Types of Wastewater from Metal Processing Facilities and Their Effects
General
Wastewater from metal processing facilities is essentially a processing agent that has been heavily diluted through rinsing operations, yet still contains usable substances. Therefore, in all cases, it should be examined whether treatment and reuse are more economical than disposal as wastewater.
Particularly in the case of continuous or semi-continuous processing of the solutions in question, the materials can be processed in a much more uniform manner than through the use and disposal of chemicals. Moreover, processing times can be kept constant and operations can be more easily automated.
Finally, the savings in chemical materials can be explained as follows: for example, a solution that is added once and used until saturation with dispersed metal salts—that is, used until it becomes unusable—usually receives excess chemicals.
However, due to the higher concentration in the solution, a higher proportion also enters the subsequent rinsing waters. In addition to the increasing loss of the chemical in question, this also means greater consumption in wastewater treatment, that is, an increase in the consumption of chemical materials.
With continuous regeneration, however, an average concentration results in lower excess in the rinsing water and consequently lower chemical discharge and, as a result, lower chemical costs. The latter can be offset in favor of a regeneration unit (see Figure 1).
- Wastewater mechanically contaminated with solid pollution, for example mill scale and annealing scale, sand, dust, metal wear.
- Wastewater contaminated with substances that are liquid and do not mix with water; for example, oils, other fats, solvents (gasoline, halogenated chlorinated hydrocarbons such as Tri, Per, etc.)
- Pickling with acid for ferrous and non-ferrous metals; rust removal baths, pickling oils (for example, 10 to 15 hydrochloric or sulfuric acid containing 2 to 5 percent emulsifier) anodizing baths, etc.
- Alkaline solutions for pickling aluminum, zinc, etc.; degreasing solutions; drum descaling solutions, etc.
- Various electroplating treatment solutions with partly strong acidic to strong alkaline properties and partly significant amounts of cyanide, chromate and metal salts.
- Chromic acid solution and chromate solutions from ether baths and anodic solutions for aluminum; solutions for aluminum, magnesium chromating (chemical oxidation); anodic polishing baths, post-treatment solutions for iron, zinc and other metals (chromating, "passivation").
- Mineral acid-based polishing baths (phosphoric acid, perchloric acid, sulfuric acid, chromic acid, nitric acid with partial organic acid addition such as acetic acid, all containing metal salts after use.
- Phosphating and passivation baths containing partly chromic acid or hydrofluoric acid, nitric acid, nitrites, chlorates and phosphoric acid containing heavy metal salts such as iron, zinc, manganese, nickel.
- Wastewaters discharged from hardening facilities containing cyanides, nitrates, nitrites, barium compounds; paint baths with ammonium or potassium sulfite and polysulfides.
- Electroless metal plating baths; for example, from the manufacture of circuit boards with distinctive high-level complex builders, high organic matter content and sludgy (coagulated binders and pigments) from painting facilities with some solvent content.
Wastewater types can be distinguished according to the following criteria, for example (see also Table 1):
Therefore, excessive dilution of rinsing water is generally only a complication, since simple disposal of wastewater without special treatment is neither sufficient nor appropriate, because large dilutions usually slow cleaning reactions and also require large collection and treatment tanks.
In many cases, concentrated wastewater (enriched pickling solutions, electroplating baths that have become unusable, and similar solutions, substances separated from ion exchangers) still contain a certain value due to the metals and other chemicals they contain.
However, it is not always economically possible to recover valuable components from these solutions or to make them usable.
Today, the costs of the regeneration process are sometimes even higher than the value of the recovered material. Selling the recovered materials is also usually difficult.
However, from the perspective of direct economic benefit, processing of concentrates and semi-concentrates should always be checked and, if necessary, carried out at higher costs in the Federal Republic of Germany at the request of the legislator.
Recovery, detoxification, precipitation, protects flowing waters and sewage treatment facilities, prevents waste, and conserves resources. The efficiency of treatment often becomes apparent only when damage that occurs in cases of neglect for which the facility owner is fully responsible and reduced waste are taken into account.
However, in the meantime, processes that make it possible to achieve immediate economic benefit through the processing of previously unusable concentrates have increasingly been developed.
In the case of diluted rinsing water, recovery of the substances it contains is rarely profitable and is considered almost exclusively for wastewater containing precious metals. In some cases, for example in the case of direct detoxification, recovery of transition metals such as nickel, copper, tin may be possible at reasonable cost.
Today, ion exchangers offer the possibility of recovering valuable materials even from highly diluted wastewater, but in such cases the economy question depends on many factors and cannot be answered generally.
Therefore, detoxification of diluted rinsing water is primarily carried out with regard to its harmfulness to the public waterways into which it is discharged.
However, the use of ion exchangers generally allows for economic efficiency. Because wastewater treatment is no longer a waste of value, but real savings from fresh water costs and wastewater fees can even generate actual profit.
When reusable materials from wastewater are used again, the possible profit margin is usually very small and is also subject to considerable fluctuations, because slight price fluctuations in any sector, such as transportation costs, can completely change the picture and turn a possible small profit into a loss.
Therefore, the calculation of wastewater treatment should never be based solely on a profit that could result from the evaluation of components, but should also take into account the necessity or administrative restrictions and the avoidance of significantly higher costs due to wastewater damage.
However, since roughly the 1960s, increasing costs and difficulties in fresh water supply, and often quite significant wastewater fees, have increasingly led to general treatment with the circular flow circulation of treated water, thus creating economic solutions.
Increased efforts to protect the environment may lead to a development in the wastewater sector aimed at finding a complete solution to the wastewater problem; thus ensuring that not only toxins, but also salt water solutions or sludges do not leave the factory.
Ion exchanger enables almost complete circular flow of process water and the salt load of escaped wastewater is greatly reduced.
On the other hand, there are already various processes that allow work with the recovery of some metal hydroxides otherwise obtained as sludge, metals, or their pure compounds. In this way, the problem of sludge disposal and storage will be alleviated.
However, complete avoidance of wastewater or sludge is limited to special cases; as a general wholesale demand, it remains impractical, utopian.
Legislators today increasingly demand the use of low-emission processes. Wastewater concentration through evaporation with concentrated and pure water recovery is economically partially possible if cheap waste heat is available.
Sources:
Wasser und Abwasser, 2. Auflage, Peter Winkel Frage und Antwort der Galvanotechnik, 6. Auflage Umweltschutz, 2. Auflage
İzzet Aydın General Manager Hillebrand Chemicals Chemical Marketing Ltd. Co.
