Comparison of Environmental Impacts of Sulfuric Acid Process and Chlorination Process in Producing Titanium Dioxide

For a long time, people have a view that the sulfuric acid method has more emissions than the “three wastes” of the chlorination method, and it is simple to conclude that the sulfuric acid method has a large environmental pollution, and the chlorination method has little environmental pollution, but this is only from the amount of emissions. The conclusion, even if the chlorination process itself, depends on what raw materials it uses, and the emissions from the use of different raw wastes are also significantly different (see Table 1).

Table 1 Waste discharge per ton of titanium dioxide in two methods / (kg / t)

In recent years, many experts have carried out environmental assessments on the toxicity of waste by-products, the toxicity of raw materials and wastes, the degree of comprehensive utilization of energy, and the consumption of energy. It is believed that the impact of the two methods on the environment is basically the same. According to the survey results published in the "Environmental Affairs", once the sulfuric acid method solves the problem of waste disposal, its environmental impact is much better than that of the chlorination method.

The waste discharged from the sulfuric acid titanium dioxide plant is sulfate, and the chloride white titanium dioxide plant emits chloride. The sulfuric acid method emits much more than the chlorination method, but the treatment method is simple, the treated product is non-toxic and most of it can be reused. The problem is that the treatment cost is high, and the small-scale factory cannot afford it economically. Chlorination waste is low in emissions. At present, 90% of the chlorine in most plants can be recycled, but the recovery rate of low-grade ore is lower. DuPont Company of the United States has 1.2 tons of waste per ton of chlorinated titanium dioxide. It was discharged into the Atlantic Ocean before 1978. After 1978, it was partially used to produce ferric chloride and iron salt flocculants under the control of the environmental protection department. In the past, most of the tail chlorine could only produce ferric chloride before the chlorine recycling technology was perfected. In the 1980s, DuPont only produced a by-product ferric chloride of 30X10 ⁴ t/ in a Delaware plant. a, while the annual demand in the United States was only 20X10 ⁴ t / a, the rest can only try to develop other products, in addition to the chlorination method and some discarded metal chlorides are toxic, there is no good treatment, only Can be injected into deep wells under pressure. In the standard set by the European Commission in July 1989, each ton of chlorinated iron oxide allowed only 130-450 kg of chloride ions containing waste by-pass, while per ton of sulphuric acid titanium dioxide allowed to discharge 800 kg of sulphate containing waste by-product. The toxicity of chemical wastes and the degree of harm to the environment are greater than those of sulfuric acid.

Recently, the British Titanium Dioxide Group (Tioxide) used the Bioreactor Assessment (LCA) to discuss six production options (see Table 2), from raw material extraction, production processes, waste emissions and environmental impacts, and treatment options. The energy consumption is comprehensively inspected and evaluated, and the water quality of the harbor is monitored through a dedicated vessel for 20 years. The final research results are concentrated on two main factors: waste by-product emissions and energy consumption. The result is a sulphuric acid process plant based on ilmenite. The energy consumed by the waste secondary neutralization treatment (Scheme B) is 74 MJ/kg, which is 106 MJ/kg of the chlorination method using the titanium slag as the raw material, and lO1MJ/kg of the scheme F using the synthetic rutile as the raw material. And the 102MJ/kg of the sulfuric acid process plant (Scheme D) concentrated with spent acid is much less.

Table 2 6 production scenarios for discussion

In addition, the former factories have tried their best to expand the scale of the economy in order to obtain more profits, and they all believe that only the large scale of the economy can digest the huge cost of “three wastes” governance, but recently people have found that after the “three wastes” treatment of the sulfuric acid law The added value of waste by-products is low, the market capacity is limited, and it is subject to the transportation radius. Any sulfuric acid plant with a size of 4x104t/a or above, no matter where it is built, what raw materials and treatment methods are used, and what by-products are processed, The stacking of wastes, the transportation and sale of by-products are more difficult, and the success of the treatment of sulfuric acid plants in Western Europe and Japan is also related to their small size.

An interesting phenomenon in recent years is that due to the depletion of natural rutile storage and the high production cost of synthetic rutile, many chlorination plants are studying the use of low-grade ore to produce, in contrast, the sulfuric acid plant in order to get rid of the "three wastes" "The heavy burden of governance has been tried to produce high-grade acid-soluble titanium slag.