Niobium-tantalum concentrates processing

Tin slags account for a sizeable fraction of the world s tantalum production. These slags are melted in electric arc furnaces, together with coke and lime (as flux), and this pyrometallurgical process leads to the production of synthetic niobium/tantalum concentrates. The waste products of this operation are mechanically separated slags which can be used, for instance, as landfill. The exhaust gases from this process are of minor consequence if dust is eliminated by the use of filters. 

These three Nb-Ta sources can be processed to produce niobium-tantalum concentrates. 

Opa.nte. There are two methods used at various plants in Russia for loparite concentrate processing (12). The chlorination technique is carried out using gaseous chlorine at 800°C in the presence of carbon. The volatile chlorides are then separated from the calcium—sodium—rare-earth fused chloride, and the resultant cake dissolved in water. Alternatively, sulfuric acid digestion may be carried out using 85% sulfuric acid at 150—200°C in the presence of ammonium sulfate. The ensuing product is leached with water, while the double sulfates of the rare earths remain in the residue. The titanium, tantalum, and niobium sulfates transfer into the solution. The residue is converted to rare-earth carbonate, and then dissolved into nitric acid. 

Concentrates of columbate-tantalate are most suitable for processing and are most frequently used in the production of tantalum, niobium and their compounds. 

The purpose of the decomposition of raw materials is to convert tantalum and niobium compounds into a soluble form and prepare the solution for use in subsequent procedures. Fig. 116 presents the process flow chart. The most typical and frequently used raw materials are columbite-tantalite concentrates with the general formula (Fe, Mn)(Nb, Ta Cfo. 

Hydrofluoric acid, at relatively high concentrations and at elevated temperatures, dissolves columbite-tantalite concentrates at a reasonable rate. The dissolution process is based on the fluorination of tantalum, niobium and other metal oxides and their conversion into soluble complex fluoride acids yielding complex fluoride ions. 

The raffinate from the selective extraction process contains mostly niobium. The tantalum extract is treated by steam stripping to obtain a tantalum strip solution. The method results in the effective separation and relatively high concentration of tantalum and niobium in the respective strip solutions. 

Fig. 128. Extraction of tantalum and niobium versus H2SO4 concentration. Reproduced from [473], A. Agulyansky, L. Agulyansky, V. F. Travkin, Chemical Engineering and Processing 43 (2004) 1231, Copyright 2004, with permission of Elsevier.

The second step in the process is the definition of the optimal mixing time and solution temperature. It was found that for both niobium and tantalum, the mixing time for extraction and stripping must not exceed one minute. No concentration changes were observed in the temperature range of 25-50°C. 

Optimal parameters for the extraction, washing and stripping of niobium were determined to be number of stages for all three processes - 4, volumetric ratios Vorg Vaqu are 1 1, 20 1 and 8 1, respectively. Additional fine purification of the extractant was recommended by stripping of tantalum and niobium remainders using a 0.5% wt. ammonia solution. This additional stripping leads to final concentrations of both tantalum and niobium in the extractant that are < 0.001 g/1. Table 62 shows the purity of niobium oxide prepared by the described method. 

The opposite process, i.e. pouring the strip solution into the ammonia solution, significantly reduces the fluorine concentration in the hydroxides formed. Bludssus et al. [495] developed a process comprising the introduction of tantalum- or niobium-containing acid solution to an ammonia solution until achieving pH = 9. It is reported that this method enables the production of tantalum or niobium hydroxides with fluoride contents as low as 0.5% wt. with... 

The most efficient washing of the hydroxide was achieved applying a three-step process using an ammonium carbonate solution as the first step, followed by an ammonia solution, and water as the final step. This washing process brings about a ten-fold reduction in the concentration of fluorine compared with laboratory and industrial experience, in which a 2-4 fold reduction in the fluorine content of tantalum or niobium hydroxides following a one-step washing process was obtained. 

Uchino and Azuma [504] developed and proposed a two-step calcination process of tantalum and niobium hydroxides to obtain oxides. The first treatment is recommended to be performed at 500-700°C, and the second- at 750-1000°C. It is reported that the above method ensures the production of oxides that contain only negligible concentrations of fluorine and silicon impurities. 

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