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.

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.

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.

Adsorption is one of the common stages in the hydrometallurgy of rare metals. Its efficiency is largely determined by the quality of the sorbent used. The purpose of this work was to create an activated sorbent based on rice husk cellulose for the extraction of rhenium from aqueous solutions. Two types of cellulose were obtained by treating rice husk with a 1.5% NaOH solution (alkaline cellulose) and a nitric acid solution in ethyl alcohol (Kürschner and Hoffer cellulose). They were tested by IR, SEM, TA, TPD-MS, and XRD methods. It was found that Kürschner and Hoffer cellulose does not contain lignin and retains structural order to a greater extent. By means of this cellulose carbonization at 600 °C and activation by physical, chemical, and combined methods, a series of sorbents were prepared and studied by different methods. It was determined that the sorbent KHC4-600VA obtained by combined activation of cellulose carbonizate by water vapor at 850 °C, followed by an alkaline treatment, has the best textural characteristics: S—~1200 m²·g⁻¹, V—1.22 cm³·g⁻¹, and R—2.05 nm. KHC4-600VA provides 90% recovery of Re (VII) ions from aqueous solutions. According to the Freundlich model, sorption proceeds favorably on the sorbent’s heterogeneous surface.

In recent years, research in the field of the sustainable production of refractory ceramics has become topical. Significant attention has been paid to the use of secondary raw materials for obtaining high-quality materials. The purpose of the current study was to develop new high-temperature porous materials based on the magnesium sulfate-refractory clay–chamotte–aluminum system using environmentally friendly raw components. To synthesize porous refractories, rice husk and the by-products of its thermal processing were used as substitutes for ingredients usually introduced into the composition of high-temperature materials. Ground rice husk was used as both a burnout additive and a silica source. It was added to the mixture instead of chamotte. An organic condensate from rice husk pyrolysis was used as a binder. A sodium silicate solution, after activating pyrolyzed rice husk with alkali, was also tested as a binder. These liquid ingredients served as replacements for lignosulfonate and liquid glass. The new raw material components and the porous refractories obtained with their use were studied using methods of chemical analysis, XRD, GC-MS, TA, SEM, and EDS. Standard methods for studying the properties of refractories were used to evaluate the physicomechanical and thermal characteristics of the experimental materials. The sample with the maximum content of rice husk (14.4 wt.%) and organic condensate from its pyrolysis (10.5 wt.%) demonstrated promising properties as a light porous refractory: an apparent porosity of 44%, a volumetric weight of 1.1 g·cm⁻³, compressive strength of 2.1 MPa, tensile strength in bending of 4.5 MPa, bond strength of 0.01 MPa, thermal shock resistance of 155 thermal cycles, and thermal conductivity of 0.05 W (m·K)⁻¹. It can be used as a prospective thermal insulating material.