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Galvanizing processes

The electroplating industry is concerned with coating metallic and non-metallic parts with a thin layer of a more noble metal than the basic one, through chemical changes produced by the electric current. In general, this process is carried out in a reactor, where a determined electrolyte is stored so that the electrical energy is transferred by an anode, providing it with ions in solution.

Pickiling bath wastewater treatment

In Electroplating, different unitary operations are used, with the aim of preparing, treating and coating the piece. These processes are as follows:

Preparation of the piece surface and degreasing

In this operation, the piece surface is prepared by removing burrs and rough points, to create the conditions for subsequent chemical treatment of the optimum surface, for chemical adhesion of the metals to be coated. Degreasing can be carried out at two levels: macro degreasing where heavy fats are removed and micro degreasing where a
refining process occurs with fats that form adhesion films.

Greases and oils found on the metallic pieces are removed from their surface. The pieces are treated by immersion while using the minimum agitation to avoid deformations or breaks. The pieces need to be able to be cleaned well for later treatments of nitriding surface oxidation or the electroplating processes themselves. This degreasing operation
can consist of different processes:

Immersion treatments with alkali

Fat removal by immersion in strong bases, e.g. NaOH or KOH. This can be done at high temperatures. Over time, this bath produces a residue that has to be neutralized with oils in solution and metallic sludge.

These treatments require subsequent rinsing baths to remove the salts resulting from the saponification reactions.

This leads to emulsification of the fluids, which requires bath changes, water recharges, and an emphasis on the “time” factor in the whole process. Degreasing may be impaired in parts or loads with positional impediments, complex geometries, highly porous materials or sintered metals, for example. Normally these baths are additive.

Surfactants

These are used in addition to alkaline solutions as neutral non-ionic degreasers. Their effect is produced by the formation of micelles and they are mostly used in spray systems.

Their main disadvantages lie in the difficulty of interacting with pieces with difficult geometries, sintered materials or loads with positional impediments that prevent easy access of the degreaser to the interior of the load, as well as the ability to emulsify oils and polluting fluids; thus,frequent bath changes are required.

Degreasing by organic solvents

Chlorinated organic solvents have traditionally been used in these types of baths.

Currently, different EU directives impose limits on their use due to their climate change impact. These solvents dissolve fats and leave the metal parts practically dry. They do not attack the piece or alter the material coloration.

These organic solvents can be recovered by distillation.

Electrolytic degreasing with alkalis

This is one of the most effective degreasing procedures. A strongly alkaline electrolyte is used with the help of the cathode electrical current and rarely the anode. Organochlorine dispersants and strong alkalis are used which have a VOC problem.

Due to the use of organochlorine solvents being prohibited, these are currently being replaced by:

  • Paraffin solvents (VOC): These are volatile and flammable organic compounds which require special safety controls.
  • Oxygenated solvents: These are compounds that have a flammability and toxicity problem. They are compatible with many solvents used in paints.
  • Fluorinated solvents: Like most chlorinated solvent-based products, most are non-flammable (except when mixed with other solvent types) and highly volatile, with the products being more like chlorinated solvents. They have the disadvantage of a highly specific application due to their solubility and high vapor pressure. This series of factors impact on their cost.
  • Paraffin solvents (non-VOC): These are high boiling point paraffin solvents and so are not VOCs. A special application is to use them as an intermediate process, since their excellent cleaning capacity, low volatility and viscosity, together with the possibility of making them emulsifiable, mean they can be easily removed with water-based processes and systems (this is often not possible to do with the remains of fats and viscous oils, for example, that need to be degreased).

Degrease washing

This is washing the pieces after the previous phase with water, to remove irregular stains or deposits on their surface.

Water is used in this stage with the result that waste from the previous stage is incorporated.

Pickling

Deoxidation process. Its purpose is to remove the oxides present on the piece surface. Pickling can be carried out in an acid or alkaline bath.

The basic solutions used are hydroxides (sodium, potassium or calcium) and carbonates (sodium carbonate), organic and inorganic additives and surfactants.

The acid solutions used may be sulfuric, hydrochloric or, in certain cases, hydrofluoric acid. As a result of this stage, sewage and sludge are formed from the removal of the oxides.

Pickling washing

This consists of rinsing the metallic or plastic pieces in a tank with water to prevent the carryover of acid to the following process stages. Contaminated wastewater is produced from the pickling process.

The object is to remove any acid remaining from the previous process and to prevent subsequent oxidation of the pieces. The wastewater obtained is rinsing water for the neutralization treatment.

Mechanical preparation of the piece

This consists of preparing the piece to leave it smooth, polished and bright to facilitate deposition of another layer of metal on the surface.

This stage is important for the good quality of the piece. This is divided into roughing, grinding and polishing. Roughing is done by abrasive discs of different sizes and hardness, refined with grain or with medium structure ceramics.

Grinding can be done by hard disks of medium structures or ceramic medium structures.

The polishing can be mechanical or electrolytic by brines that work the surface of the metal leaving it shiny. For this stage of the process, sulfuric, phosphoric, chromic, nitric and citric acids or combinations of them are used for the case of electrolytic polishing, as well as cooling water to prevent overheating of the heat-sensitive parts.

The waste produced in this stage is basically the packaging for the chemicals used, hot water, particulate polishing material and very acid metallic salt solutions, with chromium (VI) being particularly important in the case of chromium.

Physical Cleaning

Removal of particles that remain in the form of lumps on the metal parts. This stage requires cleaning materials (of wool or synthetic fibers) and water at room temperature to remove particles difficult to separate.

Electrolysis

The electrolyte coating is properly produced in this stage. The pieces, fixed like the cathode, are covered with the appropriate metal, leaving some sludge from the metal deposition, the salts and the reduction oxide processes that take place at the cathode and anode (production of oxygen and hydrogen).

To perform this task, metallic coating materials, such as sulphates, chlorides, nickel cyanides, chromium and tin are used. Additional chemical agents are also used, such as sodium naphthalene trisulfonate and formaldehyde.

The waste produced is mainly liquid waste from the nickel, chromium and tin solutions, additivated solutions, cyanide solutions and empty chemical containers.

Hot Washing

The piece is washed with a dilute hydrochloric acid solution, producing an acidic residual solution.

Drying and oiling

The metal pieces must be dried after the electrolytic process to prevent staining on the metallic deposits produced. The drying process can be carried out on drying supports, in drying ovens or by spraying with air at high temperature, 80-90°C to remove surface humidity.

Then, a thin layer of oil is applied to the metallic piece to protect it from humidity and to prevent oxidation. This process is carried out by means of an electrostatic oiling process.

Zinc plating process in strongly acidic medium

Electrogalvanized coatings (electroplating) are created by applying zinc to the steel sheet and stripping by electrodeposition. As with the galvanized sheet, the operation is continuous, and the coating thickness is minimal.

For a steel rolling plant, the sheet or strip is entered with the appropriate equipment in a series of washes and rinses, then in the zinc bath.

Grain refiners can be added to help produce a uniform and well-bonded zinc coating on the steel. Electroplating is applied to steel sheets and wires, and are therefore used in applications similar to galvanizing a continuous sheet or wire.

The most common applications are in the automotive industry and equipment mountings and fasteners. In addition, to extending service life, electroplating can be done to make the coating suitable for painting, and this is often recommended due to the extremely thin zinc coating.

The galvanizing process begins with degreasing by chemical methods, by saponification of possible oils by bases or by electrolysis procedures. The resulting solutions require neutralization and emulsion treatment, if produced.

Afterwards, the piece is rinsed to remove alkalis and to prevent the effect of subsequent acids being diminished during pickling.

Once this superficial layer of oil or other surface deposits has been removed, pickling is carried out. Chemical pickling occurs with strong acids under controlled periods. The objective is to remove oxides or other coatings that may have formed circumstantially on the piece.

This bath produces a strongly acid solution with salts from the attack of oxides, sulfates and iron chlorides. To avoid further deposition, complexing agents such as EDTA are used for iron and other metals.

This dissolution leaves the piece ready for the electrolytic process.

The governing specification, ASTM B633, lists four classes of zinc electroplating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25, where the number indicates the thickness coating in microns (μm).

In the electrolytic deposition process, metallic zinc is deposited at the anode and hydrogen released. More complex effects are produced at the cathode, such as:

  • Oxidation from SO42- to S2O8 2-
  • Decomposition of S2O8 2- to SO42- and SO32-, producing oxygen as a by-product.
  • Synthesis of H2SO4
  • Decomposition of water with oxygen production.

After electrolysis, another rinse and transfer to the passivation process is necessary to produce a protective layer on the piece. Strong acids such as chromic and sulfuric, are used in this process.

For steels, the ASTM A380 and ASTM A967 standards cover a wide range of descaling and passivation cleaning processes for stainless steel parts, equipment and systems, as well as chemical passivation treatment specifications for stainless steel parts.

In the case of zinc, depending on the electrodeposition residence time, the bath pH, stirring and the temperature, different passivation structures will be obtained, such as passivated bluish iridescence (zinc-plated rainbow), passivated (olive color), passivated (zinc-plated iris) and passivated black zinc.

The most common electrolyte/zinc anode arrangement uses lead/silver or other insoluble anodes and zinc sulfate electrolytes. Soluble pure zinc anodes are also used. The coating develops as the positively charged zinc ions in the solution are reduced by electricity to zinc metal and deposited on the positively charged cathode (steel sheet).

The temperature ranges are between 18 and 30°C. After the passivation process, a rinse removes reagents and drying is performed.

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Zinc plating process and chemical reagents used

Process Reagents
Chemical or electrolytic degreasing process Na2CO3 , NaOH, Na2SiO3, Gluconates
Rinsing Water
Passivation H2CrO4, H2SO4
Rinsing Water
Zinc electrodeposition Zinc salts,sodium and potassium chlorides, sulfuric acid
Rinsing Water
Pickling H2SO4/HCl
Rinsing Water

Wastewater Treatment Process

The waste produced during electroplating can be classified as follows:

  • Acid or basic effluents, from rinses that can be neutralized.
  • Effluents with high concentrations of heavy metals; these are precipitated at suitable pH values. Normally, at a pH close to 7, most metals can be precipitated as hydroxides. These hydroxides or their decomposition to oxides by water loss can be separated in the form of sludge and specifically managed.
  • Effluents with Cr (VI) content. This ion requires specific treatment. First, it must be reduced to Cr (III) by a reducing agent such as sodium sulfite. Subsequently, chromium (III) is precipitated by neutralization of the effluent in the form of chromium (III) hydroxide that decomposes to chromium (III) oxide.
  • Organic effluents. These effluents contain oily emulsions (cutting fluids), inhibitors, EDTA and gluconates.
  • Specific effluents with cyanides in solution. In this case, as with metals, a specific treatment is required. In particular, cyanides must be subjected to a strongly basic and oxidizing medium. Due to their danger, cyanides are being replaced by other salts with less risk.

Wastewater from these treatments can be in the form of emulsions (different phases), along with a variety of heavy metals (e.g. chromium and zinc) in solution, organic matter (e.g. antioxidants, inhibitors, gluconates and detergents), acids and bases.

Typical emulsions are formed by oily water. Waste that can be treated comes, for example, from cooling, cutting, lubrication, surface coating and rinsing. The most characteristic waste is emulsifiable cutting oil (10% mineral oil in water); anionic emulsifier (sodium sulfonate), non-ionic emulsifier (mercaptobenzothiazole), anti-corrosive additives, pH 8-9.5 solutions, antifoaming agents, bactericides and fungicides.

There are different ways of treating emulsions. In general, the most usual process consists of separating the colloidal suspension of organic compounds in water (separating the oily phase from the aqueous phase) by a pH change. Energy recovery of the oily phase can then be done, while controlling the chlorine and sulfur content at all times. Phase separation is promoted by temperature changes.

The wastewater treatment process goes from removal of heavy metals to treatment of the emulsion and neutralization of effluents. This produces wastewater with a high content of calcium, sodium, sulphates, chlorides, mainly from the processes of neutralization and electrolyte use. These high salinity parameters require specific treatments. The two processes used most for the treatment of these waters are evaporation/condensation and reverse osmosis.

Evaporation and subsequent condensation of treated water (evaporation/condensation) is highly efficient in the treatment of this type of waste characterized by high salinity while, at the same time, allowing reuse of the condensed water, the reduction of waste volume and the reuse of certain salts.

A significant limitation to consider is the presence of volatile organic compounds since, in the process of evaporation, they are transferred to the vapor of the
condensable phases. In this case, an evaporation/oxidation process or alternative treatments can be carried out in earlier or condensed phases. In this process, the concentrate retains the salts while water can be obtained from the condensed vapor phase.

The evaporator/concentrator combines vacuum and pump technology with heating to obtain a low temperature distillation. Some of the important parameters considered in the process are percentage of concentrate obtained in the evaporator, the value of the flows to be treated, energy consumption, operation/maintenance costs and evaporated-concentrate management cost.

The high efficiency of this wastewater treatment via evaporation technology makes this one of the most widely used methods in the coatings industry field.

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