How to deal with waste sulfuric acid

In daily life, especially in chemical production, there is always a lot of waste sulfuric acid. Sulfuric acid is an acidic oxidant. Sulfuric acid has serious corrosive effects. If human skin accidentally comes into contact with sulfuric acid, it will be corroded and burned. Therefore, you must pay attention to safety when doing experiments. So, how to deal with waste sulfuric acid that is so corrosive? The following is a detailed introduction.

In addition to being acidic, waste sulfuric acid and sulfuric acid wastewater also contain a large amount of impurities. Based on the differences in waste acid, wastewater composition and treatment objectives, the treatment methods currently used at home and abroad can be roughly divided into three categories: recycling and reuse, comprehensive utilization and neutralization treatment.

Recycling of Waste Sulfuric Acid

The sulfuric acid concentration in waste sulfuric acid is relatively high and can be recovered and reused after treatment. The treatment is mainly to remove impurities in the waste sulfuric acid and to concentrate the sulfuric acid. The treatment methods include concentration, oxidation, extraction and crystallization.

1.1 Concentration method

This method is to heat and concentrate the waste dilute sulfuric acid, so that the organic matter in it undergoes oxidation, polymerization and other reactions, and is converted into a dark colloid or suspension, which is then filtered out, thereby achieving the dual purpose of removing impurities and concentrating the dilute sulfuric acid. This type of method is widely used and the technology is mature. Based on the widely used high-temperature concentration method, a more advanced low-temperature concentration method has been developed, which are introduced below.

Waste sulfuric acid is produced during the chloral production process at Zibo Chemical Plant, in which the mass fraction of H2SO4 is 65% to 75%, the mass fraction of chloral is 1% to 3%, and the mass fraction of other organic impurities is 1%. The plant filtered the precipitate and then directly heated it with coal for distillation. The recovered concentrated sulfuric acid was colorless and transparent, with a H2SO4 mass fraction greater than 95%, and no chloroacetaldehyde was detected. Chloroform could be recovered after alkali hydrolysis, distillation and filtration of the precipitate. The plant processes 4,000 t/a of waste sulfuric acid and generates a profit of 550,000 yuan/a from sulfuric acid recovery〔1〕.

The waste sulfuric acid concentration method of Japan's Kimura-Datong Chemical Machinery Co., Ltd. is to combine glass-lined tube rising film evaporation and segmented vacuum evaporation to concentrate the mass fraction of H2SO4 in waste sulfuric acid from 10% to 40% to 95%. The process can be divided into three stages. The first two stages use impermeable graphite tube heaters for evaporation and concentration, and the last stage uses a glass-lined tube rising film evaporator for concentration. In each stage, the mass fraction of H2SO4 gradually increases to 60%, 80% and 95% respectively. The heating process uses a high-temperature heat carrier at a temperature of 150-220°C, which can convert organic matter into insoluble substances and then remove them by filtration. The process was pilot-tested at a scale of 2t/h and operated well for 5 years. This process has strong adaptability and can be used to treat waste sulfuric acid containing a variety of organic impurities〔2〕.

The disadvantages of the high-temperature concentration method are that the strong corrosiveness and acid mist of sulfuric acid are very harmful to equipment and operators, and the actual operation is very troublesome. Therefore, in recent years, an improved concentration method has been developed, which is called the vapor-liquid separation type non-volatile solution concentration method (abbreviated as WCG method)〔3〕.

The principle and process of the WCG method are as follows: the waste dilute sulfuric acid is pumped from the storage tank into the circulating concentration tower for concentration using an acid-resistant pump, and then heated by a heat exchanger before entering the mist maker and diffuser for forced atomization and further forced vaporization. The separated gas enters the gas purifier after high-level demisting and is discharged after purification. The separated acid liquid returns to the circulating concentration tower again. After repeated circulation, concentration and distillation, when the concentration requirement is reached, it is pumped into the concentrated sulfuric acid storage tank. Concentrated sulfuric acid can be reused as production raw material. The process flow is shown in Figure 1.

The WCG concentration device is mainly composed of a heat exchanger, a circulating concentration tower and an induced draft fan. The heat exchanger is made of graphite, the concentration tower is made of composite polypropylene, and the pump and induced draft fan are both acid-resistant equipment.

Compared with the high-temperature concentration method, this method has a low evaporation temperature (50-60°C), less steam consumption and low cost (the cost of electricity and steam for concentrating each ton of dilute sulfuric acid is approximately 30-60 yuan). The dilute sulfuric acid (H2SO4 mass fraction is 20%) produced by Shanghai Dyeing and Chemical Plant No. 5 in the production of dispersed dark blue H-GL, and the dilute sulfuric acid produced by Shanghai Dyeing and Chemical Plant No. 8, Wuhan Dye Factory, and Jining Dye Factory in the production of dye intermediates were concentrated using the WCG method, all of which achieved significant results.