Explore surface treatment before drum coating (2)
Third, the surface treatment process and equipment before steel coating
The cleaning of steel drums is not only the role of cleaning agents, but also a complete cleaning process and the use of cleaning equipment to achieve. High-efficiency cleaning agents and cleaning equipment plus a reasonable cleaning process are the guarantee for good cleaning results.
(I) Steel drum cleaning process and equipment requirements
When selecting the cleaning process and cleaning equipment for steel drums, first consider the following aspects:
1, can be *
The steel drum cleaning process and equipment required to be used have a stable cleaning quality and can achieve the required degree of cleanliness of the process.
2, the impact on the steel barrel
It is required that the damage to the steel drum in the cleaning process should be as small as possible, and no new secondary pollution should be caused to the steel drum.
3, is conducive to the protection of the natural environment
The steel drum cleaning process and equipment are required to prevent or minimize the damage to the natural environment caused by cleaning liquid waste, noise, and exhaust gas.
4, efficiency
The production process and equipment for steel drum cleaning are required. According to the specific situation of the company, the process and equipment with high production efficiency and low labor intensity should be used as far as possible.
5, a good working environment
The process and equipment required to clean the steel drums can maintain a good working environment and ensure the health and safety of production workers.
6, economical
It is required to adopt cleaning processes and equipment that can meet the requirements for the degree of cleanliness of the surface of the steel drum, and at a low cost.
(II) Process and characteristics of steel drum cleaning
The surface treatment of steel drums prior to painting is usually performed by three methods of dipping, spraying and hand-wiping. Each of these three cleaning methods has its own characteristics. The specific cleaning method should be determined according to the specific conditions of the company and the production volume. Its features are:
1, the characteristics of dipping cleaning method
Using this method to clean the outer surface of steel drums, high alkalinity solutions can be used. It is not easy to form foam, so it is allowed to contain more surfactant. Its equipment structure is simple, less investment and easy maintenance. However, the processing time is longer, the temperature is higher, and the production efficiency is low. Not suitable for mass production.
2, the characteristics of spray cleaning method
Has a strong mechanical effect, good cleaning effect. Solution concentration and treatment temperature can be lower. The processing time of the steel barrel is also short, and the production efficiency is high, which is suitable for mass production. The phosphating film crystallizes finely. Easy to produce foam, the use of low-foaming surfactants. However, the equipment is relatively complicated, equipment investment is large, and maintenance is difficult.
3, the characteristics of hand-wiping cleaning method
This method is simple and flexible, not subject to environmental conditions and production site conditions. Both the alkali concentration and the operating temperature cannot be too high. However, the production efficiency is low and the labor intensity of workers is high. Not suitable for large-volume automated operations. In addition, when working, care should be taken to prevent alkalis from damaging the skin and the cornea.
The surface treatment before steel drum coating mainly includes three parts: oil removal, rust removal and phosphating. Below we discuss separately.
(III) Degreasing and cleaning of steel drum surface
The degreasing cleaning of steel drum surface is also called degreasing cleaning. The degreasing cleaning of the steel drum surface is usually carried out by using an alkaline solution, an organic solvent, a surfactant, a solvent, or a mixed solution as a cleaning agent, and dipping, spraying, wiping, and other cleaning methods are used.
1, alkaline elution fat
(1) The cleaning mechanism of steel tank lye
Alkali liquid degreasing cleaning is mainly based on emulsification, dispersion, dissolution and mechanical action. The oil on the surface of steel drums is generally mineral oil, which belongs to the unsaponifiable neutral oil. It can not react chemically with alkali. During the cleaning process, the strong alkali promotes the dissociation of oil from the surface of the steel drum, and then dissociates. Oily * colloidal base glue is dispersed stably.
There is no need to worry about corrosion of the steel plate on the surface of the steel drum due to the alkali, because the steel has excellent alkali resistance, so the steel drum cleaning and degreasing is widely used alkali cleaning agent.
(2) Types and properties of commonly used alkali cleaning agents for steel drum cleaning
a. Sodium hydroxide, also called caustic soda, is a strong alkaline compound. When cleaning, it mainly acts as a dissociating oil. It does not have the buffering effect of alkaline salt, and it has no corrosion effect on steel drums at normal temperature and medium temperature, and it can produce slow corrosion at high temperature. Sodium hydroxide is inexpensive, but it has a strong corrosive effect on human skin, so it should be handled with great care.
b. Sodium carbonate, also known as soda ash, is an inexpensive alkaline salt. It has a buffering effect. Unlike strong alkalis that erode steel drums, sodium bicarbonate (also known as soda) and tribasic sodium carbonate are weaker in alkalinity, but they react with calcium ions in water to form soluble salts, which have a certain amount of hard water. Softening effect.
c. Silicates include sodium metasilicate, sodium orthosilicate and water glass. The silicic acid produced by the hydrolysis of silicates is insoluble in water and colloidally suspended in the bath to disperse solid dirt. The basicity of sodium silicate is close to that of caustic soda. Because of the colloidal nature of the silicic acid contained in it, the dispersion of the dirt and the ability to maintain it are superior. Therefore, the steel drum is often used together with caustic soda. The general characteristics of silicates are that the lye is excellent in wetting and penetrability, keeps the dispersibility of the dirt and hard water resistance, and has a certain anti-corrosion effect on the surface of the steel drum.
d. Phosphates used as alkaline cleaners include trisodium phosphate, tetrasodium pyrophosphate and sodium tripolyphosphate. Phosphate has a more pronounced dispersion effect and can disperse large-grained dirt into small particles that approximate colloidal particles. Tetrasodium pyrophosphate also has a surface-active effect. Phosphate can also be used as a sequestering agent. It is combined with calcium and magnesium ions in hard water to be removed as a water-insoluble salt. The common characteristic of phosphate is that it can inhibit the corrosion of the metal surface of steel drums, but due to its low alkalinity and the relatively high price in alkalis, it is rarely used as a cleaning agent by steel drum manufacturers, and it is often cheaper. The alkali is used together.
(3) Requirements for Alkaline Cleaner in Steel Tank Cleaning
a, cleaning agent should have a high degree of detergency, to be conducive to the surface of the drum sprayed with high quality, high added value, the stability of the steel drum cleaning is higher.
b. When the steel drum is cleaned, the amount of cleaning fluid should be small, which can reduce the load of wastewater treatment.
c. When cleaning, it is required that the alkali cleaning agent has the lowest toxicity and is easy to be decomposed, and the waste liquid is easy to handle.
d. The cost of the cleaning agent should be low, and the corrosivity of the steel barrel should be small.
(4) Alkali washing process commonly used for steel drums
Alkali washing process commonly used in steel drums includes immersion cleaning and spray cleaning.
a, dipping and cleaning
The steel drum is immersed in the washing tank so that the entire surface is in contact with the alkaline liquid. In order to increase the degreasing efficiency, the heating time and the cleaning time can be increased, and the flow speed of the alkaline washing liquid can also be accelerated, such as stirring with a mixer and circulation pump circulation.
b, spray cleaning
Using a spray cleaning device, the alkaline cleaning liquid is sprayed on the surface of the steel drum for cleaning. The spray cleaning includes the saponification, emulsification, and the like of the alkaline detergent, and also has a spray-impact action. The greater the injection pressure, the greater the proportion of mechanical detergency in the degreasing.
(5) Influence factors and common problems of lye during steel drum cleaning
a, the composition of alkali
Alkaline composition is the main factor affecting the deoiling quality of steel drums. Alkali liquid degreasing is * saponification, wetting, emulsification, dispersion and other effects to remove oil, because a single material does not have these properties at the same time, requires a variety of material combinations. The formula number is not strictly required, but the matching is appropriate to obtain the best overall performance.
b. Concentration of lye
The high lye concentration favors the saponification reaction, and the higher the lye concentration, the lower the surface tension between the oil and the solution, making the oil easily emulsified. Practice has proved that when the pH of the solution is below 8.3, the solution will hardly deoil.
c. The temperature of the lye
Practice has proved that in the drum degreasing cleaning process, the high temperature of the lye is conducive to the saponification reaction, but also can make the higher melting point of the oil softening, is conducive to infiltration, emulsification, and thus a high temperature steel drum cleaning effect . In general, the temperature of the solution is between 70 and 100 °C. ,
d. Mechanical function
The mechanical action helps to remove the oil on the surface of the steel drum. The mechanical action is strong and the cleaning effect is good. Dipping and cleaning should be properly stirred solution. Spray cleaning should maintain the solution to the steel drum has a certain degree of scouring force, usually pressure requirements between 50kPa ~ 400kPa.
e. Surfactant
Adding an appropriate surfactant to the alkali cleaning solution can reduce the interfacial tension, improve the wettability and emulsification, and improve the cleaning effect. However, surfactants are prone to foaming.
With dip cleaning, more cationic and non-ionic surfactants can be added without affecting the cleaning operation due to foaming. However, when spray cleaning is used, due to the strong spraying effect, even if a small amount of surfactant is contained, a large amount of foam is often overflowed out of the tank, which not only pollutes the work site, but also causes a large consumption of the cleaning solution, so that the production cannot be performed continuously.
Foams are formed by gas dispersion in liquids and solids. Foaming depends on the ability of the material to foam and the stability of the resulting foam. In order to overcome the large amount of foam generated during spray cleaning, the following measures can be taken:
1 Use a low foaming surfactant and control its amount.
2 Add antifoam agent. Silicone oils, higher alcohols, poorly water-soluble benzenes, and volatile petroleum solvents all have defoaming effects.
3 Reduce the structure of the device to bring air into the lye to reduce foam generation.
f, washed
After the steel drum is degreasing and washed in the alkali solution, rinse with water to remove the cleaning agent remaining on the surface of the steel drum. Because the residual cleaning agent will affect the adhesion of the coating, the absorption of alkalis and salts in the cleaning agent will cause early blistering of the coating.
Washing can be done by dipping or spraying. Steel drums often use two-stage washing, one for hot water and the other for cold water. Hot water temperature should not be too high, generally controlled at about 60 °C. When using two-stage water washing, which level is more suitable for hot water, it should be determined according to the specific conditions when the steel drum is cleaned. If the used cleaning agent is not easy to wash off with cold water, the first-stage washing should use hot water. If the used cleaning agent is easy to wash off with cold water or there are many cleaning agents attached to the surface of the steel drum, it needs to be washed with flowing water, and the first-stage washing should use cold water to avoid consuming a lot of heat.
If a multi-stage reverse cleaning method is adopted, the quality of water washing can be improved, water consumption can be saved, and the amount of wastewater discharged can be reduced.
After the steel drum is rinsed with water, it shall go directly to the derusting and phosphating process and then dried. If it does not enter the rust removal and phosphating process directly, it should be dried first. Before drying, the surface of the drum can be blown with compressed air or water. Generally drying at 100 °C 1min ~ 10min can be. If yellow rust occurs in the drying process, add a small amount of rust inhibitors, such as triethanolamine, to the water that was last rinsed. Adjust the pH of the water to 8-9 to effectively prevent the yellow rust.
2, solvent degreasing
The use of organic solvents to remove oil from various oils has a wide range of applications in the barrel making industry. The characteristics of organic solvents are high in oil removal efficiency, but they cannot wash away inorganic salts and alkalis. In the coating process, the commonly used organic solvent for oil removal is white alcohol (also known as paint gasoline, boiling point 100 ~ 200 °C, flash point 30 °C) or cleaning gasoline (boiling point is 80 ~ 120 °C) and various Chlorine solvents such as trichloroethylene. However, due to the toxicity of chlorine-containing solvents, which are not suitable for use in physical cleaning, most of the barrel companies use gasoline scrubbing.
Gasoline cleaning is usually done by hand scrubbing with a brush or rag, or by dipping directly into the solvent tank. Manual scrubbing is labor intensive and labor conditions are poor. Petrol cleaning stations should have good ventilation and firefighting facilities.
3. Degreasing Cleaning of Surfactant Aqueous Solution and Mixed Solvent
Since hydrocarbon solvents such as gasoline used for solvent cleaning are flammable and explosive, and halogenated hydrocarbon solvents are limited due to their toxic or destructive effects on the ozone layer, safe, non-toxic, mild surfactants or mixed solvents are used for degreasing steel drums. The new direction.
Surfactant aqueous solution and mixed solvent are used as steel drum cleaners to clean the surface of steel drums with low surface tension, good wetting and moisturizing properties, and strong emulsifying power. With the development of various new types of synthetic surfactants, especially after the development of a non-foaming or less foaming cleaning ability and a biodegradable surfactant, surfactants have been widely used in the cleaning of steel drums.
The characteristics of surfactants such as aqueous surfactants and mixed solvents are as follows:
1. Under the same conditions, the cleaning ability is stronger than that of lye cleaning, the oil removal quality is good, and the alkalinity is very weak, and the erosion of the steel drum is small.
2. It can be combined with other steel drum surface treatment processes, such as the composition of de-oil cleaning and phosphating two-in-one process, and can simplify the process under certain conditions.
3. After the use of biodegradable surfactants, it is beneficial to the surface treatment process of sewage treatment.
(1) Deoiling Mechanism of Surfactant
The deoiling mechanism of a surfactant generally consists of the following three stages.
1. Wet the surface of the steel drum to make the oil stain away.
Due to the adhesion of oil stains and the adsorption of trace amounts of air, the surface of the steel drum has a large interfacial tension and is not easily wetted by water. When surfactants are added to water, surfactants tend to adsorb at the steel-water interface and the oil-water interface, lowering the interfacial tension, replacing the gas on the surface of the steel drum, and penetrating between the surface of the steel drum and the oil stain and the oil stain. Between the oil particles, the surface of the steel drum is preferentially wetted, so that the oil film is curled into oil beads and leaves the surface of the steel drum.
2. Transfer the detached oil to the solution
Adsorption at the oil-water interface by emulsifying surfactants results in the formation of emulsions with a certain stability, or by the solubilization of surfactants to dissolve water-insoluble oils into the surfactant micelles. in. Both of these methods can transfer the detached oil to the solution.
3, the oil in the solution will be trapped in the solution
Since the adsorption membranes surrounding the oily particles have sufficient mechanical strength so as not to rupture due to collisions, the oily dirt will not re-aggregate, and the scattered oily particles are trapped in the solution and will not be deposited on the surface of the steel drums to cause secondary pollution.
(2) Surfactant degreasing cleaning
Oil-removing surfactant cleaning agents consist of surfactants, cleaning aids, and additives. Surfactants are generally formulated from anionic surfactants with strong detergency and nonionic surfactants. Cleaning aids are generally alkaline substances such as tripolyphosphate steels. The purpose is to make the cleaning liquid alkaline to improve the degreasing ability. The surfactant does not cause corrosion to the steel barrels, but it has poor ability to remove the old viscous oil stains. .
(3), mixed solvent degreasing cleaning
The mixed solvent used for the degreasing cleaning includes various forms such as a mixture of a lipophilic organic solvent and a surfactant, a mixture of an organic solvent and an alkali aqueous solution, and the like.
When the lipophilic organic solvent exists in the emulsified or solubilized state in the aqueous solution of the surfactant, the two exert a synergistic effect so that the cleaning ability is better exerted. The degreasing ability of a mixed solvent formed by an alcohol or ether-based hydrophilic organic solvent and an alkaline aqueous solution is higher than that of the solvent alone, and the flammability is greatly reduced. In addition, there is an emulsion cleaning agent, which is an emulsifier to make organic. The solvent is dispersed into a mixed solution formed in water. The organic solvent in the emulsified cleaning solution is usually a petroleum fraction having a boiling point of 60 to 210°C. Emulsifiers are anionic surfactants and nonionic surfactants. In order to make the emulsifier have certain solubility in organic solvents, some water-soluble coupling agents are often added.