Over the years, the use of new alternative lixiviants for gold extraction has been investigated to overcome the environmental concerns resulting from the cyanidation process. Moreover, with global economic factors causing a decline in gold prices, it is crucial that novel hydrometallurgical methods of extracting gold minimise operational costs by using low-priced reagents such as thiosulfate. In the current study, the response surface methodology (RSM) approach is used to optimize the kinetic factors (temperature and copper, ammonia, and thiosulphate concentration) affecting gold leaching. Gold ore assayed at 16 g/t was characterized through X-ray fluorescence and X-ray diffraction spectrometric analysis as well as scanning electron microscope-energy dispersive spectrometric technique. Gold ore was predominantly siliceous with minor pyritic content. The results indicate a strong relationship between the actual gold leaching recovery data and the RSM model. Correlation coefficients R2 and adjusted R2 are equivalent to 0.9869 and 0.9817. Gold leaching in copper-ammonia-thiosulfate media is best described as a surface chemical reaction-controlled process, suggesting that gold dissolution in thiosulfate is considerably affected by the increase in temperature. The effect of temperature is mostly significant, contributing up to 64.65% of the gold recovery response model. The contribution percentages of the effects of time, thiosulfate [S2O3], ammonia [NH3], and copper [Cu] concentrations were calculated as 12.81%, 5.88%, 5.19%, and 4.65%, respectively. All investigated kinetic parameters were found statistically significant with p value <0.05. The optimal concentrations of gold leaching media to achieve potentially complete dissolution of gold from its ore in copper-ammoniacal thiosulphate media based on the effect of the investigated parameters were 0.5 M S2O3, 3 M NH3, and 0.003 M Cu2+ with a desirability value equivalent to unity (d = 1.000).

In the current study, fly ash modified basic oxygen furnace slag geopolymers were synthesized and used as geofilters to remove heavy metals from wastewater. Batch experiment based on the removal of heavy metals from synthesized wastewater was conducted so as to investigate the influence of contact time and porosity on the metal removal efficiency and adsorption capacity of the geofilters. Adsorption kinetics was evaluated using Lagergren's pseudo-first-order model, Ho's pseudo-second order and Elovich models. Effect of contact time between geopolymer and wastewater was investigated within the first 15 minutes of adsorption with time interval of 3 minutes. The highest percentages for the removal of Fe2+, Cu2+, Al3+ and Ni2+ions with 1%H2O2 – FA-BOFS based geopolymer obtained after 15minutes were 100%, 86.87%, 84.64% and 84.63% respectively. The trend based on metal absorption capacities in an ascending order given as Fe >Cu>Ni>Al. The adsorption capacity of metal within the 15miniutes was relatively constant possibly due to sufficient number of sites upon which metal ion adsorption may have possibly occurred on the outer surface of geopolymers irrespective of H2O2 content. Metal ion adsorption rates in the current study strongly adhered to the Ho's pseudo-second-order model based on linear regression fitting method. The correlation coefficients R2 were 0.999. FA-BOFS based geopolymer can be considered as emergent and versatile material with inherent chemical microstructural property able to remove heavy metals from polluted effluent and metalliferous drainage.

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The worldwide supply constraints and the anticipated high demand for sodium silicate as well as environmental issues associated with the use of sodium silicate has given dawn to the need to opt for alternative activating solutions such as alkali-hydroxides. The current study mainly focuses on the evaluation of mechanical and chemical properties of Granulated Blast Furnace Slag (GGBFS) Binder-Spend Foundry Sand (SFS) based material toward the development of a durable material for building applications. Activated GGBFS was synthesized using a NaOH solution as the sole GGBFS activator. Uniaxial compressive strength (UCS) tests were conducted on the GGBFS-SFS based specimens to investigate the influence of varying amount of GGBFS binder (15 %- 45 %) cured at 80°C. Results showed that the specimen consisting of 45% GGBFS and 55% SFS at a solid to liquid of 0.17 yielded the high UCS equivalent to 11.07MPa. Increase in UCS has been attributed to the presence of calcium silicate hydrate phase confirmed by XRD analysis. In conclusion, GGBFS-based binder waste foundry-based material sand can be considered as a promising and efficacious building material as per ASTM C34-13, C129-14a and South African standard (SANS227: 2007).