Methods for removing oil from ferrous metal products

Due to the need for rust prevention in the machining and intermediate process of metal products, the surface is generally adhered to or coated with a variety of calendering oil, cutting oil, polishing paste and anti-rust oil and other greasy substances, before carrying out a variety of surface treatment, these oil should be cleaned, so as not to affect the effect of surface treatment.

Oil pollution can be divided into saponifiable oil and non-saponifiable oil according to its nature. Saponified oil is the oil from animals and plants, which can chemically react with lye to produce soap, and non-saponified oil is the mineral oil that does not chemically react with lye, such as petroleum jelly, lubricating oil and paraffin. Both types of oils are not dissolved in water, so they cannot be cleaned directly with water, and are generally removed or degreased by organic solvent treatment, lye treatment or electrochemical treatment.

(1) organic solvent oil removal. The use of organic solvents for oil removal can remove both saponifiable and unsaponifiable oils. The solvent requires strong solubility, non-flammable and explosive, low toxicity, slow volatilization, and low price. In fact, the organic solvents used are difficult to fully meet the above requirements. The organic solvents commonly used in production are gasoline, solvent gasoline, turpentine, toluene, xylene, dichloroethane, trichloroethylene, tetrachloroethylene and so on. Among them, chlorinated hydrocarbons are widely used, its advantages are strong solubility, non-flammable and explosive, the disadvantage is toxic, high cost, in the presence of water can be decomposed into hydrogen chloride, there is corrosion, in use should pay attention to moisture and humidity.

The simplest way to remove oil with organic solvents is to scrub the metal surface with cotton yarn dipped in organic solvents (suitable for large workpieces). The product can also be immersed in a tank containing organic solvents and the oil can be removed by dipping, which can be carried out in a special degreasing machine. A number of tanks are placed in the degreasing machine, respectively equipped with organic solvents, by mechanical transmission, so that the workpiece in turn immersed in the degreasing.

In the degreasing machine, the solvent is used more trichloroethylene, because it has a strong degreasing capacity at low temperatures, and can increase the degreasing capacity with the increase of temperature. In addition, due to its low boiling point, trichloroethylene, which already contains a lot of oil, can be reused after fractionation and purification, so that the cost of degreasing treatment is reduced. The most commonly used petroleum solvent is 200 solvent gasoline, which is generally degreased by dipping or scrubbing. Using organic solvent degreasing, after the solvent volatilizes, the surface of the product often leaves a thin layer of oil film, so the workpiece must be immersed in lye for supplementary treatment.

The advantage of using organic solvents for oil removal is high efficiency. In the case of degreasing machines, the solvent consumption is less, so the cost is not high. If the degreaser is not used, the solvent consumption is large, the cost increases, and it is flammable and toxic. Due to these shortcomings of organic solvent oil removal, in the absence of the use of degreasing machine, should try not to use organic solvent method oil removal.

(2) lye oil removal. Lye oil removal is completed by the saponification and emulsification of lye, saponification can remove the animal and plant oil on the surface of the product, emulsification can remove the mineral oil on the surface of the product. Because the emulsifying effect of lye itself is weak, the emulsifying oil removal time is longer, because various emulsifiers are often added to the lye, such as soap, water glass, OP emulsifier, etc., to improve the efficiency of emulsifying oil removal.

When the formulation and process conditions of lye degreasing are selected properly, both saponified and unsaponified oil can be smoothly cleaned from the surface of the product. The concentration of alkali should not be high when the alkali is removed from the oil, because the concentration is high, it will reduce the solubility of the soap and the stability of the emulsion, and generally use 5%-10% caustic soda solution is more appropriate. The formula of lye oil removal includes the following components: sodium hydroxide, sodium carbonate, trisodium phosphate, water glass and soap, etc., of which sodium hydroxide plays the saponification role, sodium carbonate and sodium phosphate maintain a certain alkalinity of the solution, water glass and soap mainly play the emulsification role.

Identify whether the metal surface is clean or not, according to the surface is wet by water. When the oil on the metal surface is removed, there is a continuous film of water, otherwise the water will gather into droplets. At present, the common method used in the metal surface pretreatment degreasing process is alkaline solution chemical degreasing. Production practice has proved that the use of this oil removal method is reasonable, although the application of this oil removal process, in time than the use of organic solvents to remove oil longer, but non-toxic and non-flammable is one of its major advantages, the production equipment used is simpler, and economic.

(3) electrochemical degreasing. Electrochemical degreasing is the electrical degreasing of a product by placing it in an alkaline solution as a cathode or anode. Generally, the method used is to remove oil from the cathode or to remove oil on the cathode first, and then to remove oil on the anode. The lye used for oil removal contains sodium hydroxide, sodium carbonate, sodium phosphate and sodium silicate and other components.

The second electrode for electrochemical degreasing is preferably nickel-plated steel plate or nickel plate. When removing oil from the cathode, such as using an iron plate as the anode, the iron will appear dissolved and dirty solution, and produce deposits on the cathode.

The principle of electrochemical oil removal is that the hydrogen precipitated on the cathode or the oxygen precipitated on the anode has a stirring effect on the solution on the metal surface, promoting the oil from the metal surface, and the solution on the metal surface is constantly updated, accelerating the saponification and emulsification. The gas precipitated from the electrode destroys the thin layer of oil film attached to the surface. The small bubbles are detached from the electrode near the oil drop and remain on the surface of the oil drop, and stay on the interface between the oil and the solution. As new bubbles continue to precipitate, the small bubbles gradually grow larger, so that the oil droplets are removed from the metal surface under the influence of the bubbles, and are brought to the surface of the solution by the bubbles. Therefore, electrochemical degreasing is better than lye degreasing. In electrochemical degreasing, increasing the temperature of the solution and increasing the current density can improve the degreasing effect.

The disadvantage of cathode oil removal is that the hydrogen precipitated on the cathode will seep into the metal and cause the metal to become brittle. Therefore, for hard high-carbon steel parts,

Such as springs, elastic sheets, etc., can be anodic degreasing, but the anodic degreasing speed is slower than the cathode degreasing. This is because:

① The alkalinity near the anode is low, which weakens the saponification effect;

(2) Less oxygen precipitated on the anode;

(3) Because the oxygen bubble is large, the ability to stay on the surface of the droplet is small, and the effect on the oil droplet is weaker than that of the hydrogen bubble. According to the characteristics of anode and cathode oil removal, oil removal on the cathode can be used in production, and then oil removal on the anode. In this case, the hydrogen that permeates the surface can be completely removed in almost a short time to restore the elasticity of the product.