Treatment of Concentrated Boron from Water Using Microwave Hydrothermal Mineralization Method

Boron compounds are widely-used raw materials in industries, such as ceramic, glass and liquid crystal display (LCD) process. Wastewaters from these processes often contain elevated boron concentrations, and it in aqueous systems may be harmful to human and plants.
In this study, boron recovery, calcium hydroxide (Ca(OH)2 alone), Ca(OH)2 with phosphoric acid (H3PO4) addition (P addition) and pretreated shells were used to remove and recover boron from water using hydrothermal methods. For the case of Ca(OH)2 alone and MW(Microwave) hydrothermal method, experimental results showed that boron recovery efficiency achieved 90% within 10 min, and crystals of Ca2B2O5‧H2O were found in the precipitates as indicated by the XRD analysis. For the case of P addition and the MW hydrothermal method, boron recovery efficiency reached 99% within 10 min, and calcium phosphate species (CaHPO4．H2O, CaHPO4 and Ca10(PO4)6(OH)2) were formed. For the case of pretreated shell, three types of crushed shells (oyster, hard clam and freshwater clam) were pretreated and then reused as mineralizers to remove and recover the boron from concentrated wastewater by using the MW hydrothermal method. The oyster shells pretreated by heat performed better than heated hard clam and freshwater clam shells, and the boron recovery efficiency reached around 95% within 10 min of reaction time. In order to use bio-shell as substitute of calcium hydroxide, shell pre-treated method was also considered in this study. Oyster shells were transformed by hybrid microwave heating method, and the effect of different microwave absorption materials were also investigated. Comparing with three other microwave absorption materials, CuO performed a rapider rate of heating. The temperature of CuO reached 950 oC after 3 min, and CuO-ZrO2 (1:1) microwave absorbent might have less degree of disruption. In addition, oyster shells were almost decomposed and became CaO with CuO-ZrO2 (1:1) microwave absorbent at power of 600W after 20 min of reaction time.
According to the experimental results, pretreatment of shells by composite microwave heating is a feasible method. However, some modifications and improvements should be done for practical use. In addition, an effective method for boron recovery by microwave with commercial reagents or pretreated shells was found. There are several factors that should be considered, such as pH value, initial boron concentration and interference compound before adopting this method.