Acid-base wastewater is the most common one in wastewater treatment. Acid wastewater mainly comes from steel mills, chemical plants, dye factories, electroplating factories and mines. Waste water treatment should focus on salts containing various harmful substances or heavy metals. The mass fraction of acid in wastewater treatment varies greatly, ranging from less than 1% to more than 10%. Alkaline wastewater mainly comes from printing and dyeing factories, leather factories, paper mills and oil refineries. When wastewater is treated, it will contain organic alkali or inorganic alkali. The mass fraction of alkali is higher than 5% and lower than 1%. In addition to acid and alkali, acid-base wastewater often contains inorganic and organic substances such as acid salts and basic salts [1].

source

There are many sources of acid-containing and alkali-containing wastewater, such as chemical industry, chemical fiber, acid making, electroplating, oil refining and pickling workshops in metal processing plants. Some wastewater contains inorganic acids such as sulfuric acid and hydrochloric acid, some contains organic acids such as formic acid and acetic acid, and some have both. The acid concentration of wastewater varies greatly, from less than 1 to more than 10. Alkaline wastewater will be discharged from papermaking, printing and dyeing, tanning, metal processing and other production processes. In most cases, it is inorganic alkali, and some wastewater contains organic alkali. Some wastewater has a high alkali concentration, as high as several percent. In addition to acids and bases, wastewater may also contain acid salts and basic salts and other acidic or alkaline inorganic and organic substances. Discharging waste water containing acid and alkali at will not only pollutes and destroys the environment, but also wastes resources. Therefore, the recovery and comprehensive utilization of acid-base wastewater should be considered first.

When the concentration of acid-base wastewater is high, for example:

When the acid content of acid-containing wastewater reaches more than 4 and the alkali content of alkali-containing wastewater reaches more than 2, it is possible to recover and comprehensively utilize it, which can be used to make ferrous sulfate, gypsum and chemical fertilizer, and can also be reused or used by other factories. Acidic wastewater with a concentration of less than 4 and alkaline wastewater with a concentration of less than 2 should be considered for neutralization treatment, because recycling is of little significance. It contains various harmful substances or heavy metal salts. The mass fraction of acid varies greatly, ranging from below 1% to above 10%.

Alkaline wastewater mainly comes from printing and dyeing factories, leather factories, paper mills and oil refineries. Some of them contain organic or inorganic bases. The mass fraction of alkali is higher than 5% and lower than 1%. In addition to acid and alkali, acid-base wastewater often contains inorganic and organic substances such as acid salts and basic salts. Acid-base wastewater is highly corrosive and can only be discharged after proper treatment.

job operation

Acid-base wastewater is highly corrosive, and it will corrode pipelines, ditches and structures if discharged directly without treatment. Discharging water will change the pH value of water body, interfere with it, and affect the growth of aquatic organisms and fishery production; Discharge into farmland will change soil properties, make soil acidification or salinization, and endanger crops; The loss of acid and alkali raw materials is also a waste. Therefore, the acid-base wastewater should be recycled or treated as much as possible, so that the pH value of the wastewater is between 6 and 9 before it can be discharged into the water.

The general principle of acid-base wastewater treatment is:

(1) For high-concentration acid-base wastewater, the reuse wastewater treatment method should be given priority. According to the water quality, water quantity and different process requirements, the whole plant or region shall be scheduled and reused as much as possible. If it is difficult to reuse, or the concentration is low and the water quantity is large, the concentrated wastewater treatment method can be used to recover acid and alkali.

(2) Low-concentration acid-base wastewater, such as rinsing water from pickling tank and rinsing water from alkali washing tank, should be neutralized.

For neutralization treatment, the principle of wastewater treatment should be considered first. For example, acid-base wastewater is neutralized with each other or acidic wastewater is neutralized with waste alkali (slag), and alkaline wastewater is neutralized with waste acid. Without these conditions, neutralizer can be used for wastewater treatment.

Recycle

For high-concentration wastewater containing acid (generally above 10%) and alkali (generally above 5%), firstly, according to the water quality, water quantity and different process requirements, it is scheduled in the factory or in the region to be reused as much as possible; If it is difficult to reuse, or the concentration is low and the water quantity is large, the acid and alkali can be recovered through concentration.

The recovery methods of acid-containing wastewater mainly include immersion combustion high temperature crystallization, vacuum concentration freezing crystallization and natural crystallization.

The basic process of immersion combustion high-temperature crystallization method is: the high-temperature gas produced by gas combustion is directly sprayed into the waste liquid for evaporation, the water in the waste liquid is removed, and the acidic substances are concentrated and recovered. This concentration method is suitable for treating a large amount of wastewater, and its advantages are high thermal efficiency and high concentration of recovered acid (up to 42.6%). Disadvantages are large acid fog, high anti-corrosion requirements and many combustible gas sources. The basic process of vacuum concentration and natural crystallization is to reduce the boiling point of acid-containing wastewater by vacuum decompression, evaporate water and concentrate and recover acidic substances. The advantage of this concentration method is high automation and easy to solve the problem of acid fog. The disadvantage is that the concentration of recovered acid is low (only18 ~ 20%); There are many materials that need acid resistance and corrosion resistance, and the equipment investment is large. Natural crystallization is mainly used to prepare ferrous sulfate, ammonium sulfate and other chemical raw materials and fertilizers from acid-containing wastewater. In addition, acid-base substances can also be recovered by dialysis and ion exchange. In the process of water treatment, acidic wastewater can also be used for regeneration of sulfonated coal softened by water supply and water quality stability [1].