Analytical determination of fluorides in South African chemical gypsum

Abstract

Fluoride ion is an accompanying impurity in a wide variety of chemical gypsum throughout the world. In this study, the Ion Selective Electrode (ISE) method, the Ion Chromatography (IC) method and the standard Willard and Winter method of fluoride analysis were adapted and compared for use in the quantification of fluoride in South African chemical gypsum. During the use of ISE, the pH of sample solutions was found to be a critical parameter for the results to be meaningful. An operating pH of approximately 5 was suitable for consistency of results. It was important to ensure the existence of the ionised form of fluorine in solution, because the detection was based on the sensitivity of the membrane electrode to this species. In the case of the Willard and Winter method, the traditional visual indicator titration was replaced by a more sensitive spectrophotometric detection, because of low fluoride levels in the chemical gypsum. The parameter sensitive reaction rate approach was adapted, and the reaction allowed to go to completion to enable measurement with a bench top spectrophotometer. The IC method required a good separator since fluoride ions usually eluted too early for detection on common ion exchange columns. The data handling of the chromatographic software was thoroughly examined and consistent integration of the chromatograms maintained. Sample preparation of the chemical gypsum involved particle size reduction through grinding. No trend between fluoride impurity and the particle size of the sample was observed. <p. The quantity of fluoride in Kynoch and Omnia phosphogypsum were found to be 0.10% and 0.04% respectively. The Tioxide chemical gypsum, titangypsum, was found to contain in the region of 0.02% fluoride. Generally, the level of fluorine (F) has to be reduced to about 0.2% before phosphogypsum can be used as substitute for natural gypsum in the cement industry. The aim of this study is to critically evaluate three analytical methods, namely, the Willard and Winter standard method of fluoride analysis, the ISE method, and IC as applied in the quantification of fluoride in chemical gypsum. Secondly, the efficiency of treatment of the chemical gypsum with water, lime and sulphuric acid was investigated. The ISE method was found to be faster and relatively simpler versus both Willard and Winter and the IC methods. The IC method was quite superior for indicating general complexity of the sample and it was faster than the Willard and Winter method. The standard Willard and Winter method was generally found to be long and tedious. The three methods validated one another as percentages of fluoride in the chemical gypsum samples were the same. The study indicated that a sulphuric acid pretreatment of the chemical gypsum was the most effective (90%) in the removal of fluoride impurity compared to the lime treatment and water washing (9%). However, the water washing would be inexpensive for practical treatment of chemical gypsum at plant level.