Dimensions of Environmental Protection and Occupational Safety

Environmental Protection and Occupational Safety Dimensions

Surface technology operations employ harmful substances in various quantities affecting the environment and human health. Consequently, the following approaches may be considered: • Prevention of these substances from entering the process cycle, • Recovery, • Prevention of emissions from waste air, sewage and waste, • Appropriate use and • Development of low-emission or non-hazardous technologies.

1. Renewal and Disposal of Cleaning and Degreasing Solutions 1.1. Organic Solvents

During degreasing operations, the solvent becomes enriched with dissolved matter—oils, greases, waxes and solid particles such as dust. Since the solvent does not undergo chemical change, it can be recovered. Perchloroethylene, trichloroethylene, acetone and 1,1,1-trichloroethane are the most commonly used organic solvents in the metal industry. Chips, dust and metal wear are separated by filtration, while dissolved matter is separated by distillation alone. Reduction of solvent material and consequently solvent-containing waste is achieved through sealed-capsule degreasing systems with feed-through gas leakage, integrated solvent recovery and recycling. Various separation or extraction systems operating according to the following principles are suitable for solvent recovery: • Adsorption on activated carbon (particles, particulates, fibers), silica gel, aluminum oxide, molecular sieves, polymer particles and adsorber resins, • Absorption, • Condensation, • Membrane processes. These operations have various advantages and disadvantages as shown in Table 1. The most commonly used methods are adsorption on activated carbon and polymer particles, and condensation of solvents in waste air. Adsorption operations require solvent desorption. Often, the purity obtained after desorption is insufficient for reuse, making distillation additionally necessary. Spent organic solvents can be destroyed by incineration. Since incineration of halogenated solvents at sea has been halted and on-land disposal capacity is limited, and also due to resulting environmental and health risks, efforts are focusing on the prevention, reduction, substitution, recovery and disposal of halogenated solvents in particular. Complete avoidance of solvent-based cleaners is currently not feasible due to quality requirements. Recovery-based reduction possibilities have already been mentioned. Substitution options include: • Mechanically assisted cleaning, • Aqueous-alkaline cleaning systems and • Use of high boiling point halogen-free cold cleaners (halogen-free solvents). If aqueous alkaline cleaning systems are used to substitute halogenated solvents, they additionally contain surfactants responsible for the formation of fine oil droplets. They are suitable for processes already operating in aqueous environments (see "GC PREP" product line - Hillebrand Chemicals). The advantages and disadvantages of aqueous alkaline cleaning compared to halogenated hydrocarbon cleaning are given in Table 2.

Table 2: Comparison of aqueous-alkaline cleaners with halogenated hydrocarbons

As an alternative to hot degreasing with halogenated hydrocarbons, cleaning at room temperature with high boiling point halogen-free cold cleaners is possible. Solvents typically used include alcohols, mineral spirits and mixtures of esters and glycol ethers. Use of these solvents at room temperature is recommended due to high fire and explosion risk. To achieve at least the same level of results at room temperature, cleaning must be mechanically assisted. The advantages and disadvantages of cleaning with cold cleaners compared to cleaning with halogenated hydrocarbons are summarized in Table 3.

Table 3: Comparison of cold cleaner use versus halogenated hydrocarbons.

1.2. Reduction, Renewal and Disposal of Alkaline Aqueous Cleaning Solvents

Renewal of alkaline aqueous cleaning solvents is not a common practice. During saponification of natural oils, salts of fatty acids (soaps) such as propane triol (glycerol) are produced and dissolve in water. To remove these compounds from the cleaning solution without destroying the solution itself, they must be separated. When disposing of alkaline solutions, they are mixed in equimolar ratios with acids and neutralized. In case of excess acid formation, esters may form that are insoluble in water and easily separated. With the addition of calcium chloride, poorly soluble, filterable calcium soaps form. The service life of cleaning baths containing surfactants that form emulsions with oils and greases can be significantly extended by removing oils and greases from the cleaning bath through ultrafiltration. For this purpose, cleaning baths are pumped through ultrafiltration modules available in different types. The filtrate (permeate) is returned to the cleaning bath, while oils and greases accumulate in the residue (unfiltered portion) at approximately 50% concentration. The residue is disposed of through mineralization, also called incineration. The resulting thermal energy can be utilized. In ultrafiltration, care must be taken that the cloud point of surfactants used in cleaning baths is at the highest possible concentration; otherwise, much of the surfactant remains in the unfiltered portion and is lost. Water containing surfactants can cause problems in production, for example through foaming and blockage of ion exchangers. Surfactants can be separated from solvents through adsorption on activated carbon, but this method is not considered very practical in practice because reactivation of the activated carbon filter is not possible. This wastewater produced by the metal industry is generally sent to municipal treatment plants where it is biodegraded.

1.3. Reduction, Renewal and Disposal of Aqueous Cleaning Solutions Containing Dispersing Agents

Solid matter in dispersion, such as graphite, metal wear or grinding and polishing residues, are found at the surface or as sludge on the bottom depending on their density. These solids and water-insoluble oils and greases can therefore be easily separated by mechanical separation. Sedimentation, flotation, centrifugation and filtration methods are possible. Sedimentation is used when the density of solids is considerably higher than that of the cleaning bath and forms sludge. The cleaning bath is drained by filtration, and the waste sludge is drained, for example, by evaporation. In the flotation method, materials floating on the surface are separated by mechanical retention while the cleaning bath is continuously approached and withdrawn. A combination of both methods is also possible, allowing sludge and light matter to be simultaneously separated from the liquid phase. To enable separation of matter by sedimentation or flotation, density differences are increased by precipitation or by adding gas bubbles. During centrifugation (or spinning), separation of a heterogeneous solid-liquid or liquid-liquid system occurs under the effect of centrifugal force in a rotor. Larger forces than gravity act for separation. This method is particularly suitable for removing oil from waste or separating liquids such as painting solutions, and is also used for separating suspensions and emulsions. The centrifugate can be returned to the material cycle without requiring any further preparation. Unlike the gravity method, filtration does not require a density difference between the solid matter and the cleaning bath. In filtration, only a pressure difference and a filter with pore size suitable for the solid particles are needed. The pressure difference can be applied either as hydrostatic liquid pressure or by pumps. Generally, the separated matter is drained, for example by heating, pressure or reduced pressure, reducing both mass and volume, and is then sent to the next disposal location, e.g., storage.

1.4. Renewal and Disposal of Aqueous Etching and Neutralizing Solvents

Metal etching generally uses sulfuric acid, nitric acid or hydrochloric acid or their mixtures, sometimes phosphoric, hydrofluoric or chromic acid. The etching process accumulates metal ions in the solvent. Concentrated acids can be desalted using strong basic ion exchange resins. Water is used as the regeneration agent. The process here is based not on ion exchange but on electrostatic interaction and diffusion processes. Only undissolved acid can diffuse into the resin. The recovered acid-rich, metal-poor fraction is returned to the acid. The solvent containing metal salts can be recovered for metal through an appropriate purification process, such as electrolysis or cementation. Another route is precipitation followed by filtration. Acids can be disposed of by neutralization. The resulting solutions can be evaporated; through this method, demineralized water is additionally obtained by subsequent concentration. A second possibility involves precipitation of heavy metal hydroxides in alkaline medium, followed by filtration and neutralization of the filtrate. Acids and alkaline acids often contain inhibitors that delay or prevent bare metal attack after oxide layer removal. Wetting agents that ensure rapid penetration and wetting of oxides are also frequently present. They complicate renewal or disposal.

1.5. Renewal of Rinse Waters

Rinsing means successive dilution of materials emptied from process baths many times. The rinsing criterion expresses rinsing quality. Good rinsing quality, expressed as a high rinsing criterion, requires significant amounts of rinse water. Therefore, demand for high rinsing quality conflicts with the requirement to reduce rinse water. Achieving the required rinsing criterion using less rinse water is possible only by using the water multiple times. Options include: • Recycling ion exchange process, • Recycling evaporation, • Reverse osmosis in recycling process, • Cascade rinsing and • Combinations of these methods. Wastewater and waste result from carrying process solutions to rinsing baths during product transport. This is precisely where highly effective measures for reduction are needed. Depending on the equipment used, the following possibilities are available: • Dripping, blowing, pressing, shaking, • Slow removal of product carriers, drum movement over the bath, • Reduction of metal concentration in bath solution, • Reduction of bath viscosity (temperature increase), • Appropriate design of products and product carriers for processing, • Maintenance of product carriers, perforation of drums, demetallization of racks.

1.5.1 Recycling Method Using Ion Exchange

Ions brought in by rinsing are exchanged in ion exchangers with hydrogen ions and hydroxide ions, i.e., loaded onto exchangers: Cation exchange, example: Na+ and Cu²+ (R … Resin) R – H + Na⁺ ←→ R – Na + H⁺ 2 R – H + Cu²⁺ ←→ R₂ – Cu + 2 H⁺ Anion exchange, example: CN- and CrO₄ ^2- (R … Resin) R – OH + CN^- ←→ R – CN + OH^- 2 R – OH +CrO₄ ^2- ←→ R₂ – CrO₄ + 2 OH^- When loading of the ion exchanger is complete, it must be regenerated with acid (cation exchanger) or alkali (anion exchanger). The regenerants contain all the ions previously present in the rinse water, but at much higher concentration, i.e., in a much smaller volume that can subsequently be sent for detoxification and neutralization. For the ion exchange process, large amounts of rinse water are required because rinsing is necessary between each loading, elution and regeneration. A disadvantage of the circular ion exchange flow process is salt accumulation in wastewater (salt carryover, neutral salts).

1.5.2. Circular Flow Process Using Evaporation

In the circular flow process using evaporation, there is also a high volume of rinse water flow. Ion separation occurs during evaporation with salt formation. After condensation, it is collected again; in this way high quality and balanced demineralized rinse water is obtained. Residues remaining in the evaporator must be disposed of. The advantage of the process is that the wastewater is not salty.

1.5.3. Reverse Osmosis in Recycling

In this process, rinse water is purified through a membrane using high pressures (reverse osmosis). Only relatively small water molecules pass through (penetrate) the membrane's micropores. Salt ions with voluminous hydration shells do not pass to the other side and accumulate on the (concentrated) side of the semipermeable membrane. This process operates continuously.

1.5.4. Cascade Rinsing

In cascade rinsing, the rinse water flow is small and is used many times without intermediate cleaning. In this case, rinse water flows in the reverse direction of operation, so rinsing first occurs in the most concentrated and finally in the cleanest water. For the first stage bath, the highest possible concentration is desired to be able to send dissolved matter to recycling. Ideally, the rinse water overflow rate equals the evaporation of the preceding aqueous process bath itself. If wash water requirement is greater than its own evaporation, the water overflow is artificially evaporated and the condensation water is returned to the final cascade stage. Generally, demineralized water should be used for cascade rinsing; otherwise water hardness increases. Table 4 shows a comparison of rinse water recovery procedures.

1.5.5. Metal Recovery from Aqueous Solvents

Solvents containing non-ferrous metals, particularly from electroplating, are processed electrolytically, enabling metal recovery in marketable form. The fundamental problem in recovery electrolysis is the continuous depletion of metal ions in the electrolyte. In connection with this, continuous changes occur in electrolysis conditions.

Three types of cells are available for separation from the following:

• Cyanidic solutions, e.g. Ag, Cu, Zn and brass • Sulfuric acid electrolytes, e.g. Cu, Ni • Chloride-containing solutions due to membrane electrolysis, e.g. Cu, Ni With this highly environmentally friendly method, both concentrated and dilute solutions can be consumed without chemical use.

2. Detoxification of Wastewaters

In electroplating, the use of cyanide and chromate electrolytes remains unavoidable. Due to high toxicity, toxins in wastewater must be completely eliminated through detoxification. When choosing between chemical or electrolytic detoxification or recovery process, the electrolytic method is of great importance. Use of chemical processes often leads to additional loading of the wastewater cycle and waste air, resulting in increased wastewater and waste disposal costs.

2.1 Cyanide Detoxification

The toxicity of cyanide ions results from the formation of a very stable complex with hemoglobin iron ions. This leads to loss of oxygen-carrying capability. Additionally, they complex heavy metal ions and thus complicate their separation. Cyanide electrolytes are indispensable for electrodeposition, e.g. zinc, copper, silver and gold, due to ease of use and predictable plating results. Different wastewater compositions and cyanide contents do not permit a universal detoxification process. Cyanide poisoning preferably occurs with different oxidizing agents at different oxidation states of cyanide nitrogen. 1. Oxidation with sodium hypochlorite (NaOCl) CN- → CNCl → NCO- at pH 10 Here, cyanate is formed, which is less toxic than cyanide. 2. Oxidation with hydrogen peroxide, possibly UV-assisted. Direct oxidation of cyanate without intermediate stage CNCl; in UV-assisted reaction to CO₂ and N₂. 3. Electrolytic oxidation to CO₂ and N₂ via anodic oxidation, with the advantage of no salt formation compared to 1. However, at high chlorine levels, chlorine-oxygen compounds form. By adding Fe (II) and Fe (III) salts to cyanide waste, the poorly soluble Fe₄[Fe(CN)₆]3 complex forms, for example, in excess of Fe (III) ions.

2.2. Chromium Detoxification

Chromium at the +6 oxidation state appears in wastewater as chromate CrO₄^2- or dichromate Cr₂O₇². Chromium (VI) ions are obtained from chromium plating electrolytes, chromating solutions and anodizing electrolytes. Reduction of chromate produces the chromite stage (+3), which is 100 times less toxic. Depending on the pH value of the wastewater, reduction is fundamentally carried out with three different reducing agents: 1. Sodium hydrogen sulfite in acidic range (NaHSO₃) This is the most frequently used method. 2. Sodium dithionite in neutral and weakly alkaline range (Na₂S₂O₄) This reducing agent is relatively expensive but leads to nearly quantitative reduction of chromate and is used at low chromate concentrations. 3. Iron (II) salts in alkaline range. This method is recommended if iron ions are also to be removed simultaneously. "Indirect detoxification" occurs through removal of Cr (VI) ions with the aid of ion exchangers. This way, direct recovery of chromates is possible.

3. Organic Coatings and Environmental Protection

To achieve ecologically acceptable values in coating processes with organic polymers, there are two approaches: • Reduce or prevent the use of organic solvents, • Reduce spray loss and/or • Recycle spray loss. Solvent content reduction is possible with solid paints, water-based paints and powder coatings. For example, if solvent content in water-based paints is approximately 25%, the use of powder coatings leads to solvent-free organic coating. In powder coating, waste air treatment facilities are not required. Paint sludge that would need to be disposed of as hazardous waste does not accumulate. Another feature is the elimination of pigments containing heavy metals. Cadmium and lead have not been used for years, and chromium (VI) use has been reduced by nearly half. When spray coating techniques are compared, many ways are shown to reduce spray loss, but they are always related to pigment, binder and solvent content. Thus, for example, high-pressure air application efficiency is only 40-50% compared to airless atomization, which has a maximum of 75%. However, airless atomization can only be applied with non-abrasive pigmentation. While recovery is almost impossible in wet paint application, sprayed powder in powder coating can be recycled. İzzet Aydın General Manager Hillebrand Chemicals      

Sources: • Effektive Methode zur Untersuchung von Spritzspülprozessen, Galvanotechnik, 95 (2004). • Praktische Oberflächetechnik, 3. Auflage. • Galvanotechnik, Carl Hanser Verlag. • Organischer Metallschutz, Vincentz Verlag. • Chemie und Physik der synthetischen Polymeren, Vieweg Lehrbuch.