Hydrothermal Solidification Of Boron Mud And Its Mechanical Properties

Boron mud (BM), which is mainly constituted by magnesite (MgCO3) and forsterite (2MgO·SiO2) in mineral compositions, is the industrial waste generated from the wet-process production of hydrated sodium borate, i.e. borax by making use of warwickite or vonsenite as the raw material. More than 1 million tons of BM is discharged yearly, and the total residual exceeds ten million tons in China. Only a small proportion of BM (about 30%) is being resourced for the utilizations in other industries, including the extraction of magnesia-containing chemicals, the firing of refractory/ceramics, the preparation of pelleting additive or flocculant, etc., while most of BM (more than 70%) is disposed directly by landfilling or casually onto an open storage. Some noticeable environmental problems have been aroused, such as harmful dust pollution and soil alkalization which may cause serious grain reduction in surrounding areas. Hence, an urgent requirement to develop a substitutable and environmentally acceptable recycling technology for large-scale BM has created widely concern and attention.In order to resolve the technical problems restricting the resourced utilization of BM in large scale, lightweight magnesia, gound quartz and water was preliminarily selected as the starting materials to systematic study the solidification mechanism in the system of MgO-SiO2-H2O, which in turn was applied into the hydrothermal solidification of BM. Some investigations were performed to reveal the influence of mixing ratio and hydrothermal conditions (temperature and time) upon the mechanical strength and microstuctural morphology. It has been found that the generation and aggregation of chrysotile. a kind of high-alkalinity hydrated magnesium silicate, was the critical factor to realize a dense structure and appreciable strength in the MgO-SiO2-H2O system, but a reasonable mixting ratio and optimized conditions of hydrothermal reaction were also necessary to promote the generation of chrysotile grains and to improve the mechanical properties of solidified bodies. Referring to the activity of starting materials, it is proposed that the hydrothermal solidification of MgO-SiO2-H2O system with a Mg(OH)2 content of 40 wt% should be carried out at 200? for 6h.Based on the experimental results mentioned above, hydrothermal solidification of BM was performed by using an incoopration of roasting activation and autoclaving process, in which roasting treatment at high temperature is used to improve the reaction activity of BM, so a combination reaction of roasted BM and fly ash is expectable under hydrothermal conditions to form cementitious chrysotile products. Systemic studies were carried out to investigate the influence of reaction factors, including the temperature and time of roasting activation, mixing ratio of starting materials, conditions of the hydrothermal reaction, and compaction pressure, upon the mechanical strength of solidified bodies, while X-ray diffraction (XRD) and scanning electron microscope (SEM) was employed to characterize the structural development during the hydrothermal solidification. Experimental data and their analysis suggested that roasting treatment is helpful to improve the activity of BM by the decomposition of MgCO3 to MgO, especially those operated at a relative high temperature with a shorter time, the involvement of fly ash is critical for the formation of chrysotile, but excessive content of fly ash plays a negative role by lowering the corresponding mechanical strength, so a content of fly ash is proposed to be 40 wt%; so does the water content, and the optimized content is 15wt%. Chrysotile formation is sensitive to the autoclaving conditions, including temperature and time, which consequently did a great influence on the mechanical properties of solidified BM. The most suitable conditions for BM solidification are 200?,6h in this work. Increasing the compaction pressure can upgrade the mechanical strength by lowering the porosity in specimens, but excessive pressure of compaction is not favorable to the strength development of solidified BM due to the generation of crystallization stress. A proposal compaction pressure is 20 kN (about 30 MPa). Because of the high porosity, the softening coefficient of in solidified BM in water and its resistance to freezing-thaw cycles are relative low, which can be improvee to some extent by increasing the compaction pressure.In this work, a combination of roasting activation and autoclaving precess was carried out to realize a high mechanical strength and reasonable durability from the the hydrothermally solidified BM The technology is able to utilize environmentally-harmful BM waste in large scale by transferring it into building materials with a certain commercial value. The technology also show great potential and foreseeable applications in the resourced utilization of other Mg-containing wastes or tails such as low-grade magnesium and boron slurry, etc.