Eutectic freeze crystallization of boron compounds

Boron compounds, especially borax and boric acid, are desired and important raw material in various industries. They have numerous applications from agriculture to nuclear reactors. Major usage of borax and boric acid includes borosilicate glass, ceramic glazes and detergent. Generally, borax and boric acid production is manufactured from tincal and colemanite minerals. Turkey is the world leader of tinkal (35.5% B2O3) and colemanite (50.8% B2O3) minerals. In Turkey, 803 million ton boron reserve exists, and every day several hundred to thousand tons boron waste is discharged in industrial processes. Before getting environmental awareness and consciousness, those wastes were disposed directly to the environment. Nowadays industrial wastes are enriched to reuse or wastes are discharged in appropriate way due to the negative environmental and ecological impacts. Three major options are available for the management of boron containing wastes: • recovery of boron minerals from tailings, • safe disposal of boron tailings without harming the environment, • utilization of boron tailings as raw materials in ceramics and cement industry. The continuous demand and consumption of raw materials and water in the world has increased significantly in the past decades. This leads to the necessity for sustainable processes and technologies in the development of raw material and for optimization of processes as regards recycling of wastewater, recovery of waste stream, and energy efficiency. This characterize the commonly known "Eutectic Freeze Crystallization" (EFC) technology in recent. This technology is in particular suited for the treatment of aqueous waste streams. The basis of eutectic freeze crystallization is the existence of eutectic point. The eutectic point refers to a characteristic point in the phase diagram of a salt-water mixture. At this temperature an equilibrium exits between ice, salt and a solution with a specific concentration. Thus, eutectic freeze crystallization process converts an invaluable and/or hazardous waste stream into clean water and valuable solid salt products. Besides, EFC has the biggest potential to treat aqueous streams at low cost and high yields. In this thesis, the primary purpose is to investigate eutectic points of borax-water (Na2B4O7-H2O) and boric acid-water (H3BO3-H2O) systems. The first stream, synthetic borax stream, was used for investigating the eutectic point in a 1 liter EFC experimental set-up. After crystallization process was conducted, ice product purity was detected. In addition, salt crystal size measurement was conducted for evaluating the salt crystal growth at different supersaturation process and different batch times. This aims to calculate the borax growth rates at different supersaturations under EFC conditions. In this experiment, microscopic studies were conducted by using Olympus BX51 microscope. The second stream, synthetic boric acid stream, was used for investigating the eutectic point in a 1 liter EFC experimental set-up. Similar to borax case, after crystallization process was conducted, the ice product pruty was detected. All experiments were done using starting compositions of 2 wt% borax aqueous, and 2.5 wt% boric acid aqueous solutions. The experimental investigations and feasibility of EFC applications for boron systems were discussed in result and discussion part. Detailed calculation of the processes and additional microscopic images were given in Appendices.