results show a sulphidation dependence on the pH, sulphide dosage, and metal concentration. The selective sulphi-dation of metals also showed significant dependence on the respective metal sulphide solubility order as a function of pH. According to the Department of Environmental Affairs' South African Waste Information Centre, over 42 million cubic metres of general waste is generated every year in South Africa and mining waste is by far the biggest contributor to the solid waste (about 72%)). Although alarming, these vast quantities of waste also present an opportunity for integrated economic development, particularly in the recycling sector. The major argument has always been that the mining sector generates a large number of waste streams which show strong differences in time, in their treatment methodologies, or even in their spatial distribution. This paper presents a case of a simple strategy for integrated recycling of two mining waste streams and highlights the need for the mining industry to break away from the traditional 'linear' cul-de-sac disposal of wastes and think of new sustainable ways of waste management.

Se sintetizó nitruro de vanadio (VN) mediante la reducción carbotérmica de mezclas de óxido de vanadio (ex vanadato de amonio) y carbón activado en atmósfera de N2. El método de disolución utilizado para la mezcla de los precursores NH4VO3 y C, permitió obtener VN de morfología uniforme con tamaño de cristalita submicrométrica, a temperaturas inferiores a aquellas utilizadas en procesos convencionales. El producto obtenido fue caracterizado por difracción de rayos X (DRX), microscopía electrónica de barrido (MEB), espectroscopia de fotoelectrones de rayos X (XPS) y área superficial-BET. Para posible aplicación en catálisis, el producto sintetizado fue evaluado en la hidrodesulfuración (HDS) de tiofeno a presión atmosférica y T = 400°C, encontrándose que la actividad catalítica de los sólidos sintetizados durante 120 min. a 1100ºC fue mayor a aquella de polvos de VN obtenidos con tratamientos térmicos más cortos.

It has been synthesized vanadium nitride (VN) by carbothermal reduction of vanadium oxide (ex ammonium vanadate) and activated carbon mixtures in a N2 atmosphere. The solution-derived precursor method used to mix NH4VO3 and C promoted the formation of submicrometer and morphologically uniform crystallites of VN at a lower temperature in comparison with conventional methods. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and BET-surface area. In order to consider catalytic uses the synthesized product has been evaluated in the reaction of hydrodesulfuration of thiophene under atmospheric pressure and T = 400°C. It was found that the catalytic activity of the heat treated solids during 120 min. at 1100ºC was larger than those of VN powders obtained at shorter heat treatment times.

A rotating cylinder reactor was designed for the synthesis of niobium carbide powders at 1173 K. Niobium carbide, NbC, was prepared by carbothermal reduction starting from commercial niobium pentoxide powders. The reactor was heated using a custom-made, two-part, hinged, electric furnace with programmable temperature control. The design and operational details of the reactor are presented. The longitudinal temperature gradient inside the reactor was determined. Total reaction time was monitored by a gas chromatograph equipped with an FID detector for determination of methane concentrations. The results show that time of reaction depended on rotation speed. NbC was also prepared in a static-bed alumina reactor using the same conditions as in the previous case. The niobium carbide powders were characterized by X-ray diffraction and compared with commercially available products. Morphological, particle size distribution and surface area analyses were obtained using SEM, LDPS and BET, respectively. Therefore, the present study offers a significant technological contribution to the synthesis of NbC powders in a rotating cylinder reactor.

Nanofitas de óxido de estanho com diferentes estados de oxidação foram produzidas pelo método de redução carbotérmica a 1260 ºC. As nanoestruturas foram caracterizadas por difração de raios X, microscopia eletrônica de varredura e espectroscopia por dispersão de energia de raios X. O controle do estado de oxidação das nanoestruturas foi feito pela variação da atmosfera de síntese dentro do forno. Em atmosferas sem oxigênio os materiais obtidos foram nanofitas de SnO e de SnO2, enquanto em atmosferas com oxigênio (aproximadamente 10%) obteve-se apenas o crescimento de nanofitas de SnO2.

Tin oxide nanobelts in different states of oxidation were produced by carbothermal reduction at 1260 ºC. The nanostructures were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The state of oxidation of the nanostructures was controlled by the synthesis atmosphere inside the furnace. The materials obtained in atmospheres without oxygen were SnO and SnO2 nanobelts, while atmospheres with oxygen (about 10%) only allowed for the growth of SnO2 nanobelts.

The objectives of this work are to supply a basic background on nanostructured materials and also to report about the obtaining of nanoparticles, mainly, tin dioxide nanocrystalline particles (obtained by using the polymeric precursor method) presenting a high stability against particle growth due to the usage of a metastable solid solution. The synthesis and growth of SnO2 nanoribbons by a carbothermal reduction process are also discussed.

Neste trabalho foram utilizadas matérias-primas naturais para a obtenção de pós Al2O3/SiC através da redução carbotérmica das mesmas por um agente rico em carbono. Os aluminossilicatos estudados foram caulim, cianita e ilita-moscovita, sendo que os mesmos foram reduzidos ou por negro de fumo ou por grafite. Os pós resultantes da reação foram caracterizados por microscopia eletrônica de varredura e transmissão, difração de raios X e picnometria de hélio. Durante a reação, as diferenças existentes tanto na estrutura das matérias-primas quanto na composição química produziram pós com morfologia diversa. Foram encontradas estruturas como grandes aglomerados esféricos, whiskers, partículas e aglomerados fibrosos.

In this work natural raw materials were used to obtain Al2O3/SiC powders by carbothermal reduction. The aluminosilicates studied were kaolin, kyanite and illite-muscovite and the reductor agents were carbon black or graphite. The reaction powders were characterized by transmission and scanning electron microscopy, X-ray diffraction and helium picnometry. During the reaction, the differences among the raw materials structures as well as into the chemical composition produced powders with several morphologies. Big spherical agglomerates, whiskers, particles and fibrous clusters were found.