1. Pickling process:
Pickling is mainly suitable for removing carbonate scale. The principle is to use highly corrosive hydrochloric acid or nitric acid to react with carbonate to generate soluble salts and release CO2 to achieve the purpose of descaling. Pickling can only remove carbonate scale and has basically no effect on insoluble scale such as sulfate, silicate and phosphate. For sulfate scale and silicate insoluble scale, alkali boiling transformation is adopted, and then hydrochloric acid or hydrochloric acid is added to add fluoride for cleaning. For iron oxide scale, add fluoride to hydrochloric acid or use nitric acid to clean it.
Problems with pickling:
▲The corrosion inhibition rate of pickling corrosion inhibitors cannot reach 100%. While the acid dissolves the carbonate scale, it also dissolves the metal of the corroded equipment.
▲Many pickling companies on the market currently use unqualified acid, mixed acid, waste acid, and pickling liquid during the pickling process, which will cause serious corrosion damage to the equipment being cleaned.
▲During the pickling process, if the acid addition method, heating method, dosing sequence, flow rate, temperature, acid concentration and other process conditions are unreasonably designed or operated incorrectly, it will cause serious corrosion damage to the pickled equipment.
▲ Failure to passivate or passivate unqualified equipment after pickling and descaling, using alkali neutralization instead of passivation, etc. will make the equipment active for a long time. The activated metal surface is easily damaged by various corrosive factors in water or atmospheric environment, resulting in increased equipment maintenance costs and shortened service life.
▲Insufficient water flushing, insufficient amount of neutralizing alkali, residual acid remaining in dead corners of the cleaning equipment cycle, and residual scale will lead to continued acid corrosion damage after cleaning.
▲Pickling is very dangerous to equipment and operators. The state stipulates that operators engaged in pickling must undergo strict technical training and obtain a qualification certificate issued by the China Water Treatment Association before they can engage in pickling.
▲Water washing--alkali washing (boiling)--water washing--pickling--water washing--rinsing--water washing--passivation. When encountering difficult-to-dissolve scale, alkaline washing and pickling must be repeated many times, but the effect is still not satisfactory. The entire process has high technical requirements and difficult operations. Even if there is a problem in the tracking, analysis, detection and manual operation of one link, the cleaning process cannot be carried out safely, leading to serious consequences that are difficult to recover and remedy. It happens every year that expensive equipment is damaged due to cleaning leakage.
▲Acid cleaning and descaling must be shut down, and heat exchange equipment such as boilers and central air conditioners need to be shut down for several days. This will cause large production shutdown losses to users of the equipment, especially when production is tight and cannot be shut down.
▲While removing scale from traditional pickling, the cleaning liquid will inevitably cause corrosion damage to the equipment metal, especially hydrogen penetration damage, which will destroy the metal crystal structure, leading to thinning of the equipment metal and deterioration of mechanical properties. In severe cases, hydrogen embrittlement and hydrogen-induced cracking may occur, putting the equipment in dangerous operation and causing explosions. Pickling is a very unsafe technique.
▲The toxicity and acid corrosiveness of pickling waste liquid caused by the decomposition of corrosion inhibitor due to its strong acidity is very harmful to organisms. Wherever it is discharged, fish will die and flowers will wither. It seriously pollutes the environment and is listed as one of the nine major pollution sources in China. Its use has begun to be restricted by the environmental protection department.
▲ Generally, pickling companies discharge the pickling waste liquid directly, because the cost of treating the waste liquid is 3-5 times the cleaning cost.
2. Alkaline cooking——Pickling process:
For the cleaning of difficult-to-dissolve scale, the traditional process involves alkali boiling at high temperatures to convert it into acid-soluble carbonate, which is then removed by pickling. This treatment process needs to be repeated many times to achieve results and the effect is limited, so it has high cost, long construction period and low efficiency. Alkalization in boiler cleaning is the cleaning of boilers by alkaline media at high temperatures, which causes the cleaning objects to saponify, dissolve or transform, and finally loosen and fall off, and then remove them manually.
During the alkali cooking process, the trisodium phosphate, sodium hydroxide concentration and pH value of the alkali boiling solution should be measured every 2 hours. This process is extremely complex and dangerous, and requires constant on-site tracking and inspection, and adjustments to the operating steps based on data from the changing process. Sometimes it is necessary to repeatedly alkali boil and pickle, but the cleaning effect is still not satisfactory. Cleaning a 20t/h boiler with a scale thickness of 10mm requires 120 to 160°C alkali boiling for 10 days is time-consuming and labor-intensive, and the effect is not good.
3. Some foreign companies use the EDTA method (also called complex cleaning)
It has a good effect in cleaning sulfate scale, but due to the high price of EDTA, the cleaning cost is extremely high, which is unacceptable to enterprises and has not been widely used.
