Pyrometallurgical Roasting

The pyrometallurgical roasting of sulfide ores and concentrates is an important process step in the production of metals and chemicals. In this roasting process sulfides of iron, zinc, copper and other valuable metals are oxidized with air in the temperature range from 873 to 1273 K into metal oxides and gaseous sulfur dioxide. After cleaning and cooling, the sulfur dioxide contained in the roasting gas is further processed to produce sulfuric acid. Heat recovery is essential part of the entire roasting processes, both in the complete or partial roasting of sulfides. Sulfide ores and concentrates roasting in the state-of-the-art fluidized bed reactors combine efficient heat recovery and off-gas treatment, including the process of converting the off-gas into sulfuric acid [29]. Heat is recovered from the exothermic processes such as dead roasting of Zn concentrates mainly in the form of high-pressure steam production.

Experimental Section Materials Preparation

The ternary phase AgBiS2 was synthesized from a mixture of fine powders of 99.9+ % pure Ag2S and 99.999% pure Bi2S3, which were both purchased from

Alfa Aesar (Germany). The pure binary phases in powder form were mixed in the appropriate composition, pressed into a pellet, sealed in an evacuated fused silica tube, and then annealed in a muffle furnace at T = 673 K for three days and at T = 873 K for twelve days. The synthetic phase was quenched in ice water, while the fused silica glass tube holding it was intact. A sample of the synthesized material was prepared and analyzed by the SEM-EDS techniques. The SEM-EDS analyses confirmed the existence of a single homogenous ternary phase, with an approximate composition of AgBi098S195. X-ray Powder Diffraction (XRPD) analysis of the synthesized sample also confirmed that there were no impurity phases within 0.05 mass fraction XRPD detection limits.

In order to synthesis the ternary phase Ag0.93Cu107S, appropriate compositions of fine powders of 99.9 + % pure Ag2S and 99.5% pure Cu2S were weighed and mechanically well-mixed, in excess of Cu2S. The mixture of the two binary phases were pressed into pellets, sealed in an evacuated fused silica tube and annealed in a muffle furnace at T = 573 K for three days and at T = 873 K for ten days. In an attempt to homogenize the sample as well as to make sure traces of Ag2S may not exist, it was heated up to T = 1173 K and then cooled down to T = 338 K. The sample was then annealed at 338 K for three weeks. The synthesized material was analyzed by the SEM-EDS technique. The SEM-EDS analyses confirmed the existence of a homogenous ternary phase with a composition of the mineral stro- meyerite, Ag093Cu107S, coexisting with excess of Cu2S.

Thermal Analysis

Thermal analyses of the synthesized samples were done by a simultaneous Differential Thermal Analysis (DTA)- Thermogravimetric Analysis (TGA) analyzer using TA Instruments SDT Q600. The calorimeter was calibrated with the melting temperatures of high purity zinc, aluminum, and gold. The average measurement accuracies of temperatures were determined to be ±2 K.

Two calorimetric measurements were performed for each phase. Mass change and heat flow during the linear heating and cooling in all runs were measured simultaneously. The runs were performed in a synthetic air with a composition of 20% O2/80% N2 gases. In all runs the furnace was heated to 1173 K and then cooled to 573 K. The heating and cooling rates were 5 K min 1, and the flow rate of the gas was 100 ml min-1. Al2O3 crucibles were used as a sample holder and reference in all runs. 69.1 mg of fine powders of Ag0 93Cu107S, and 50.9 mg of fine powders of AgBiS2 were analyzed separately in the different runs.

 
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