The utilization of geothermal energy remains underdeveloped, mainly due to the technical problem of silica scaling. The scaling can eventually disrupt the electricity production process due to frequent pipe maintenance. Although inevitable, scaling can be controlled by accelerating the precipitation process through the addition of silica seeds. Silica gel has an affinity to bind with dissolved silica in geothermal brine that therefore reduces the likelihood of silica to form scale on the pipeline surfaces. In the present work, brine was taken from geothermal well Unit 3A–3B at the Dieng geothermal power plant with an initial silica monomer concentration of approximately 420 ppm. Silica gel seeds were added to the brine at a precise pH and temperature and dissolved silica concentration was analyzed by detecting silica monomers with UV–visible spectrophotometry using the vanadate/molybdate (yellow) method. Experimental results showed that the silica concentration in the liquid phase could be reduced by the addition of these seeds. Silica precipitation was determined by mass transfer of silica monomers from the fluid phase onto solid surfaces, and it was found that precipitation decreased as pH and temperature increased. Calculations also showed that the mass transfer coefficient was enhanced by fluid agitation. The silica precipitation process was optimal at a pH of 7, a temperature of 40 °C and agitation speed of 800 rpm; the result was a mass transfer coefficient of 0.5924 cm/s. In a dimensionless correlation, the mass transfer coefficient can be expressed in the equation (kc·dp/DAB) = 1.4242·Re0.529·Sc0.3333.

The kinetics study has an essential role in the scale-up process because it illustrates the real phenomena of a process. This study aims to develop a mathematical model that can explain the mechanism of the leaching process of laterite ore using a low concentration of the citric acid solution and evaluate that model using the experimental data. As a raw material, this process used powder-shaped limonite laterite ores with a size of 125–150 µm. The leaching process is carried out using 0.1 M citric acid solution, F:S ratio of 1:20, and a leaching time of 2 h. The temperature parameter was varied at 303, 333, and 358 K. The experimental results showed that the higher the operating temperature, the higher the extracted nickel. The results of this experiment were used to evaluate the shrinking core kinetics model and the lumped model. The simulation results for both models show that the lumped model can provide better simulation results. Quantitatively, the percentage of errors from the shrinking core model is around 3.5 times greater than the percentage of errors from using the lumped model. This result shows that in this leaching process, the process mechanism that occurs involves the reactant diffusion step and the chemical reactions step; those steps run simultaneously.

Selective lithium recovery from a mixture of LFP-NMC spent lithium batteries presents significant challenges due to differing structures and elemental compositions of the batteries. These differences necessitate a distinct recycling pathway for each, complicating the process for the mixture. This study explored a carbothermal reduction approach combined with water leaching under atmospheric conditions to achieve a selective lithium recovery. For individual NMC black mass, at the optimal carbothermal conditions (950 °C, 15 °C/min, 2 h), lithium recovery of 95.7 ± 0.31% with 100% selectivity could be achieved. However, when the black mass was mixed with that of LFP in a 50:50 ratio, the recovery dropped to 9.78 ± 0.44%. Solid-state reactions during carbothermal process resulted in the formation of highly insoluble Li₃PO₄, and Fe-Ni-Co/Ni-Co alloys, which hinder lithium dissolution. To address these challenges, Na₂CO₃ was introduced as an additive to suppress Li₃PO₄. The addition of Na₂CO₃ to the 50:50 ratio of LFP-NMC black mass, increased lithium recovery to 59.47% with 100% selectivity. This enhancement was due to the stabilization of lithium as Li₂CO₃, a water-soluble compound. The results demonstrated that addition of Na₂CO₃ is a promising strategy for improving lithium recovery from mixed LFP-NMC batteries, providing a potential pathway for a more efficient recycling process.

Rare Earth Element plus Yttrium (REY) is an essential element in the modern industry. Due to its limited supply, REY recovery from alternative sources is needed. Coal fly ash is one of the candidates of REY sources. Coal fly ash taken from the pulverized coal power plant in West Java was used as a raw material in this research. A sequential process of alkaline digestion and acidic leaching was applied for enriching and recovering REY from this coal fly ash. The enriching step using alkaline digestion produced desilicated residue with higher REY concentration than the raw coal fly ash. The sulfuric acid solvent was used for leaching the desilicated residue in this work to investigate its effect on REY recovery. The highest REY recovery obtained in this work was 75.25% which recovery process condition using 1 M H2SO4 concentration, liquid to solid (L/S) ratio of 25 (mL/g) and 30 minutes leaching time.