Développement de procédés de purification de l’acide phosphorique industriel. Extraction liquide liquide et solide liquide des métaux lourds.

Abstract

The liquid- liquid and liquid- solid extractions are techniques of separation commonly used in the field of mineral industry and reserved to very elaborate products. Indeed, They have been frequently applied for the separation of elements that are expensive or produced in small quantities, such as uranium, rare earths, gallium, heavy metals,...Recently, the liquid-liquid and liquid-solid extraction methods were extended to the processes of purification of more conventional metals which are produced in large quantities such as copper, nickel, cobalt [1].

The introduction of these two techniques into an industry with rather “fine chemistry” as the one of phosphates may appear to be “foolish”. However, in the last ten years several processes of phosphoric-acid purification have been developed in order to prepare phosphates for either technical or food uses [2].

The toxicity of the heavy metal cations is now very well established [3] and it is essential to avoid their release in nature. Their recovery requires an extraction and/or fixing techniques. Hence, the development of better organic agents with high molecular weight and solid extractants such as bentonitic clays is recommended [4].

A pretreatment of the raw phosphoric acid for the elimination of the organic matter, seems to be an important step, not only for the production of decontaminated phosphoric acid and fertilizers but also for the recovery of heavy metals [5]. This led us to seek treatment processes that are less expensive and more realizable. Our choice was directed toward the process of adsorption by Algerian bentonitic clays [6].

In a first step, tests of extraction of zinc, cadmium and chromium in phosphoric medium by common industrial extractants such as D2EHPA, OPhPA, TBP and Kelex 100 were carried out, in order to find a system of extraction that is selective and efficient. In a second step, a study of the influence of the physicochemical parameters (pH, mass of clay, temperature) on the extraction process was also performed.

An adequate model that includes the effect of pH, mass of clay and temperature was developed in order to describe the adsorption of zinc, cadmium and chromium onto activated bentonite. The results of an experimental validation showed that the developped model fit quite well the operating conditions of the fixing process.