The metalworking sector consists of many businesses which are very different from each other, although they use the same materials and produce the same waste and effluents.
The most important businesses in this sector are metal fabrication and boilermaking.
The first includes all types of metal construction from pipes, profiles, plates as well as bolted, riveted and welded parts.
Boilermaking brings together a large assortment of parts and equipment manufactured with sheet metal, including watertight joints. Products can range from a typical tank to a heat exchanger or evaporator, through pipes and ducts or coils, for example.
Other activities included in this sector are metal processing without machining (by stamping, extracting, drawing or cupping); machining (e.g. milling, turning, grinding and cutting) and finishing, based on thermal and surface treatments. Thus, the variety of activities is very broad.
What these techniques have in common is that they use the same materials as raw materials: e.g. steel, iron, stainless steel and aluminum, and these also have similar environmental impacts.
Basically, those operations with the most impact on the environment lead to the emission of harmful gases and pollutants into the atmosphere, the production of liquid effluents that need proper treatment before dumping as well as solid waste.
Due to its nature, this type of industry is a great consumer of lubricating oils, used assiduously to facilitate lubrication and cooling in the different types of cutting, molding and mechanical treatment of metal parts.
Once these lubricating oils have been used, they become contaminated liquid waste that must be managed properly.
Of all the products used, the most important are the cutting oils, which are used mostly in the metal machining industry for its following properties:
- Lubrication: protecting the tools by reducing friction.
- Cooling: preventing parts and tools from overheating, which would lead to microwelds giving a poor surface finish.
- Removing abraded material: thus preventing it from damaging the parts.
- Preventing corrosion to machines and parts.
Thus, the use of cutting oil is necessary for processes with direct contact between the metallic part being worked and the tool used, for its properties of lubrication, cooling, removal of the shavings and filings produced, as well as prevention of rust formation.
Cutting oil properties as a pollutant
As a cutting oil is used, the effectiveness of its properties reduces, its performance decreases and it becomes contaminated with external material, such as oils, fats, metallic particles, environmental dust and microorganisms that degrade organic matter.
At this point, the cutting oil is highly pollutant waste, both for the environment and the work environment, and is considered hazardous waste by European regulations.
To its polluting potential, must be added that its proper management is very costly, due to the high proportion of water it contains, which increases the volume of original waste as well as producing a strong emulsion, which later makes separation and purification processes more difficult.
The oil included in the formulation of the cutting oils improves its lubricating capacity, while water is added to increase its cooling capacity.
Thus, there are many different types of industrial cutting oils – which in reality are oil-water emulsions – depending on which properties need to be enhanced.
As water and oil are the main components of cutting oils, a long list of additives can also be incorporated, especially the following:
- Surfactants: sodium sulfonates and glycols
- Corrosion inhibitors: e.g. amines, amides, borates and nitrites.
- Humectants: e.g. alcohols and phosphates.
- Antifoam agents: e.g. esters, silicones and ethoxylated derivatives.
- Biocides: e.g. formol, phenols, boron compounds and polyglycols.
- Additives for high pressure operation
The use of cutting oils is continuous and periodic, due to the loss of its properties and to the net consumption produced by spills and entrainment by the parts.
The continuous use of cutting oil means its properties decline with the high temperatures reached during the machining of metals, while the most volatile components evaporate.
The loss of cutting oil from spills, entrainment by the parts, along with the wastewater produced in the plant form the effluent known as “oily water”.
In addition, cutting oil baths are contaminated (with metallic impurities) the more they are used and become subject to microbiological degradation processes; thus, they need to be replenished periodically, which leads to liquid waste called “exhausted cutting oil” being produced.
In companies with a high consumption of cutting oil, there are cutting oil recovery units that extend the useful life of the cutting oils bath a little by a process to separate metal filings and shavings.
Thus, processes that use cutting oil produce toxic and irritating effluent which contains heavy metals, biocides and toxic decomposition products cataloged by European regulations as hazardous waste which cannot be evacuated to the public sewage network because of the serious problems they would cause in purification facilities.
Most conventional processes used in the treatment of cutting oils must be reviewed and updated due, mainly, to two factors.
Firstly, discharge legislation which is increasingly strict and restrictive; and, secondly, more and more difficulties becoming apparent for effective treatment. These difficulties have their origin in recent changes in cutting oil formulation, to the detriment of oil emulsions, which have become more synthetic in nature, making it more difficult to separate the oil fraction of the cutting oil.
In addition to the cutting oil, the wastewater produced in the metalworking sector contains suspended solids, various metals, phosphates and surfactants and is highly conducting.
Cutting oil treatment
There are different effluent treatment methods, which can be classified as destructive or non-destructive.
Non-destructive methods include chemical treatment, membrane treatment and evaporation.
While among the destructive methods are biological treatment, incineration and advanced oxidation processes (wet oxidation and supercritical water oxidation, SCWO).
Below, the different treatment alternatives are analyzed separately:
This was one of the most common treatments in the last century, as the base is well known and is easily scalable over a wide range of flows to be treated.
The treatment philosophy is based on breaking the emulsion by neutralizing the surface charges. Traditionally, this was achieved by adding inorganic acids like sulfuric or hydrochloric and salts such as sodium, iron, magnesium or calcium chloride or ferric or aluminum sulfate.
The addition of a sufficient amount of cation results in the demulsification process.
However, cutting oil formulation has changed to obtain more stable products to resist the attack of the cations released in the metal cutting and machining processes that tend to break the emulsion.
This has resulted in this cutting oil treatment method becoming less effective, due to the addition of emulsifying and dispersing agents in the formulation.
An alternative to adding inorganic salts is the use of polymers to break down the emulsion. The principle is the same, with the high charge cation polymers used to destabilize the negative charges of the oil droplets.
The rest of the contaminants present in the wastewater accompanying the cutting oils can also be removed in this process. However, the physical-chemical process must be modified according to the exact composition of the effluent.
Ultrafiltration membranes are used, as microfiltration ones do not have sufficient retention capacity, and nanofiltration and reverse osmosis ones are easily contaminated with high molecular size organic compounds.
Ultrafiltration has provided good results working at low pressure, although there are certain operating conditions that the membranes will not tolerate, such as moderate-high temperature (above 60°C), extreme pH values, high amount of solids, large quantities of non-emulsified oils and the presence of solvents.
Furthermore, it should not be forgotten that low molecular weight molecules can easily cross the ultrafiltration membrane.
Vacuum evaporation is an effective treatment for those effluents unable to be treated by conventional methods.
It is a simple, robust and mature technology for efficiently treating oily waters which adapts well to changes in both the volume and concentration of the effluent to be treated.
Technology has evolved in recent decades to the point where energy consumption is moderate, and this is one of the most competitive treatment alternatives. This contributes to both the quality of the separated water and the small volume of waste produced.
In fact, it is the only treatment alternative that is capable by itself of reducing effluent volume to very small quantities without the need for additional processes.
Due to the inclusion in cutting oil formulation of antimicrobial agents which protect the product from being degraded by microbiological action, such as boron derivatives, phenols, formals and polyglycols, biological treatment of waste water is not highly effective when used as the only treatment.
Biological treatment needs to be combined with a preliminary physical-chemical process to achieve acceptable purification yields, with a tertiary treatment stage sometimes needed to purify the treated effluent.
Direct combustion of oils and fats is a feasible alternative due to the energy released; however, wastewater produced from this type of industry contains a high proportion of water, making incineration of this effluent not a viable process, unless preceded by an evaporation stage.
Advanced oxidation processes: wet oxidation and supercritical water oxidation (SCWO)
Wet oxidation is performed in the aqueous phase at moderate-high pressures and temperatures (50-200 bar and 100-300°C). Effluents with high loads or toxic compounds which cannot be treated viably with conventional methods are viable under these conditions.
Supercritical water oxidation differs from wet oxidation in that the pressure and temperature conditions exceed the critical point of the water (221 bar and 374°C).
Under supercritical conditions, the efficiency of destruction of contaminants is very high even with reduced reaction times.
Both technologies have a high destruction capacity of refractory compounds, with SCWO better than wet oxidation; however, the latter’s high investment, operation and maintenance costs make it a not very competitive technology.
In addition, once all the organic pollutants have been oxidized, an additional process is needed to treat the other contaminants, such as metals.
The following table compares the most relevant variables for selecting from the different cutting oil effluent treatment alternatives:
|Chemical method||Ultrafiltration membranes||Biological process||Vacuum evaporation||Wet oxidation||SCWO|
|Oil removal efficiency||Variable||Very high||Low||Total||Very high||Total|
|Changing inlet flow||Bad||Bad||Bad||Very Good||Good||Good|
|Changing inlet concentration||Very Bad||Good||Bad||Very Good||Good||Good|
|Waste production||High||Moderate||Moderate||Very low||Low||Low|
|Reagent consumption||High||Very low||Very low||Very low||Moderate||Moderate|
|Additional treatment needed||No||Yes, for the reject||Yes||No||Yes||Yes|
Thus, the metalworking sector includes a large group of businesses with a number of common features, especially their use of cutting oils to facilitate work when there is direct contact between the machined part and the tool used.
Cutting oils are water-oil emulsions containing many additives that lose their properties as they are used and have to be replenished.
As a result of their use, spills and entrainment they can end up in sewage and cleaning water.
Not all processes are viable to treat these liquid effluents, which contain other contaminants in addition to cutting oils, such as suspended solids, metals, phosphates, surfactants and are highly conducting.
Among the most competitive treatments are ultrafiltration membranes and vacuum evaporation, with the latter being the only treatment process capable of reducing the effluent to a minimum amount of waste to be managed.