Kentse Motsetse

In recent years, technical processes for the sequestration of CO₂ through industrial waste mineral carbonation have been explored and developed. There is a large portfolio of carbon capture, utilisation, and storage (CCUS) techniques that have been employed in laboratories and at pilot scale. These include geological storage, ocean storage, and mineralisation by carbonate ores. In view of this, the main purpose of this research was to investigate and explore chemical variables, particularly ammonium salts as lixiviants for calcium mineral extraction from iron-making slag. The slag in use was acquired from a steel mill in the Vaal Triangle Region in Gauteng, South Africa. The experimental test work was conducted using different ammonium lixiviants, namely, NH₄NO₃, NH₄Cl, and CH₃COONH₄, to understand the influence of anion type as well as possible differences in mechanisms of interactions. Lixiviant concentration as well as reaction time were varied in this research study. The three selected ammonium-based lixiviants showed different extents of calcium extraction owing to differences in the anion groups. NH₄NO₃, NH₄Cl, and CH₃COONH₄ were found to be capable of dissolving 50% to 80% of the calcium from the selected slag for different molar concentrations. Anion type and leaching time also had significant influences on the leaching of calcium from the slag. Rapid pH degradation resulted in better calcium extraction capabilities. This work has shown that the selected ammonium salts have the potential to be lixiviants for calcium dissolution from iron-making blast furnace slags. These lixiviants would, therefore, be important to consider during calcium mineral carbonation for CO₂ sequestration.

An ironmaking blast furnace slag was prepared and dissolved in lixiviant solutions of known concentrations for the leaching of calcium. A portable laboratory orbital shaking incubator was used for the leaching experiments. In addition, XRF and AAS were utilised for the sample characterization of solid residues and leach liquor, respectively. This study demonstrated that increasing the concentration of the solvent enhances the efficiency of calcium extraction; however, it also causes the leaching of other elements, such as magnesium, which can impede the carbonation stage. The research also revealed that the extraction of calcium from the largest particles is constrained by mass transfer and the accessibility of calcium from the particle. The authors hypothesize that the reaction products build up and hinder the reaction on the surface layer of the particles. However, with the smallest particle size fraction, more calcium can be extracted before the surface is obstructed, thus ensuring maximum extraction efficiency. Based on the activation energy value of 70.51 kJ/mol, it is less likely that the leaching of blast furnace slag in ammonium nitrate is a product-layer-diffusion-controlled process, as the activation energy for this type of process is usually below 20 kJ/mol. Instead, the higher activation energy suggests that the leaching process may be controlled by surface chemical reactions or a mixed mechanism. However, more detailed analysis and experimental data would be required to confirm the reaction mechanism.

In recent years, technical processes for the sequestration of CO2 through industrial waste mineral carbonation have been explored and developed. There is a large portfolio of Carbon Capture, Uti-lisation and Storage (CCUS) techniques that have been employed on a laboratory and at pilot scale. These include geological storage, ocean storage and mineralisation by carbonate ores. In view of this, the main purpose of this research was to investigate and explore chemical variables, particularly ammonium salts as lixiviants for calcium mineral extraction from iron making slag. The slag in use was acquired from a Steel Mill in the Vaal Triangle Region in Gauteng, South Af-rica. The experimental test work was conducted using different ammonium lixiviants, namely, NH4NO3, NH4Cl and CH3COONH4 to understand the influence of anion type as well as possible differences in mechanisms of interactions. Lixiviant concentration as well as the reaction time were varied in this research study. The three selected ammonium based lixiviants showed different extents of calcium extraction owing to differences in the anion groups. NH4NO3, NH4Cl and CH3COONH4 were found capable of dissolving 50% to 80% of calcium from the selected slag for different molar concentrations. Anion type and leaching time also had significant influences on the leaching of calcium from the slag. Rapid pH degradation resulted in better calcium extraction ca-pabilities. This work has shown that the selected ammonium salts have a potential as lixiviants for calcium dissolution from iron making blast furnace slags. These lixiviants would therefore be important to consider during calcium mineral carbonation for CO2 sequestration.

Using iron and steel slags as feedstock for a mineral carbonation reaction using carbon dioxide gas is an excellent technique because they are easily accessible, contribute to land pollution, and have a reasonable quantity of lime and magnesia. The rate at which ironmaking blast furnace slag dissolves in an aqueous solution of ammonium acetate was investigated in relation to pH, stirring speed, solvent concentration, and temperature. A one-factor-at-a-time experiment was conducted, pH was monitored to the maximum value of 11, stirring speed ranged from 100 to 200 rpm, solvent concentration was adjusted from 0.01 to 1 M, whereas the reaction temperature was maintained between 25 and 80 °C. The dissolution kinetics of ironmaking slag were calculated by fitting experimental data to a model of a diminishing core. Using Atomic Absorption Spectroscopy, the leach liquor was characterized under various experimental conditions. The results of the trial revealed that this reaction is driven by a chemical reaction model equation. A semi-empirical model was also developed from the experimental data to better describe the dissolution kinetics.

This study aims to evaluate the grindability and energy requirements for the liberation of quartz from blast furnace ironmaking slag. Furthermore, the study investigates the efficiency of gravity concentration method by using a shaking table and Falcon concentrator for the separation of quartz from the slag. The grindability of the slag was evaluated using the Bond’s Work Index (BWI) method. The energy required for the liberation of quartz was determined using a Modified Bond’s Work Index (MBWI) method. The results showed that the BWI of the slag was 13.5 kWh/t and the MBWI of the quartz was 22.3 kWh/t. Gravity separation tests were carried out using a shaking table and a Falcon concentrator. The results showed that the shaking table was able to recover 91.2% of the quartz with a grade of 99.5% SiO₂. The Falcon concentrator was able to recover 98.3% of the quartz with a grade of 99.7% SiO₂. In contrast, the study found that the quartz in blast furnace ironmaking slag can be physically separated using gravity separation techniques such as shaking table and Falcon concentrator. The study also provides valuable information on the grindability and energy requirements for the liberation of quartz from the slag, which can be used in the development of more efficient separation processes.

Inorganic electrolytes present in the process water used during froth flotation may have both beneficial and detrimental effects. These effects are said to be ion specific, as some ions may result in enhanced froth stability, increased mineral recoveries and decreased concentrate grades, while others may bring the opposite effects. Onsite process water quality variations have intensified the need to understand the relationship between inorganic electrolytes and flotation reagents on flotation performance. The use of mixtures of thiol collectors in sulfide flotation is a common practice across the globe; however, very few investigations have considered these in process waters of varying compositions. This study considers the effect of common cations, Na⁺ and Ca²⁺, in process water on the behavior of mixtures of thiol collectors. Single-salt solutions of NaCl and CaCl₂ at an ionic strength of 0.0213 mol·dm⁻³ were used to investigate the behavior of mixtures of two thiol collectors. These were carefully selected to understand how mixtures of thiol collectors behave in the presence of a monovalent cation versus a polyvalent cation. Bench-scale froth flotation tests were conducted using a Cu-Ni-PGM ore from the Merensky Reef. The results have shown that the divalent cation, Ca²⁺, resulted in higher %Cu and %Ni recoveries at all collector mixtures compared to the monovalent cation, Na⁺. The concentrate grades were, however, slightly compromised, as slightly more gangue reported to the concentrate in the presence of Ca²⁺. This behavior is attributed to the effect of polyvalent cations on bubble coalescence and froth stability.

Beneficiation of iron and steel slags have a great potential in mitigating high levels of anthropogenic carbon dioxide (CO₂) gas emissions owing to their high contents of calcium and magnesium. Carbonation of calcium and magnesium leaching solutions produce precipitates that promise to act as fluxing agents in various industries. There is also a need to study the physico-chemical factors affecting the extraction of these elements and their kinetic behaviour. A calcium-rich blast furnace ironmaking slag milled to 80%—75 µm was used in an experiment to study the effect of reaction temperature on the dissolution of calcium. An orbital shaking incubator was used by applying the one-factor-at-a-time method for a 2 h leaching test. Reaction temperature was controlled at 30 °C, 50 °C, 70 °C, and 100 °C at constant agitation speed of 200 rpm, 0.5 M of lixiviant concentration and solid-to-liquid ratio kept at 200 g/L. CH3COOH, (NH₄)₂SO₄, and NH₄Cl were tested individually as lixiviants. AAS, XRD and EDX were used to analyse the leach filtrate and residue. Consequently, the findings showed that temperature is the critical operating parameter that yielded exceptionally high calcium extraction. 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 use of mixtures of thiol collectors is reportedly beneficial in sulfide flotation. This is becoming standard practice for many concentrators, but process water recirculation and re-use in flotation circuits may compromise the behaviour of such mixtures owing to changes in physicochemical interactions occurring in the pulp phase as a result of water quality variations. It is expected that changes in the pulp chemistry would in turn affect both the pulp and froth phase phenomena, thereby affecting flotation performance. Thus, this study considers mixtures of thiol collectors, sodium iso-butyl xanthate (SIBX), and sodium di-ethyl dithiophosphate (SEDTP) in degrading water quality. Bench-scale flotation tests were conducted on various molar ratios of the selected thiol collectors under different ionic strengths (0.0242 mol·dm⁻³ and 0.1212 mol·dm⁻³) of synthetic plant water. Increasing the ionic strength of synthetic plant water and SEDTP molar ratio resulted in an increase in water, solids, Cu, and Ni recoveries. Cu-Ni grades decreased in increasing SEDTP molar ratios. The highest Cu-Ni grades were obtained in degrading water quality. The increase in water and solids recoveries in increasing SEDTP and ionic strength of plant water is attributed to an increase in froth stability. It can be concluded that the increase in the ionic strength of plant water increased water recoveries and therefore froth stability in parallel with SEDTP’s froth stabilizing effect, thus suggesting an additive interaction on the froth stabilisation effect seen.