The choice of cleaning agent in industrial cleaning

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(1) Requirements for water

Generally speaking, fire water can be used for flushing water during cleaning. Fire water and industrial water can also be used for alkali washing, alkali water washing, acid washing and acid washing. Desalted water should be used for rinsing and passivation of industry websites.
The cleaning water is preferably colorless and transparent. If the acrobatics in the fire water will affect the cleaning effect, it cannot be used.
(2) the acid commonly used in cleaning

Acid is the most commonly used chemical to treat metal surface fouling. The acids commonly used in cleaning include inorganic acids and organic acids.
Inorganic acids commonly used in cleaning

⑴ sulfuric acid sulfuric acid is a hydrate of sulfur trioxide, is a colorless viscous liquid.

The mass fraction of sulfuric acid used in chemical cleaning is generally above 15%, and it is usually used after diluting 95% to 98% concentrated sulfuric acid. Diluted sulfuric acid will generate a lot of heat, so concentrated sulfuric acid should be slowly added to the water, and while adding acid while stirring to dissipate the heat in time to prevent splashing. Moreover, the dilution sequence cannot be reversed. Never add water to concentrated sulfuric acid. When a small amount of water is added to concentrated sulfuric acid, the heat released by the women's shopping website can rapidly increase the water temperature and cause boiling, which will cause acid splash and cause harm to people. In addition, sulfuric acid is dangerous to the human body and equipment, so pay attention to safety when using it.

The advantage of using sulfuric acid as a cleaning fluid is that it is cheap, not corrosive to stainless steel and aluminum alloy equipment, and is suitable for cleaning these special metal equipment. Sulfuric acid is a strong acid that is not volatile, so it can be heated properly to speed up the cleaning. Generally, when sulfuric acid with mass fraction of 5% ~ 15% is used as cleaning solution, it can be heated to 50 ℃ ~ 60 ℃ to speed up the cleaning speed.
When sulfuric acid is used for cleaning in industry, nonionic surfactants are usually added to improve its rust removal ability. Adding an appropriate amount of nitric acid to sulfuric acid can remove tar, coke, algae and other biological dirt, in order to reduce the corrosion of sulfuric acid on metal objects to add a certain amount of corrosion inhibitor in the cleaning agent.
(2) Hydrochloric acid Hydrochloric acid is an aqueous solution of hydrogen chloride gas. The solubility of hydrogen chloride in water at 15 ℃ can generate 42.7%(mass) hydrochloric acid. The concentrated hydrochloric acid sold on the market is 37.2%(mass).

Hydrochloric acid is a volatile acid, so when Hangzhou website construction is used above 40 ℃, hydrogen chloride gas will volatilize from the hydrochloric acid solution, which brings difficulties to the use.
When hydrochloric acid is used as a cleaning solution, it is generally used at a concentration of less than 10% and used at room temperature, and try to avoid heating up to prevent acid mist. Since most chlorides are easily soluble in water, alkaline carbonate scales, rust, copper rust, and aluminum rust can be well dissolved in hydrochloric acid. Hydrochloric acid is cheap, so it is widely used in the cleaning of the above dirt. Hydrochloric acid cleaning fluid is suitable for equipment cleaning of carbon steel, brass, copper wall, iron wall and other copper alloy materials. It is the most effective and economical cleaning of carbonate scale and rust, so the industry has been widely used to clean heat exchangers, various reaction equipment and boilers.
Note: hydrochloric acid as cleaning stainless steel and aluminum surface dirt cleaning fluid. Because hydrochloric acid has a strong corrosive effect on many metal materials such as steel, corrosion inhibitors should be added to hydrochloric acid in order to ensure that the equipment is not corroded during cleaning.
(B) Nitric Acid The industrially used concentrated nitric acid is a 65% by mass aqueous solution. It is one with sulfuric acid and hydrochloric acid

It is widely used as three common strong acids.

The primary ecological atomic oxygen produced by the decomposition of nitric acid has strong oxidation, which is an important characteristic of nitric acid. Therefore, nitric acid has a wide range of dissolution ability for many other metals besides precious metals (such as gold and platinum). When cleaning and removing the dirt on the metal surface, it can not only remove the organic dirt, but also form a dense oxide film on the surface of some metals to protect the metal from corrosion. However, it should be noted that the brown-red nitrogen dioxide gas produced by the decomposition of nitric acid is very toxic, and it should be used with great attention to safety, and it should be operated in a well-ventilated environment.
A mixture of 1 part of concentrated nitric acid and 3 parts of concentrated hydrochloric acid is called aqua regia, which is a strong acid with strong corrosion ability and can dissolve precious metals such as gold and platinum.
The mass fraction of nitric acid used in industry for pickling is generally about 5%. Under the condition of low attempt, nitric acid is relatively stable, not easy to decompose, and the oxidation is weakened, mainly playing the role of acid. Because nitric acid is relatively stable, not easy to decompose, nitric acid itself has a certain oxidation, so the boiler scale and metal oxide has a strong solubility. Some metal oxides and scales that cannot be dissolved by hydrochloric acid are usually cleaned by nitric acid solution, so nitric acid is widely used in industry as pickling agent. In particular, nitric acid is used to clean equipment that does not use stainless steel as a substrate, which will not cause pitting corrosion like hydrochloric acid, and nitric acid has a particularly good effect on cleaning copper rust. Therefore, nitric acid is often used in cleaning stainless steel and copper equipment. In industry, nitric acid is mainly used to clean stainless steel, carbon steel, brass, copper and carbon steel-stainless steel equipment and brass carbon steel welding assembly equipment.
At present, Lan-826 and Lan-5 corrosion inhibitors are widely used, and the effect is very good, while the general corrosion inhibitor is easy to be decomposed by nitric acid and failure.
(4) Hydrofluoric acid Similar to hydrochloric acid, hydrofluoric acid is a solution of hydrogen fluoride gas dissolved in water. The hydrofluoric acid sold in the market is generally 30% to 60% (mass). It is a weak acid, and the strength of the acid is similar to that of formic acid in organic acids.

The biggest feature of hydrofluoric acid is that it can react violently with silica and dissolve it. Silica is very stable and resistant to all other inorganic acids. Hydrofluoric acid on the human body has a strong toxic and sexual and corrosive, take should be very careful, to wear rubber gloves, protective masks or masks. An alternative to hydrofluoric acid for the same purpose is ammonium hydrofluorate, which is an acidic buffer solution formed by the reaction of hydrofluoric acid with ammonium fluoride. The effect of ammonium hydrofluoric acid is milder than that of hydrofluoric acid. When hydrofluoric acid is used in industry as pickling agent, the content is generally below 5%. Due to the increase of temperature, the reaction speed is obviously accelerated, so the temperature is often controlled at about 50 ℃. The use of hydrofluoric acid to clean rust and dissolve oxide has the characteristics of short cleaning time and high efficiency. This is because hydrofluoric acid has a strong ability to dissolve iron oxide, which is due to the special effect of ions such as fluorine-oxygen exchange when hydrofluoric acid contacts with ferroferric oxide, followed by F-ion complexation reaction to dissolve the oxide skin. Hydrofluoric acid is corrosive to metals, and its corrosion to high alloy steel containing 13% to 15% chromium is about 10 times higher than that of low alloy steel, so it is often used with appropriate corrosion inhibitors.
Hydrofluoric acid cleaning has the advantage that the residual liquid after pickling is easy to handle, and the precipitation of iron hydroxide and calcium fluoride generated when the residual acid is neutralized with lime water can be removed from the water. Industrial hydrofluoric acid is usually not used alone but with ammonium fluoride, hydrochloric acid, nitric acid and other substances, such as hydrofluoric acid-ammonium fluoride cleaning agent is mainly used to clean silicon scale, but also to add hydrochloric acid and nitric acid for cleaning rust. The hydrochloric acid-hydrofluoric acid cleaning liquid is mainly used to remove the mixture of carbonate scale, silicate scale and iron oxide scale, in which hydrochloric acid dissolves carbonate scale quickly, but can not dissolve silicate scale, while hydrofluoric acid can dissolve silicon scale and iron oxide. Sometimes a certain amount of ammonium bifluoride is added to the hydrochloric acid pickling solution, and the purpose is to use the reaction of ammonium bifluoride and hydrochloric acid to generate hydrofluoric acid to form a hydrochloric acid-hydrofluoric acid mixed cleaning agent to speed up the dissolution of carbonate scale, silicate scale and iron scale mixture. However, nitric acid-hydrofluoric acid has good dissolving ability for carbonate scale, silicate scale and various rust, and has fast scale removal speed. It is especially suitable for cleaning equipment of carbon steel-stainless steel and carbon steel-copper combination that are not resistant to hydrochloric acid. It has the advantages of low corrosion rate, no hydrogen permeation, fast cleaning speed at normal temperature, energy saving and convenient source of raw materials, it is the best pickling agent for carbonate scale, silicate scale and iron scale in domestic heat exchanger, boiler and various chemical equipment. In order to reduce the corrosion of the metal substrate, the corrosion inhibitor should also be added to the hydrofluoric acid pickling agent.

Organic acids commonly used in 2.2 cleaning

There are many organic acids used for pickling, such as sulfamic acid, glycolic acid, citric acid, ethylenediaminetetraacetic acid, etc. Compared with inorganic acid, pickling with organic acid has relatively high cost, needs to be operated at a higher temperature, and the cleaning takes a long time, which is its disadvantage. However, organic acids are often less corrosive, and some organic acids have chelating ability and can be used in equipment cleaning without stopping, so they have their own characteristics and use value.
(1) Aminoiodic acid Sulfamic acid molecular formula is NH2SO3H, commercially available as a solid. It has the characteristics of non-volatile, no odor and minimal toxicity to human body.
Sulfamic acid and aqueous solution have the same strong acidity as hydrochloric acid and sulfuric acid, so they are called solid sulfuric acid. The aqueous solution of sulfamic acid is more stable at room temperature, but at higher temperatures, it will be hydrolyzed into ammonium sulfate and ammonium bisulfate.

In the case of washing with sulfamic acid, the temperature is generally controlled below 60°C. Sulfamic acid is not volatile, no special odor and less toxic to human body.
Alkaline earth metal salts of sulfamic acid have good solubility, and sulfamic acid reacts violently with calcium and magnesium scale. Usually industrial use of 7% ~ 10% content of sulfamic acid aqueous solution as a cleaning agent, in the following 60 DEG C scale, generally within 1h can be 90% of the calcium and magnesium scale into soluble amino acid salt and removal. Ammonium sulfonic acid has a slow effect on rust, and some chlorides such as NaC1 can be added to slowly produce hydrochloric acid, thereby effectively dissolving rust.
Due to the high solubility of most metal salts of sulfamate in water, do not produce precipitation in the cleaning solution, and sulfamic acid is less corrosive to metals, so it is often used to clean steel, copper, stainless steel, aluminum and ceramic materials such as the manufacture of equipment on the surface of the hammer and scale. Sulfamic acid is the only acid that can be used for cleaning galvanized metal surfaces.
(2) Citric acid Citric acid, also known as citric acid, is a lemon fruit flavor, soluble in water crystals. Citric acid is the most used organic acid in the cleaning process. It can dissolve iron oxide, copper oxide and other rust scales. The principle of its action is to use H ions and alkaline metal oxides on the one hand, and the complexation of citric acid on the other hand (Citric acid is a complex masking agent commonly used in analytical chemistry). It and rust generated citric acid solubility in water is small, so in the citric acid solution to add ammonia, the valley called ammoniated citric acid, then it through complexation to generate high solubility of ferrous citrate and citric acid ammonium salt and improve the effect of removing iron oxide. The reaction process is that citric acid reacts with ammonia to form monoammonium salt of citric acid, and then the complexation reaction occurs. Mono-ammonium lemon reacts with iron oxides to form soluble substances such as ferrous ammonium citrate and ferric ammonium citrate to dissolve the rust.
When there are rust and copper rust in the equipment at the same time, it can be removed by controlling the pH value (adding ammonia water) to make it equal to 3.5, so that iron ions that are easy to complex form ferrous ammonium citrate and ferric ammonium citrate, and also prevent the precipitation of Fe(OH)3 with very small solubility product. After the iron compound is dissolved, the pH of the solution is controlled to 9, and the citrate ion concentration in the solution is increased to complex the copper ions to remove the patina scale. Citric acid in chemical cleaning is often used to remove rust-based rust, in order to speed up the cleaning speed, shorten the pickling time, often maintain a higher temperature, in addition, in order to prevent the corrosion of acid on the metal is also added corrosion inhibitor. Usually, after the boiler is cleaned with hydrochloric acid, it is usually rinsed with dilute citric acid to remove the remaining iron salt, which can make the cleaned metal surface more easily passivated.
(3) Common alkali

1. Sodium hydroxide

Sodium hydroxide, also known as caustic soda, caustic soda or caustic soda, white solid, melting point 318.4 ℃. Because concentrated alkali can corrode glass, solid sodium hydroxide is usually stored in plastic bottles or tin drums.
Solid sodium hydroxide has a strong water absorption, and its solubility in water at 0°C is 42%. In chemical cleaning engineering, about 5% sodium hydroxide aqueous solution is generally used as alkaline chemical cleaning solution, and sometimes some sodium carbonate or sodium phosphate is added to make a solution. Aqueous sodium hydroxide has two main uses as a cleaning fluid.
NaOH can react with animal and vegetable oils to generate glycerin and soap. The soap and glycerin generated by the reaction are easily soluble in water. Soap as a surfactant can also produce emulsification, which can improve the wetting performance of the aqueous solution on the surface of the scale layer.
Calcium magnesium sulfate is a strong acid and alkali precipitation salt, which can not be dissolved directly by strong acid. Sodium hydroxide can then react with calcium magnesium sulfate to form calcium hydroxide or magnesium hydroxide. The Na2SO4 produced by the reaction is easily deep, and Mg(OH)2 and Ca(OH)2 are easily dissolved by acid.
Sodium hydroxide can dissolve proteins to form alkaline protein compounds, which have a significant corrosive effect on human tissues and can cause burns when in contact with skin. The greater the concentration of the alkali solution and the higher the temperature, the stronger the burning ability. When opening the sodium hydroxide drum, it should not be neglected because it is solid. You must wear overalls, rubber gloves and protective glasses, and use special tools. When breaking the lump of sodium hydroxide, wrap it with waste cloth or do it in an uncovered vat to prevent splashing of the lump.

2. Sodium carbonate

Soda ash (Na2CO3) is a strong alkali weak acid salt. Because its aqueous solution is alkaline, in practical use, has been used as a base. In chemical cleaning, Na2CO3 can be used in alkaline washing, alkaline cooking, neutralization, passivation and other steps.
(1) In the process of alkaline washing, the use of Na2CO3 can loosen, emulsify or disperse oil and fat substances and become soluble substances.
⑵ Alkali cooking alkali boiling by Na2CO3, CaSO4, CaSiO3 and other substances insoluble in acid reaction at a higher temperature, so that it is converted into CaCO3 soluble in acid, thus laying the foundation for pickling.
(3) After the neutralization and pickling, the residual acid inside and on the surface of the equipment is neutralized with weakly alkaline Na2CO3 to prepare the conditions for passivation.
(4) passivation of weak alkaline Na2CO3 to the metal surface of the active metal ion hydrolysis, the formation of a layer of precipitation film, attached to the metal surface. In this way, the metal ions are passive, and at the same time, this layer of precipitation film protects the metal substrate from the erosion of the external environment.
3. Phosphate

Phosphate has a softening effect and helps to disperse or peptize dirt. Phosphates are commonly used in orthophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, polyphosphate, etc. Trisodium phosphate has good degreasing ability, commonly used in degreasing alkali washing formula, can also react with iron to generate iron phosphate precipitation, so it can be used in passivator formula.
Polyphosphate, also known as polyphosphate, is one of the most valuable in the formulation of water-based metal cleaners. It is usually made after the following phosphate is heated and dehydrated. Its main varieties are sodium tripolyphosphate (commonly known as pentasodium), tetrasodium pyrophosphate, sodium hexametaphosphate.

Sodium tripolyphosphate is almost the best universal phosphate, most used in large water-based metal cleaners. Its main features are as follows.
① Sodium tripolyphosphate has a strong chelating ability to polyvalent metal ions, which can complex insoluble polyvalent cations to generate soluble complex ions.
② sodium tripolyphosphate on the fine inorganic particles or fat particles with dispersion, emulsification, peptization, can improve the dirt suspension capacity, to prevent dirt redeposition.
The phosphate can not only play the role of builder, but also is a water-soluble corrosion inhibitor. Phosphates as water-soluble corrosion inhibitors mainly include sodium hexametaphosphate, sodium tripolyphosphate, trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, etc. Phosphate is mainly used as a sodium iron corrosion inhibitor. Disodium hydrogen phosphate is effective for steel, cast iron, lead, etc., but has a corrosive effect on total brass; sodium hexametaphosphate is mainly effective for steel, cast iron, and lead, and has a corrosive effect on copper, zinc, brass, and aluminum. So phosphate is a good industrial cleaning aid.
(4) Organic solvents

The biggest feature of organic solvents is the dissolution of oil fast, high oil removal efficiency, the dissolution of polymer swelling effect is strong, but can not dissolve inorganic dirt.
The organic solvents used for cleaning can be divided into two categories: petroleum solvents and halogenated hydrocarbon solvents, and can also be divided into three categories: petroleum solvents, chlorinated hydrocarbon solvents and fluorocarbon (Freon 113 series) organic solvents.
4.1 petroleum solvent

The commonly used words for oil removal are white alcohol, gasoline for cleaning (boiling point 80~120 ℃), kerosene and toluene.
4.2 chlorinated hydrocarbon solvents

Chlorinated hydrocarbon solvents used as organic cleaning fluids in cleaning works are mainly dichloromethane, trichloroethylene and carbon tetrachloride. Their common feature is the ability to dissolve oil pollution is very strong, non-combustible, waste liquid is easy to regenerate repeated recycling, in addition to aluminum and magnesium on most metals without corrosion, but high toxicity. Therefore, chlorinated hydrocarbon solvents are commonly used as recyclable cleaning fluids for vapor cleaning equipment, with high oil removal efficiency and suitable for industrial cleaning. However, the tightness of the steam cleaning equipment is better. These solvents are not suitable for manual cleaning.