Water Is Water,,, Or Is It?

Transcription

1 Reprinted From Water Is Water,,, Or Is It? c Reminted Fnr MASTER CHEMICAL CORPORATION 610 West Boundary, Box 361 Perrysburg, Ohio Phone :

2 By WILLIAM A. SLUHAN General Manager Master Chemical Corporation Perrysburg, Ohio 3, h With the ever-tightening environmental regulations facing today s metalworking industry, managers of American metalworking 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 dra- matically 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 relatlon to the 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 -2-

3 I the yrt-.,iler 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, carbonates and sulfates) affect 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 watermiscdible fluids, and are frequently the cause of corrosion problems asso-. ciated with these fluids. Sulfates (sodium, potassium, calcium and magnesium) all act as oxygen sources for the sulfatereducing bacteria, which are responsible for liberating hydrogen sulfide gas-commonly referred to as Monday-morning stink in metalworking 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, cut- ting 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 (CI- or SO,=) of the dissolved minerals in the water supply. However, the corrosion inhibitors will tolerate only certain concentrations of these salts before their 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 or 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 -3-

4 in the sump to a poor quality water. Figure 1 shows the increase in mineral content of a coolant sump for a 90,000-gallon central system run three shifts per day, six days per week. Coolant makeup required was 10,000 gallons every twenty-four hours-8,800 gallons of water lost by evaporation and 1200 gallons that adhered to removed chips and parts. In 40 working days the mineral content of the coolant solution increased from 250 ppm (14 grains) to 1000 ppm (56 grains). This increase is typical of most coolant systems. 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 metalworking plants have done nothing to improve water quality. Typically, they continue to use raw 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 a half 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 (calcium and magnesium ions primarily) are exchanged for nonhardness mineral ion (sodium) by passing the 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 adsorb 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 -4-

5 and magnesium salts have been exchanged for sodium salts and the sticky, yummy 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 water or demineralized water, as shown in Figure 2. Fig. 1 -Mineral accumulation versus time in a water-miscible metalworking fluid. - 5-

6 I 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 the drawbacks of requiring high initial investment and being both energyand 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 semipermeable 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 ihe 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 much lower installation, operating and maintenance costs than distillation equipment. The Deionization Process Deionizers, shown in Figure 3, 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 exchanger 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 about one-fourth to two cents per gallon (1/2 cent per gallon is about average). 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 with I

7 tlw atnount of water required. Since a deiorii/er has a fixed minimum cost, the greater the amount of water that is deionized the less the equipment cost per gallon. The average turret lathe requires approximately 500 gallons of water per year and, assuming an unusually high cost of one cent per gallon, deionized water for that turret lathe Fig. 3-Deionizers can produce water equivalent in quality to distilled water for an average of one-half cent per gallon. Of course, thecost varies with the initial quality of the water being deionized and with the amount of water required for plant usage. -7-

8 I c would cost only five dollars per year. 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. If that average turret lathe used $40.00 worth of cutting fluid concentrate per year, the use of deionized water would save three to six times the cost of the deionized water yearly. Deionized water can also greatly extend the sump life of watermiscible 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 fluid sump life from three months 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. Again, assuming deionized water for this average lathe costs $5.00 per year but saves one cleaning and recharging per year, the net savings would be $31.00 per year. In addition to lowering coolant consumption and reducing machine cleaning and pump-out costs, deionized water can reduce machine corrosion problems. Simutaneously, it will reduce workpiece corrosion problems and 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. Sluhan directs the design,manufacturc and marketing of Master Chemical's closed loop coolant systems. Upon graduation from Ohio Wesleyari University in 1964 with majors in chemistry and business admlnistration. Mr Sluhan joined the Master Chemical Corporation as a salesman. After serving two years in the U.S Army, he returned to Master Chemical Corporation as Assistant to the Sales Manager arid Director of Product Servlce 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