New Technology: Recycling Spent Cleaners via Ozonolysis
by Mike McGinness
Manufacturers, rebuilders, and those in related service industries have been progressively driven to address one new environmental mandate after another since the early 1970s. The general practice until recently has been to do only what one was required to do to meet the new regulations, however, those in the industry are now actively looking for more permanent, long-term answers. To be truly complete, these answers must be capable of avoiding future "end of process" waste control regulations and their associated liabilities.
Conventional Waste Control Options
Conventional waste control options include disposal, end of process treatment, switching to an entirely different process or method, and redesigning the part itself. Disposal has become the least favored option and end of process treatment is a close second. An example of waste minimization due to part redesign is material substitution, where mild steel, which is oiled to avoid rust and friction, is replaced with Teflon, radically changing the need for part cleaning. Such a part redesign is not feasible in many situations.
A less conventional method of waste minimization is redesigning cleaning processes so that they account for waste issues. This is the type of waste control solution discussed here. The purpose of the process discussed is to reduce and, where possible, eliminate cleaning process waste treatment and disposal costs entirely.
The New Process
Many technologies that separate known constituents, such as spent cleaner and rinsewater, at known constant concentrations from each other in a feed stream are available on the market
today. These separation processes may be effective for large volumes. Low volume and highly variable content streams are not likely to be cost effective applications for these separation processes.
Process data from a variety of cleaning operations reveals that the type and quantity of soils, contaminants, and cleaning solution concentrations can vary on the same part in the same facility and in the same cleaning process from day to day. These variations can easily exceed 50% and even reach 90%. Most failed attempts at extending cleaning solution bath life have resulted from failures to recognize the highly variable dynamics of real world cleaning processes.
The basic premise used to develop the process introduced here has six steps: They are:
Step 1: Redesign the cleaning process and chemistry so as to eliminate the need for any waste disposal.
Step 2: Make sure any "waste" produced is a designer waste designed to be a product itself and not a waste.
Step3: Redesign the process to reduce the waste volume of any waste to be produced.
Step 4: Redesign the process to reduce or eliminate the toxicity of any waste produced.
Step 5: Consider ways of applying new technologies with old technologies in ways that were not previously possible or economical.
Step 6: Consider opportunities made available by Step 5 for miniaturizing and automating large-scale proven processes.
This new process is based on on-site reaction chemistry. Instead of trying to selectively separate the organic contaminants from the cleaning solution, which would result in the creation of a new waste stream, the process uses a limited amount of reagent (primarily ozone) to chemically convert organic contaminants. These reaction products are less or non-hazardous compounds that also function as useful cleaning agents. As organic contaminants are dragged into the cleaner and converted, the ability of the system to clean and emulsify contaminants actually increases for a period of time. For systems with overflowing rinses, the drag-out of cleaning agents eventually equals the creation of cleaning agents from the drag-in of organic contaminants.
One of the advantages of this approach is its lack of dependence on separation efficiency. Another is its lack of dependence on non-emulsifying aqueous cleaner chemistry to achieve optimum performance. Many new environmentally friendly stamping and machining fluids are highly water soluble oil emulsions. Non-emulsifying oil splitting cleaning formulas may not split as effectively when water-soluble oils are being removed from the parts.
Cleaning Chemistry Manipulation
In order to understand the chemistry behind the process that turns spent cleaner contaminates into usable cleaning agents, it is useful to briefly explore some of the chemistry behind cleaning. Cleaning processes depend on one or a combination of three basic processes:
Elevated temperatures are used to lower soil viscosity, increase saponification rates, increase transport rates of surfactant migration to part soil interfaces, and to lower the surface tension of water-based cleaning products. Increased mechanical pressure is used to increase mechanical action removal rates. The only remaining control variable beyond temperature and pressure manipulation is cleaning agent chemistry manipulation. Cleaning chemistry manipulation is assumed to include any reactive chemistries and solid abrasive compounds for the purpose of this discussion.
Aqueous cleaners also depend on additives that sequester hard water ions so that they stay in solution at concentrations beyond their normal solubility and do not precipitate on the parts being cleaned. Chelating agents keep metal ions, like iron that has been removed from parts, from plating back out of solution on to parts as a black iron oxide smut. Wetting agents reduce the surface tension of the cleaning solution to increase cleaner wetting action on the part to be cleaned. They can also reduce the dynamic (time-dependent) surface tension of the cleaning solution. Solubilizers increase the solubility of certain species in the cleaning agent.
Ethylene glycol butyl ether, which is soluble in water and oil, is an example of a solubilizing agent. By adding "butyl" to water, one can increase the solubility of oil in water. Emulsifiers increase the capacity of a cleaner to emulsify non-soluble compounds in the cleaner. Anionic soap surfactants are an example of an emulsifying agent (as opposed to a solubilizing agent).
Many cleaning compound agents perform several functions at once. Butyl, for instance, can serve as a wetting or surface tension reducing agent as well as a solubilizing agent. It also can contribute to emulsifying capabilities when combined with anionic surfactants or soaps (alkali-metal salts of carboxylic acids).
Chemistry of the New Process
Ozone reaction chemistry and mechanisms are already well-documented in many publications of the International Ozone Association (IOA). Until recently, typical ozone usage has been limited to sterilization or disinfection processes and destruction of organic contaminants in drinking water, ground water, and cooling tower water. These applications always require large excesses of ozone on the order of 10-50 times the stoichiometric requirements. By using only limited amounts of ozone it is possible to convert certain organic contaminants, primarily hydrophobic organics removed from parts by the cleaning solution, into a mixture of surface-active agents that include all but the inorganic sequestering agents. These reaction products include organic compounds with alcohol, carbonyl, ester, and carboxylic acid functionalities on the backbone of the organic compound. Reaction with ozone adds the alcohol, carbonyl, ester, and carboxylic acid functional groups to the previously unsaturated hydrocarbons.
After repeated exposure of recycled cleaning solutions to limited amounts of ozone, new surfactants begin forming that are amphoteric (soluble in acids and bases) with alcohol, glycol, and carboxylic acid functional groups, all on the backbone of the hydrocarbon. These surfactants variably exhibit high solubilities in water, the ability to chelate and sequester metal ions, and can increase the cleaner's ability and capacity to emulsify and solubilize hydrophobic contaminants. Increased emulsification and solubilization are the result of the formation of micelles. Abrupt changes and discontinuities in solution conductivity, solution turbidity, and surface tension / concentration curves are observed at the Critical Micelle Concentration (CMC) point. Micelles, CMC, and cloud points will be discussed in more detail in future articles.
The resultant mixture of surfactants that are hydrophobic on one end and hydrophilic on the other end increase the cleaner's ability to emulsify hydrophobic compounds, thus increasing the cleaning agents useful lifetime. Reduced cleaning cycle times and reduced operating temperatures are also possible when compared to typical low-cost cleaning formulas. Furthermore, implementation of this concept can lower energy costs by allowing users to maintain steady-state line cleaning speeds no longer dependent on frequent bath dumps to get line speeds back up, reduce washer operating temperatures, and reduce the need for energy-consuming waste treatment cleanup steps.
Ozone consumption and feed requirements for this process are generally three orders of magnitude (on a stoichiometric basis) less than what is typically used for standard ozone applications such as drinking water disinfection. The overall efficiency of the process can be further increased by a synergistic combination with any bioreactor technology. Using bioreactors to convert excess soap (surfactants) and emulsified oil into fertilizer (biomass) can result in a zero-discharge process. Conventional activated-sludge municipal wastewater treatment systems and small industrial systems can readily biodegrade all ozone reaction products.
Ozone reacts directly with organics by attacking unsaturated carbon to carbon double bonds. Some reaction products of ozonation include organic peroxides, hydrogen peroxide, and superoxide anion. These organic peroxides can react non-selectively with other saturated or unsaturated compounds in the recycled cleaner through a free radical reaction mechanism, thus forming new compounds through a coupling process that is similar to polymerization. This results in the creation of a wide variety of surface active compounds in the recycled cleaner/ozone process.
The process is not expected to produce any one primary surfactant or cleaning agent compound in any significant quantity. This is because the process is a dynamic one that varies depending on an assortment of factors, including contaminant concentration and the chemical structure and reactivity of the contaminants and cleaning agents in the cleaning bath. Additionally, the number of individual compounds that become available reactants from the soils washed off parts into the cleaner is fairly large.
As a result of these factors, dozens of compound species varying from a molecular weight of about 90 to 5000 atomic mass units (AMUs), with varying degrees of increased hydrogen bonding capability are produced. Such species include oxygen bearing functional groups such as alcohols, glycols, carbonyls, esters, and carboxylic acid functional groups.
The added oxygen atoms create a hydrophilic region (water loving) on the backbone of the previously hydrophobic (water hating) hydrocarbon resulting in the formation of a surfactant molecule (hydrophobic on one end and hydrophilic on the other end). The result is a mixed surfactant system with excellent steady-state parts cleaning capabilities.
Further details of ozone reaction chemistry are already available in the IOA literature and are beyond the scope of this article. Due to the unusual nature of the entire process and highly varied reaction products, standard cleaning concepts such as hydrophobic-lipophilic balance (HLB) are not tested directly. They can be estimated with empirical methods based on the soil being removed but will surely vary with the contaminant. The process is somewhat self-regulating with regard to final HLB values of the mixture.
In addition to the basic reactor, in-process sensors can be added to determine the cleaners dynamic surface tension, temperature, pH, turbidity and Total Organic Carbon. These parameters can be set as control points for process control which can regulate the cleaners critical properties by varying the ozone feed rate, pH make up additive addition, and the bleed rate to a bioreactor conversion stage to remove accumulated excess organics and salts.
The entire process can actually result in a positive (net gain) energy balance if used to increase or maintain line speeds at optimum levels, thereby increasing net production of clean parts per hour, or if used to lower operating temperatures at the same time. The process also minimizes volatile organic compound emissions, ozone precursor gas emissions, ozone depletion gas emissions and minimizes green house gas emissions.
With the advent of concerns about global warming, Ozone depletion, limited resources and landfill availability, and the need to address all these issues in order to maintain a sustainable economy, (the new buzz word today is sustainability), the need to look for and try new methods and ideas for producing more with less will be with us for some time. The new process and the 6 step method (described above) used to develop the process have the potential to address all the issues above at the same time. The old days of just replacing one outlawed cleaner and waste with another are coming to an end. Most environmental regulatory agencys are now focused on air, water and solid waste issues as a connected "bubble" where moving an air pollutant (scrubbers that produce solid waste) to land, a land pollutant (burning solid waste) to air, and moving waste water problems to landfills as sludge or evaporating them away to the air are rapidly becoming unacceptable. The emphasis on zero discharge is moving towards zero discharge to all three medias. Thus the need to work towards 100% product and zero waste in new production proceses.
About the Author
Mike McGinness is Vice President of Technology at EcoShield Environmental Systems (Houston, Tex; 877 326-7443; www.ecoshieldenv.com) and holds a degree in Chemical Engineering from the University of Houston. He has 26 years of experience in industrial metal finishing processes and chemistry.
EcoShield Environmental Systems, Inc. · P.O. Box 1476 · Houston TX, 77251 · USA
Toll Free (877) 326-7443 · Fax (877) 326-9090 · www.ecoshieldenv.com