In this study, we investigated the removal mechanism of total petroleum hydrocarbons (TPH) from soil by microwave heating. TPH contaminated soil was investigated to determine the desorption behavior of five carbon number-based fractions of TPH. The applied operating microwave power density influenced the final temperature that was reached during heating. For low operating power density applications, microwave effectiveness was limited due to the soil’s dielectric properties, which exhibited a direct relationship with temperature variation. Soil particle distribution could be attributed to permeability, which significantly influenced the evaporation of contaminated soil during the microwave treatment. The results indicate that the activation energy was correlated with the influence of particle size. The removal efficiency of the coarse soil reached 91.1% at 15 min, whereas that of fine soil was low. A total of 30 min had passed, and a removal efficiency of 71.2% was found for the fine soil. Residual TPH concentration was decreased when irradiation time was increased with a removal rate dependent on soil temperature variation. The surface functional groups of the contaminated soil were influenced by microwave irradiation, and changes in the hydrocarbon fraction affected contaminant removal.

Microwave technology has been confirmed to be suitable for use in a wide range of mineral leaching processes. Compared to conventional leaching, microwave-assisted leaching has significant advantages. It is a proven process, because of its short processing time and reduced energy. The purpose of this study was to enhance the gold content in a refractory gold concentrate using microwave-assisted leaching. The leaching efficiencies of metal ions (As, Cu, Zn, Fe, and Pb) and recovery of gold from refractory gold concentrate were investigated via nitric acid leaching followed by microwave treatment. As the acid concentration increased, metal ion leaching increased. In the refractory gold concentrate leaching experiments, nitric acid leaching at high temperatures could limit the decomposition of sulfide minerals, because of the passive layer in the refractory gold concentrate. Microwave-assisted leaching experiments for gold recovery were conducted for the refractory gold concentrate. More extreme reaction conditions (nitric acid concentration > 1.0 M) facilitated the decomposition of passivation species derived from metal ion dissolution and the liberation of gangue minerals on the sulfide surface. The recovery rate of gold in the leach residue was improved with microwave-assisted leaching, with a gold recovery of ~132.55 g/t after 20 min of the leaching experiment (2.0 M nitric acid), according to fire assays.

In this research, we studied the use of microwave pretreatment to enhance the efficiency of Au leaching from gold concentrate. The gold concentrate was pretreated using microwaves with different irradiation time. The sample temperature was increased up to 950 °C by the microwave irradiation. A scanning electron microscope-energy dispersive spectrometer showed the evolution of microcracks and the reduction of sulfur on the mineral surface. X-ray diffraction data also showed the mineral phase shift from pyrite to hematite or pyrrhotite. A leaching test was conducted for the microwave-treated and untreated gold concentrates using thiourea. Although the thiourea leaching recovered 80% of Au from the untreated concentrate, from the treated concentration, the Au could be recovered completely. Au leaching efficiency increased as the microwave irradiation time increased, as well as with a higher composition of thiourea.

In this study, we investigated the feasibility of using a solution of sulfuric acid and phosphoric acid as an extraction method for soil-washing to remove Cu, Pb, Zn, and As from contaminated soil. We treated various soil particles, including seven fraction sizes, using sulfuric acid. In addition, to improve Cu, Pb, Zn, and As removal efficiencies, washing agents were compared through batch experiments. The results showed that each agent behaved differently when reacting with heavy metals (Cu, Pb, and Zn) and As. Sulfuric acid was more effective in extracting heavy metals than in extracting As. However, phosphoric acid was not effective in extracting heavy metals. Compared with each inorganic acid, As removal from soil by washing agents increased in the order of sulfuric acid (35.81%) < phosphoric acid (62.96%). Therefore, an enhanced mixture solution using sulfuric acid and phosphoric acid to simultaneously remove heavy metals and As from contaminated soils was investigated. Sulfuric acid at 0.6 M was adopted to combine with 0.6 M phosphoric acid to obtain the mixture solution (1:1) that was used to determine the effect for the simultaneous removal of both heavy metals and As from the contaminated soil. The removal efficiencies of As, Cu, Pb, and Zn were 70.5%, 79.6%, 80.1%, and 71.2%, respectively. The combination of sulfuric acid with phosphoric acid increased the overall As and heavy metal extraction efficiencies from the contaminated soil samples. With the combined effect of dissolving oxides and ion exchange under combined washings, the removal efficiencies of heavy metals and As were higher than those of single washings.

The purpose of this study was to liberate gold from sulfide minerals in a gold concentrate through microwave-nitric acid leaching and to separate the light minerals in an insoluble residue using a hydro-separation process. The representative sulfide minerals in the gold concentrate were pyrite with minor galena. Mineralogical characterization was conducted on the gold concentrate using 1715.20 g/t based on lead-fire assays. During the leaching experiment, the effect of nitric acid concentration was studied. The results indicated that the metal leaching rate of the gold concentrate increased with increasing nitric acid concentration. After the microwave-nitric acid leaching, the resulting main feature was consistent with the increased exposure to reactive sulfide minerals and decrease in weight. Characterization techniques such as X-ray diffraction (XRD), scanning electron microscope energy dispersive spectroscopy (SEM-EDS) and scanning electron microscope backscattered electron imaging (SEM-BSE) were performed to characterize the minerals in the insoluble residue using microwave-nitric acid leaching and the hydro-separation process. The XRD patterns of the insoluble residues were compared. The intensities of the pyrite peak decreased and disappeared under different nitric acid concentrations, whereas intensities of the quartz peak increased. The hydro-separation process focused on the separation of heavy (e.g., native gold) and light (e.g., quartz) minerals from the insoluble residues. After the hydro-separation treatment process, the heavy minerals exhibited typical diffraction lines of gold, as obtained using the XRD analysis.