Energy Conservation and Pollutant Reduction Yet to Be Promoted in Sulfuric Acid Industry

By China Sulfuric Acid Industry Information Station   

1. Waste emission and energy consumption in sulfuric acid production

Sulfuric acid production in China mainly uses sulfur, pyrite or smelting flue gas as raw material. Sulfuric acid production using sulfur as the raw material generates only small amounts of waste gas sulfur dioxide owing to the clean raw material and the simple process flow, so emissions can meet the standard. Due to poorer characteristics of pyrite and smelting flue gas, sulfuric acid production using them as raw materials generates lots of wastes.

    (1) Sulfur dioxide tail gas

A. The purification process in pyrite-based sulfuric acid production is rather complicated. Some small and medium sulfuric acid producers cannot get a stable supply of pyrite. Even with the use of the double-conversion and double-absorption process, the emission of tail gas can hardly be kept within the standard tolerance. Also, when low grade pyrite is used by some small and medium sulfuric acid producers that are capable of only the single-conversion and single-absorption process, tail gas emission is even harder to limit.

B. Sulfuric acid production using smelting flue gas as raw material tends to be defined as an environment-protecting technique for nonferrous metal smelters. Large nonferrous metal smelters can easily keep tail gas emission within the national standard in their sulfuric acid production. Nevertheless, flue gas generated in some nonferrous metal smelting has a low concentration of sulfur dioxide and large variations in flow, so it cannot be used in conventional sulfuric acid production processes and is difficult to control.
   To meet the ever more stringent requirements for environmental protection, some new sulfuric acid units using smelting flue gas as raw material use cesium-containing catalysts to increase the conversion rate of the sulfur dioxide. Some sulfuric acid units using the double-conversion and double-absorption process are equipped with tail gas absorption units to further reduce the content of sulfur dioxide in tail gas to less than 0.3 grams per cubic meter.

    (2) Acidic waste water

In the production of sulfuric acid using pyrite or smelting flue gas as raw materials, the furnace gas contains great quantities of hazardous solids and gases. Dry-process and wet-process collecting equipment must be used to cleanse it. The wet process generates some acidic waste water emission. Most sulfuric acid producers use the dilute acid circulation scrubbing process today. In this way fresh water consumption is greatly reduced and waste water emission is also down. Producing one ton of sulfuric acid generates 30 - 50 kilograms of acidic waste water. Some small and medium sulfuric acid units, however, still use the direct water scrubbing process, in which the production of one ton of sulfuric acid generates 10 - 15 tons of acidic waste water. Besides sulfuric acid, sulfurous acid and mineral dust, acidic waste water also contains hazardous impurities such as arsenic, fluorine, lead, zinc, copper, mercury and cadmium, and therefore contributes to a wide range of environmental problems.

    (3) Waste residue

Major solid residues from sulfuric acid production using pyrite include roasted pyrite slag and neutral slag (mostly gypsum). Roasted pyrite slag is mainly used as a cement additive. Some roasted slag from the sulfuric acid production is used as raw material for iron smelting. Major solid residues in the sulfuric acid production using smelting flue gas include filter cake bearing copper, lead or arsenic, and neutral residue. Filter cake is usually taken to dressing plants after casting to recover the metal or arsenic. The recovery rate of metals and arsenic by large sulfuric acid producers is quite high, leaving basically no problem of pollution. Only a few sulfuric acid producers use neutral slag to make construction materials. Most sulfuric acid producers take neutral slag to a stack yard, causing a problem of secondary pollution.

    (4) Energy consumption

Among the three processes for making sulfuric acid, the sulfur-based process consumes the least energy. Energy consumption in the pyrite process is equal to that in the smelting flue gas process. Appropriate energy conservation measures have been taken in the sulfur-based process in recent years, such as the use of technologies like turbine-driven air blowers and the dry absorbers being placed in a lower position in large sulfuric acid units, so power consumption per unit of product is declining overall. In spite of energy conservation measures where pyrite or smelting flue gas is used as the raw material, power consumption per unit of product is rising overall because the pressure losses of high-effect equipment such as kinetic-wave scrubbers and high-effect gas heat exchangers is extreme and energy consumption is increased, .
   Sulfuric acid production is an exothermic process. The high-temperature and medium-temperature waste heat is recovered in China when sulfur is the raw material. The high-temperature waste heat is also recovered when pyrite or smelting flue gas is the raw material. Due to restrictions in the process, however, only a few producers using pyrite or smelting flue gas recover the medium-temperature waste heat. Low-temperature waste heat is seldom recovered. The average amount of steam per ton of sulfuric acid is 0.935 tons in China today and the recovery rate of waste heat is only 60% - 70%. The recovery rate is even lower where smelting flue gas is the raw material.


2. Major energy conservation measures in the sulfuric acid industry

    (1) Optimize the system and reduce system pressure losses
    (2) Use high-effect blowers and acid delivering pumps
    (3) Increase the recovery rate of waste heat


3. Major emission reduction measures in the sulfuric acid industry

    (1) Increase the conversion rate of sulfur dioxide
    (2) Recover and make use of smelting flue gas with low sulfur dioxide concentration
    (3) Recover arsenic from acidic waste water


4. Joint efforts to promote energy conservation and pollutant reduction

Although energy conservation and pollutant reduction measures in the sulfuric acid industry of China are quite well developed, they have failed to gain extensive applications. The main reason is that these measures cannot bring immediate benefit to producers but require certain investments and operating expenses. To achieve energy conservation and pollutant reduction in the sulfuric acid industry, therefore, there should be:

    (1) Policy support from the government

Government policies and regulations should encourage energy conservation and pollutant reduction. For example, with the guidance of the "Clean Production Evaluation Index System in the Sulfuric Acid Sector," the policy of granting tax rebates rather than collecting pollutant emission fees should be implemented for sulfuric acid producers. The state should accelerate the formulation and perfection of tax policies for energy conservation and pollutant reduction.
   For energy, the state should quickly establish and perfect effective incentives for conserving energy and re-using waste heat, and help power managing departments balance the cost of power from conventional generating units against the cost of power from waste heat. For sulfuric acid producers, the cogenerated power sold to the State Grid Corporation of China (