Rice husk is a large-tonnage waste left from rice production. It is not subject to humification and therefore becomes a serious environmental pollutant. Due to the presence of two essential elements—carbon and silicon—in its composition, rice husk is a promising organo-mineral raw material. The known methods for processing of rice husk are associated with the formation of even more aggressive waste. The creation of a waste-free technology for processing this plant material requires a detailed study. Rice husk of Kyzylorda oblast was studied using IR, SEM, TA, TPD-MS, EPR, and TEM methods. It was determined that under a temperature up to 500 °C, the ligno-carbohydrate component of rice husk decomposes almost completely. Three main peaks are recorded during the decomposition: hemicellulose at 200 °C, cellulose at 265 °C, and lignin at 350–360 °C. This process is endothermic. However, above of 300 °C the exothermic reactions associated with the formation of new substances and condensation processes in the solid residue begin to prevail. This explains the increase in the concentration of paramagnetic centers (PMCs) in products of rice husk carbonization in the range of up to 450 °C. Further increase in temperature leads to a decrease in the number of PMCs as a result of carbon graphite-like structures formation. The silicon–carbon product of rice husk carbonization (nanocomposite) is formed by interconnected nanoscale particles of carbon and silicon dioxide, the modification of which depends on the temperature of carbonization. The obtained data allow management of the rice husk utilization process while manufacturing products in demand based on ecofriendly technologies.

Adsorption is one of the main methods of water purification. Novel advanced, eco-friendly, cost-effective adsorbents with high adsorption capacity and selectivity are required to remove pollutants from aqueous solutions. Plant polymers are viewed as both prospective adsorbents and as raw materials to produce them instead of conventional adsorption materials. There is widespread interest in using rice husk as a universal sorbent to remove different contaminants from aqueous media because of its surplus availability, low cost, and high content of oxygen containing functional and silanol groups as active sites for adsorptive extraction. Different methods of heat and chemical treatments have been developed to improve the sorption properties of raw rice husk. Unmodified rice husk and rice-husk-based sorbents have been tested to uptake non-ferrous, ferrous, minor, precious, rare, and rare-earth metals and radionuclides from artificial and industrial solutions, natural contaminated water, and industrial wastewater. This review summarizes the results of numerous studies and characterizes the current state of work in this area, with recommendations for further development.

Organization of environmentally-friendly production of refractory materials based on the principles of cost-effective use of energy and material resources through use of energy-saving technologies and replacement of natural raw materials with industrial and agricultural waste is gaining relevance. Scientists are increasingly interested in creating high-temperature materials using silica of plant origin. Its source is rice husk, a multi-tonnage waste from rice production. Organo-mineral in its nature, rice husk determines the uniqueness of the structure and properties of the materials obtained from it. Use of this waste allows us to produce porous, high-strength silicon carbide refractories with properties corresponding to classical analogs, while benefiting from environmental, economic and technological aspects. The lack of industrial production of refractories using rice husk ash, despite the positive results of scientific research, indicates insufficient study of the issue with certain gaps in this area. This review is intended to help researchers to identify existing problems and outline further actions necessary to ensure that the scientific results are implemented in production.

Lignin processing products have an extensive using range. Because products properties depend on lignin precursor quality it was interesting to study lignin isolated from rice husk being a large tonnage waste of rice production and its structural transformations during carbonization. Lignin isolated by the thermal hydrolysis method with H₂SO₄ 1 wt % solution and its carbonized products prepared under different carbonization conditions were characterized using elemental analysis, IR, TPD-MS, XRD, TEM, and EPR. It was shown lignin degradation takes place over the wide (220–520 °C) temperature range. Silica presenting in lignin affects the thermal destruction of this polymer. Due to the strong chemical bond with phenolic hydroxylic group it decreases an evaporation of volatile compounds and as a result increases the temperature range of the lignin degradation. Rice husk hydrolytic lignin transformations during carbonization occur with generation of free radicals. Their concentration is decreased after condensation of aromatic rings with carbon polycycles formation, i.e., the graphite-like structure. Quantity and X-ray diffraction characteristics of the graphite-like phase depend on carbonization conditions. Morphology of the lignin-based carbonized products is represented by carbon fibers, carbon and silica nanoparticles, and together with another structure characteristics provides prospective performance properties of lignin-based end products.

Lead sludge from copper production is a source of rare metals, such as rhenium and osmium, whose content reaches 0.06–0.08% and 0.0025–0.0050%, respectively. The base of the sludge consists of lead sulfate. A method of reductive smelting of lead sludge from copper smelting production at 1000–1100 °C has been developed. Coke was used as a reducing agent and sodium sulfate as a slag-forming material. Optimal conditions for selective extraction of rare metals in smelting products were found: osmium in the form of metallic form into raw lead and rhenium in the form of perrhenate compound Na₅ReO₆ into sodium-sulfate slag. The developed technology makes it possible to extract rhenium with a high degree of extraction in the form of water-soluble compounds for the subsequent production of commercial salts of rhenium by the known hydrometallurgical methods. The content of rhenium in the slag phase is 0.18–0.25%, with its initial content in the slime of 0.06–0.08%. The degree of rhenium concentration at the first stage of processing is 3–3.2 times in the form of water-soluble perrhenate. Osmium and lead do not form solid solutions; osmium in crude lead is mainly concentrated in the lower zones of lead. A method of obtaining a concentrate containing 53–67% osmium from raw lead with an initial content of 0.0025–0.0050% in the slurry and a concentration number of 13,000–21,000 times has been proposed.

Coke-based carbon sorbent (CBCS) was produced using special coke fines with the following characteristics: ash, 4.5%; iodine adsorption capacity, 52%; specific surface area, 600 m² g⁻¹; and total pore volume, 0.4 cm³ g⁻¹. Gold adsorption from real production cyanide solutions in batch and column laboratory experiments was studied. The optimum adsorbent/solution ratio was 0.2 g/20 cm³. Sorption equilibrium occurred after 60 min of phase-time contact. The CBCS maximum adsorption capacity for gold was found to be 1.2 mg g⁻¹. Both the Langmiur and Freundlich isotherm models confirmed that gold adsorption by CBCS proceeds favorably, but the Freundlich isotherm best describes the adsorption equilibrium. The CBCS dynamic exchange capacity (100 g t⁻¹) and full dynamic exchange capacity (4600 g t⁻¹) for gold were determined in column tests. It was revealed using SEM that adsorbate was retained in sorbent pores. The possibility of completely eluting gold from CBCS was demonstrated. A CBCS pilot test to recover gold from 200 dm³ of the cyanide solution containing (mg dm⁻³) 2.6 Au, 0.42 Ag, and 490 Cu was carried out. The total amount of noble metals (Au + Ag) adsorbed was 99.99% and gold ions was 94%. The CBCS maximum adsorption capacity for gold reached 2900 g t⁻¹.