An investigation into the effect of physico-chemical factors affecting the extraction of calcium from iron and steel slag

Abstract

During steel production, a large quantity of waste residue commonly known as slag is produced with a reasonable amount of metals such as calcium and magnesium. Iron and steel slags have various economically viable applications such as fertiliser production, cement production, road construction, land fill and soil reclamation. However, other applications respond to the issue of controlling and degrading high levels of anthropogenic carbon dioxide (CO2) gas emissions by carbonating calcium and magnesium leach solutions from iron and steel slags. One such technological method is the extraction of calcium by means of leaching from iron and steel slag whereby CO2 sequestration is a long-term goal. In contrast, calcium and magnesium leach solutions can produce precipitated carbonates for various uses when carbonated. In addition, it is a crucial step to study the leaching parameters and reliable solvents for the dissolution of calcium and magnesium owing to their contents in iron and steel slags. The aim of this research project was to determine the physico-chemical factors affecting leaching of calcium from blast furnace slag and thereby improve the extent and kinetics of calcium extraction. The chemical and mineralogical study by X-ray fluorescence (XRF) and X-ray diffraction (XRD) respectively, has shown that the blast furnace slag sample under study contained calcium in the form of aluminoakermanite ((Ca,Na)2(Al,Mg,Fe2+)). The minerals identified through XRD are classified as silicates with the host mineral being calcium in recoverable quantities. The initial XRF analysis showed that the slag contained 23.47% Ca, 5.21% Mg and 1.24% Fe with the rest being minor elements. The slag was received at a particle size of 20 mm. Size reduction was conducted in a dry rod mill process to produce a size of 80% -75μm. Particle size distribution was analysed by a simple laboratory sieving method. The leaching tests were conducted in an incubated orbital shaker by applying the one-factor-at-a-time method for a two-hour leaching test. Reaction temperature was controlled at 30, 50, 70 and 100˚C. Ionic strength varied at 0.5, 1.0, 1.5 and 2.0 molar (M). In addition, the solid/liquid ratio was observed in the range 200, 300, 400 and 600 gram/Litre (g/L). Acetic acid (CH3COOH), ammonium sulphate ((NH4)2SO4) and ammonium chloride (NH4Cl) were tested individually as lixiviants under varying temperatures, ionic strengths and solid/liquid ratio. The results showed that extraction of calcium from ironmaking slag by leaching is feasible. 100% Ca was extracted using CH3COOH at 100˚C, 0.5 M, 200 g/L and 120 minutes. As the reaction temperature was increased from 50˚C up to 100˚C, silica gelling was observed, which hindered filtration of a leach liquor from a residue. Silica gelling did not compromise calcium recovery. The increase in lixiviant ionic strength and solid-liquid ratio did not influence calcium recovery. The role of anions during leaching by using ammonium sulphate, ammonium chloride and acetic acid as lixiviants was identified. Sulphate ions were found to have the least influence in the increase in calcium recovery. Acetate ions have a significant role on dissolution of calcium as well as chloride ions. Consequently, the findings showed that temperature is the critical operating parameter that yielded exceptionally high calcium extraction percentages. CH3COOH yielded 80%-90% of calcium extracted under high reaction temperatures. The kinetic data for the effect of reaction temperature leaching data fitted one shrinking core model equation, which described diffusion-controlled leaching reactions. The diffusion controlled by a product layer model was fitted and the activation energy were calculated as 19.4930 kJ/mol for NH4Cl solvent. (NH4)2SO4 and CH3COOH did not fit into any model because of undefined kinetic data and formation of gelatinous silica layer during leaching process. The Arrhenius equation, confirmed that the leaching reaction by NH4Cl solvent fitted diffusion-controlled mechanism.