BISMUTH TELLURID WASTE PROCESSING

The main stages of bismuth telluride processing comprising sintering with NaOH, leaching and precipitation were investigated. Bi2Te3 samples produced by "ADV Engineering" were used as starting compounds. The studies revealed regularities of tellurium behavior during the sintering of Bi2Te3 with NaOH and the resulting the solid residue leaching. It was noted that annealing at 350-450°C with NaOH transforms tellurium into Na2TeO3, which is an appropriate form for further dissolution and separation from bismuth. Increasing temperature results in Na2TeO3 oxidation and formation of the water-insoluble compound Na2TeO4. Thus, it decreases tellurium extraction degree during the leaching. It has been shown that increasing temperature from 8 to 25°C at the step of tellurium hydrolytic precipitation slightly affects the extraction degree, the value of which is 93.5-98.2%.

Precipitation and Complexation

In Chapter 2, a method for the construction of single-element E–pH diagrams has been presented. No agent which could produce an insoluble compound nor any agent which could complex with any of the simple ions in the single-element diagrams except OH− has been included. However, it is of interest in many cases to derive E–pH diagrams for systems which involve the precipitation or complexation of one or more of the simple ions present in the single-element diagram. One method of deriving such diagrams is to recognize that precipitation or complexation of a simple cation or anion reduces the concentration of the simple ion in solution considerably. If this reduced simple-ion concentration is calculated, it can then be used to construct a new E–pH diagram for the simple ion. Therefore, since the predominant species in the region labeled as the simple ion is no longer the simple cation or anion due to the precipitation or complexation, the region is re-labeled with the precipitated compound or the complex. E–pH diagrams which involve the precipitation or complexation of one or more of the simple ions present in the single-element diagram may also be obtained using one of the available computer programs. For complicated systems, the hand calculations become time consuming and it is often better to employ a computer program. In order to determine the changes in a single element E–pH diagram that occur due to the addition of a precipitating species and the resulting formation of an insoluble compound, the following five steps may be followed. (1) Select an element of interest showing a simple cation or anion and construct the E–pH diagram for the element at a soluble species equilibrium concentration of 10−4.0 M. (2) Select a precipitating agent which will form an insoluble compound with a simple cation or anion of the element of interest. In order to determine which species will form insoluble compounds with the simple ions of an element, it is often useful to consult a list of solubility rules.