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Reprint from Machine Shop Magazine

Water is Water, Or is it ?

With the ever-tightening environmental regulations facing today's metal working industry, managers of the American metal working plants are recognizing what those in the aerospace industry , the plating industry and the electronics industry have known for a relatively long time - water is not just water. Rather the quality of the water used in a manufacturing process can dramatically affect the quality of the product, the efficiency of the process, the maintenance costs of the process machinery and the useful life of any process chemical bath.

It is primarily in relation to this useful life of chemical baths that managers are beginning to recognize the effect of water quality, generally the purer the water the greater the useful life of the chemical bath. And the greater the useful life, the less material which must ultimately be processed for disposal.

CUTTING FLUIDS

Water soluble cutting and grinding fluids are chemical baths; that is they are composed of fluid concentrate diluted normally from one to ten percent with water. Water then amounts to 90 to 99 percent of the cutting or grinding fluid in the machine coolant sump. Poor quality ( High mineral content ) water dramatically affects the performance of the fluid, as follows:

Hardness minerals ( primarily calcium and magnesium chlorides and sulfates ) affect the chemical, semichemical and emulsion type fluids, and result in gummy, sticky, residues, separated emulsions and tend to foster microbial growth.

Nonhardness minerals ( Primarily sodium and potassium chlorides and sulfates ) affect all types of water miscible fluids, and are frequently the cause of corrosion problems associated with these fluids.

Sulfates ( sodium, potassium, calcium and magnesium ) all act as oxygen sources for sulfate reducing bacteria, which are responsible for liberating hydrogen sulfide gas - commonly referred to as "Monday-morning stink" in metal working shops.

Virtually all water-miscible fluids are formulated to overcome some of the detrimental effects of minerals dissolved in the water with which the concentrate is mixed. Generally, cutting fluid emulsifier systems contain both anionic and nonionic wetting agents; the latter are unaffected by hardness salts present in the water. But nonionic wetting agents, while able to overcome some effects of hardness salts, make poor cutting fluid lubricants and tend to form stable foams, which can cause operational problems with some coolant systems.

Anti-corrosion systems in the fluid will suppress the corrosive effects of the acid radical (Cl4 or SO ) of the dissolved minerals in the water supply. However, the corrosion inhibitors will tolerate only certain concentrations of these salts before there effectiveness is overwhelmed. The effect of these chloride and sulfate ions in the coolant is perhaps most noticeable when machined parts are stacked wet and allowed to dry in tote boxes. Staining of corrosion is frequently found on mating surfaces of these parts after a relatively short time and this staining is usually directly attributable to the presence of these ions in the coolant solution trapped between parts.

This latter point is important because a machine tool coolant sump acts like a still at room temperature and much of the daily coolant usage is actually replacement of water lost by evaporation. Whatever salts were present in the evaporated water are left behind in the coolant solution in the machine sump. Consequently, although a coolant solution starts off with a relatively good water the accumulation effect rapidly converts the water in the sump to a poor quality water.

WATER IMPROVEMENT

Since water quality affects the performance of water-miscible cutting and grinding fluids and the performance of the fluid affects the efficiency of the manufacturing operation, what options does the manager have in regard to the quality of water?

To date, the majority of metal working plants have done nothing to improve water quality. Typically, they continue to use raw water or untreated water supplies, either private wells or public water systems. The quality of such sources ranges from very good to very bad. One metalworking plant uses a private artesian well, which produces water containing about 9 ppm total dissolved solids, about half a grain, and is almost as pure as distilled water while another plant's water system contains over 110 grains per gallon total dissolved solids. The majority of metalworking plants mix coolant concentrates with waters varying from four to five grains( considered to be moderately hard ) to 15 to 20 grains ( considered to be very hard).

In some metalworking plants with extremely poor quality water, managers have chosen to improve water quality by installing water softening equipment. Water softening is a process in which hardness mineral ions are exchanged for the nonhardness mineral ion ( sodium ) by passing water through an ion exchange resin bed. The ion exchange bed is a pressure-tight tank filled with ion exchange resin ( tiny, porous, plastic beads which carry a negative electric charge ), and the necessary plumbing and controls to affect water flow through the bed as well as periodic regeneration of the bed. As water flows through the bed, calcium and magnesium ions absorb onto the resin particles and in so doing, replace sodium ions present on the resin particles. Thus, calcium and magnesium ions are exchanged for sodium ions.

Periodically, the ion exchange bed is regenerated with a saturated salt ( sodium chloride ) solution. The highly concentrated sodium ions replace the calcium and magnesium ions previously removed from the water, and the bed is rinsed to remove excess salt. The resin bed is now recharged or regenerated with sodium ions and ready to soften more water.

The total amount of dissolved solids present in softened water is not appreciably different from the hard water. But the nature of the water is different in that the calcium and magnesium salts have been exchanged for sodium salts and the sticky, gummy residues which result when mixing coolant concentrates with hard water are no longer a problem. However, coolants mixed with softened water have a greater tendency to cause corrosion than coolants mixed with either hard or demineralized water.

Three processes are available to remove dissolved minerals from water: (1) distillation, (2) reverse osmosis, and (3) Deionization.

Distillation is the process by which dissolved minerals are removed by first evaporating the water ( thereby leaving the dissolved solids behind ) and condensing the water vapor. Distillation is extremely effective in producing high quality water but has drawbacks of requiring high initial investment and being both energy and maintenance intensive. The cost of water produced by distillation is relatively high.

Reverse osmosis is a technique whereby relatively pure water is produced by forcing water through a semipermiable membrane under high pressure. Water molecules pass through the membrane while the majority of dissolved ions are filtered out by the membrane. While the process does improve water quality, it does not produce water of sufficiently high quality for use with water miscible fluids; typically, only 90 percent of the minerals are removed from the water supply. Further, the membranes have relatively unpredictable lives and relatively high replacement costs and approximately half of the water fed to the system goes down the drain as waste.

Deionization is the process by which dissolved minerals are removed from water by passing the water through ion exchange beds. Both negatively and positively charged ions are removed to produce the equivalent of distilled water with a much lower installation, operating and maintenance costs than distillation equipment.

THE DEIONIZATION PROCESS

Deionizers are similar in operation to the water softeners described previously. The major difference is that softeners consist of a single ion exchange bed wherein sodium ions are exchanged for calcium and magnesium ions, whereas deionizers are composed of two ion exchange beds:

1. A Cation exchanger wherein hydrogen ions are exchanged for all cations present in the water supply, normally sodium, potassium, calcium, magnesium, iron, and aluminum.

2. An anion exchanger wherein hydroxyl ions are exchanged for all anions present in the water, normally sulfates, chlorides and carbonates.

Whereas softeners are regenerated with sodium chloride, deionizers normally utilize hydrochloric acid to regenerate the cation and sodium hydroxide to regenerate the anion exchanger, although other regenerating chemicals can be used in certain instances.

Generally, deionizers can produce water equivalent in quality to distilled water for pennies per gallon. This cost includes the equipment purchase price amortized over five years and the cost for regenerant chemicals used over that same period. The cost per gallon varies with the relative quality of the water being Deionized ( the better the quality, the lower the cost ) and the amount of water required. Since a deionizer has a fixed minimum cost, the greater the amount of water that is Deionized the less the equipment costs per gallon. Deionized water can reduce coolant consumption as much as 80 percent and normally 30 to 40 percent compared to mixing coolant concentrate with raw water.

Deionized water can also greatly extend the sump life of water-miscible fluids. Theoretically, if the coolant sump can be kept relatively free of tramp lubricating and hydraulic oils and other contaminants, the fluid could have virtually unlimited sump life. However let us assume Deionized water would only extend sump life from three to four months. Such an extension would save one machine pump-out and cleaning per year. If machine cleaning is done during production time and requires three hours for each cleaning, it would cost $36.00 to clean a manual turret lathe, assuming the machine carries a burden rate of $12.00 per hour.

In addition to lowering coolant consumption and reducing machine cleaning and pump-out costs, Deionized water can reduce machine corrosion problems. Simultaneously, it will reduce the tendency for bacterial growth in coolants. In summary, a small expenditure for improved quality of the water used to dilute water-miscible coolants can produce major savings in overall plant maintenance costs as well as major increases in overall plant efficiency. MMS

ABOUT THE AUTHOR

As general manager of Master Chemical Corporation's Systems Equipment Division, William A. Sluham directs the design, manufacture and marketing of Master Chemical's closed loop coolant systems. Upon graduation from Ohio Wesleyan University in 1964 with majors in chemistry and business administration, Mr. Sluthham joined the Master Chemical Corporation as a salesman. After serving two years in the US Army, he returned to Master Chemical Corporation as assistant to the Sales Manager and Director of Product Service and Evaluation, responsible for the field testing and evaluation of research and development products. In 1973 he was promoted to Vice President Operations, and assumed his present responsibilities in 1976.