Coolant Conservation Through Choice and Care

The Need

In many metalworking facilities, water dilutable coolants with oil can generate high costs because of waste treatment and disposal costs. Efforts to reduce this cost by lengthening the coolant's performance life are ongoing and in fact intensifying because of a new fluid's rising prices and stricter environmental regulations. When a new coolant is introduced into many plants, often the first document to be scrutinized is the Material Safety Data Sheet (MSDS) for hazardous components and waste treatment issues. The ideal goal for any metalworking facility is to use a coolant with an effective cleansing system that allows the coolant to last indefinitely and minimize losses due to the drag-out on machined parts.

New technology coolant formulations, modern equipment, and planned maintenance programs for coolant recycling and rejuvenation are experiencing a solid business growth. The economic and operational benefits are certainly strong enough to warrant serious consideration by any plant that generates so much coolant waste that it impacts the company's bottom line. Even small shops frequently can experience serious production losses when they are forced to shut down even a single machining center for a lengthy cleaning of degraded or rancid coolant.

Some major metalworking manufacturers continue to experience large wasteful costs with cheap soluble oils or poor performing semi-synthetics. For example, we recently interviewed a plant that was routinely treating its soluble oil coolants within its facility with chemicals to split the emulsion and then sending out the waste oil for "rejuvenation" back into a soluble oil so that it could be returned to the plant for reuse. The fluid volumes and treatment chemicals costs were staggering. Other plants have resorted to evaporators to drive off the water and reduce the volume to a disposable sludge. Although this method reduces the volume of fluid for disposal, it generates high energy costs and increased coolant consumption. The solution to coolant management problem systems begins with chemical education provided by a competent coolant supplier supported by recycling equipment suppliers.

Coolant manufacturers must face and accept today's cost reduction challenges by producing long life high performance coolants and accepting the market trend of reduced coolant consumption. They must also be able to supply a single product or the minimum number of compatible products for the many different machining requirements within a metalworking facility. Recycling equipment manufacturers need to closely cooperate with coolant suppliers to achieve the most efficient and cost effective contamination removal results.

Two additional trends that exemplify reduced flood coolant consumption are minimum coolant misting and dry machining. While the use of these methods are growing and have some advantages for certain applications, they also exhibit some serious drawbacks. Neat oil usage and recycling typically found in screw machine shops are also important topics that will not be dealt with in this article. However replacement of oil coolants by new high-performance synthetic water based coolants is occurring and should grow.

Chemistries in Recycling

This article compares the potential of three basic coolant chemistries -- soluble oils, semi-synthetics and synthetics -- for allowing the coolant to last indefinitely by recycling. Although coolants are not commodity items because of significant differences in composition, quality and pricing, there are some generic properties that can be used to differentiate the three classes when analyzing long-term monitoring, care and longevity.

Companies that have either large or numerous small coolant systems want to not only reduce coolant waste but also monitor and care for the coolant with the minimum amount of effort and time. Those that set up and routinely follow sound basic coolant programs of monitoring, care and maximum coolant reuse can experience many of the following benefits:

1. Reduced new fluid purchases

2. Reduced fluid disposal costs.

3. Less liability for on-site or off-site environmental issues.

4. Fewer worker complaints concerning odors, dermatitis, coolant mists.

5. Significantly improved tool life, finishes and production quality.

6. Reduced coolant treatment costs, machine downtime and maintenance time.

7. Better and more predictable coolant costs regarding production volumes.

The three basic types of coolants are typically defined as:

a. Soluble Oils (consisting of petroleum hydrocarbon oil, emulsifiers and other additives for extreme pressure lubricity, corrosion protection, biological control, wetting, defoaming, etc.).

b. Semi-Synthetics (soluble oil and synthetic hybrid typically containing less than 30% mineral oil)

c. Synthetics (no oil, chemical lubricity agents and other additives for corrosion protection, biological control, wetting, defoaming, etc.).

Any type of coolant could last indefinitely if not for the changes continually occurring while the coolant is being used such as: contamination ingress, microbiological activity, water evaporation, and non-uniform depletion of coolant constituents. Often the first components to be depleted are those that impart corrosion protection and lubricity.

Contamination

Sources of contamination are:

• Hydrocarbon based lubricants and process oils (tramp oils)
• Metal and materials from fines and chips (swarf)
• Dissolved water constituents
• Biological agents (bacteria and fungi)
• Dissolved gases from the air entrainment
• Fluids carried in by metal and material surfaces from previous processing
• Other foreign matter mistakenly introduced into the coolant such as cleaners, concrete dust, food scraps, paper, cigarettes, etc.

Because coolants are dynamic changing systems, coolant manufacturers must have real world experience and knowledge of complex chemistry and formulations. They must be able to manufacture with exacting skills, utilize quality control measures with repeatable results, and work closely with additive suppliers so that their products can withstand the rigors that coolants face while at the same time be maintained, controlled, and recycled. Thus, modern coolants have become truly high technology products whose ingredients are intricately combined and yet are cost driven to keep prices realistic in the marketplace.

Most manufacturers of recycling equipment are familiar with basic chemistries as well as the effects of contamination and water evaporation that can concentrate contaminants. In designing their equipment, manufacturers understand the contaminants' separation properties that their technical components take advantage of. However, they also need to know how coolant and contamination dynamics interact, and what complexities may work against their separation equipment.

Some contaminants impacting the coolant's life and its continuing effectiveness can be controlled or modified by the user employing control measures, additives and high quality separation equipment. Others cannot. These contaminants are summarized in the table below.

Tramp oils may appear in coolants as free floating, mechanically dispersed or emulsified. Primary sources of tramp oil are lubricating oils, grease, or oil residues (process lubes or corrosion inhibitors) on parts coming into the machining operation. Excessive oil can contribute to misting, residue buildup, odors, grinding burn, non-uniform lubricating action and bacterial growth. Tramp oil can be removed by skimmers, centrifuge, coalescing, and sometimes by settling along with fine solid particles. Oil skimmers work quite effectively whether the system is down by allowing tramp oil to float or while running by using gravity as a separation force.

Solid particles such as metal chips, grinding fines, cast iron dust, rust (iron oxides), plastics, loose fungi, coagulated or clumped grease or sludge and foreign material (ex., cigarette butts, paper, candy wrappers, etc.) should routinely be removed by filtering, centrifuge, settling, coalescers, dredging, vacuum/pressure or magnetic systems and other mechanical means. These solid materials can selectively pull out loose emulsified particles. Sometimes special polymers that are being machined can actually "wick" oil out of the coolant.

Bacterial or fungal growth can be reduced with biocides, fungicides, ozone generators, aeration, ultraviolet, chemically treated beads and pasteurization. Fungus is a difficult microbiological issue that can create special mechanical problems because of its biomass. Fungus often competes with the same food sources as bacteria. Usually both bacteria and fungi do not plague a coolant at the same time. Fungi become evident when they grow on the walls of sumps near the fluid's surface or in coolant feed lines and sometimes produce a "sweaty sock" odor. A biomass can turn an oil layer into a mucus-like substance thus sometimes preventing oil from being picked up by a mechanical skimmer. Both mechanical and chemical cleaning is normally required to rid the system of fungal growth.

Extraordinary cleaning agents and methods often need to be employed to effectively and totally remove fungi. Fungi will often go dormant when encountering harsh conditions such as cleaners. When fluid conditions become more favorable, they will reemerge and contaminate the new coolant over again.

After extended use and recycling over a period of time, some coolants experience sufficient selective additive depletion to create serious operational problems such as: corrosion, reduced lubricity, foaming, odors from microbiological growth products and increasing tool wear. Coolant formulations that use multi-functional components can reduce the probability of selective depletion.

Differentiating Coolant Types

When considering the three different types of coolants and the effects that usage and contamination play, it becomes obvious that oil containing products are prone to destabilization because of the oil's properties of emulsification and distribution of its oil droplet particle size. The more oil in the product, the more difficult it is to remain stable over time due to the contamination onslaught of hard water's constituents, microbiological activity and tramp oil entrainment. Oil and its emulsification components have been the traditional "Achilles Heel" of coolant formulations. Oil has remained popular because of its historically low price and good lubricating quality. But lubrication technology, rapidly rising oil prices, and increasing environmental problems with oil disposal are driving an attitude change in the marketplace.

Oil containing products are often difficult to mix and emulsify into plant water especially if the water is hard. Some oil solubles require special mixing equipment to achieve a decent oil "bloom." In certain cases coolant suppliers need to promote mixing equipment or time consuming techniques in order to achieve a decent dispersion. Mixing through proportioners is a sound practice to control uniform concentration additions and is encouraged, but semi-synthetics and synthetics are much easier to solubilize and disperse in water than oil solubles.

Products containing oil naturally tend to absorb some tramp oil because of the excess emulsifiers built into the product. Some recycling systems that remove tramp oil may also remove some of the product's inherent oil. This means that recycling soluble oils or even semi-synthetics can artificially raise costs by removal and discarding some of the product's oil with the tramp. Even some metal ions can have a negative effect on the emulsifiers and the bond they form with oil. Weak emulsification systems often can entrain metal particles and return them to the metal removal process such that quality can be reduced through shorter tool life or poorer surface finishes. Customers who have switched from soluble products to non-oil containing synthetics sometimes are surprised at the increase in tool life and better finishes. Although some of the cause for this improvement is newer technology in synthetic formulations, there often is an improvement due to the more efficient and quicker removal of solids and tramp oil which previously contributed to non-uniform lubrication.

When soluble oils or semi-synthetics "pull in" foreign oil, they yield higher refractometer readings, thus creating a false indication of higher concentration. The additional emulsified oil can destabilize emulsions, and also serve as additional breeding grounds for bacteria to grow, multiply, lower the pH and create objectionable odors. In addition to oil splitting, oil containing coolants tend to show higher consumption rates due to increased drag out rates.

Oil containing products often cannot be fine treated by micro-filtration methods because the filter will take oil out along with other entrained ingredients. Because oil containing products often have a wide distribution of oil droplet particle sizes and a disparity in electrochemical forces within the fluid, they are more subject to selective depletion of ingredients. This occurs not only in the recycling process but also during carry out on parts because of their affinity to certain metal surfaces.

Recently there has been a movement in the coolant industry to promote micro-emulsion solubles and semi-synthetics without petroleum sulfonates. Claims are being made that these products are bio-stable and do not foster foul smelling hydrogen sulfide gas odors resulting from sulfonate degradation caused by bacterial growth. While this claim can be valid in some cases, after prolonged use the micro-emulsion solutions often allow significant bacterial activity that destabilizes the emulsion and reduces rust protection. Although these micro-emulsion chemistries can be recycled and offer longer solution life than traditional formulations, they still can pull in a significant amount of tramp oil thus creating operational problems such as: smoke, oil misting, fine metal particle suspension, oily surfaces, inaccurate refractometer readings and selective depletion of additives (problems already described for macro-emulsion products).

Another innovation in coolant chemistry is to substitute vegetable oils for mineral oils in soluble oil formulations. While vegetable oil imparts a coolant feature of being more rapidly biodegradable in waste treatment systems, this property presents bacteria with prime food sources thus promoting faster bacterial growth in sumps. Although these solubles offer higher lubrication than mineral oils, they do not improve the associated difficulties of tramp oil absorption and separation. Vegetable oils are also harder to emulsify and their emulsions tend to exhibit poor stability. As with mineral oil they can also become destabilized with heat caused oxidation and will yield oily residues and mists. Promoting vegetable oil based coolants can sometimes mislead the user into thinking that once used as machining coolants they can readily be poured down the drain and disposed of without care. TMD