Improving Tank Life

Q. We have a recirculating system for our mass finishing solution. The flow rate to the finishing units is 200 gal/hr. Our system consists of two settling tanks, each capable of holding 1,000 gal, but usually there is about 800 gal in each tank. These are cone shaped tanks with baffles to cause the solution to circulate counterclockwise, and downward, settling the solids in the bottom of the cones. Solution is removed near the top of the cone, from a central riser tube. The first tank gets the dirty discharge from two vibratory bowls and centrifugal disk finisher. The solution from this tank then goes to the second tank for additional settling. As a final step, the solution from the second tank is pumped through a 25-micron filter and on to the finishing room. We tap the sediment from the bottom of each cone, at a rate of one gallon an hour from each cone and send that material to an evaporator. The system works well as far as delivering acceptable solution to our finishing machines, but the problem is a foul odor within just a few weeks of freshly charging the system. Can you give us suggestions for improving the tank life of the solution? S.C.

A. Bio-contamination is a common problem in all closed loop systems. It can occur in machine shops, finishing departments, cooling towers, diesel fuel systems, swimming pools and anywhere a volume of liquid is stored. Jet aircrafts have a critical problem because biomass can plug their fuel system with deadly results. In metalworking machine shops and mass finishing departments, the method for controlling bacteria, mold and fungus has two primary concerns that don’t necessarily apply to other systems: personal safety and protection of metal parts.

Personal safety first involves reducing the exposure of operating personnel to a biomass that may invade their bodies. Good fluid management is essential and it is properly a joint venture between the company using a product and the manufacturer of that product. Even properly formulated compounds can, after lengthy exposure to contaminating environments, start to degrade. Contaminating environments cannot be avoided. They include airborne bacteria, people-borne bacteria, mill oil on stock, fluids coming from prior operations and so on. It is essential that all fluid reservoirs, coolant systems in cutting and grinding machines, water towers, mass finishing systems or whatever—be checked regularly for biomass growths.

The second concern is protection of metal parts. When bacteria grow beyond manageable limits, one result can be corrosion of metal parts. This includes the parts you are manufacturing, the parts of the machines and the fluid handling system.

While odor is often the first thing you observe when a fluid is going fowl, it is better not to wait for that indicator. It is better to have routine testing. The simplest way to do this is to use the commonly available paddle testers. The paddle is dipped in the solution and sealed in its clear plastic tube. Put the tube in a warm, dark place such as a desk drawer in the shop office. In two or three days the surfaces of the paddle will show the presence of common bacteria, mold and fungus. I recommend this as a weekly test.

Controlling the growth of a biomass begins with the formulation of the compound you are using. The control ingredients are expensive. If your buying criterion is price, you will probably get what you paid for and that can be some very costly problems. So, start with quality compounds. Then, if you detect a biomass growth, immediately involve your supplier and apply treatments that are compatible with his products and with your parts and machinery.

The more effective control agents can be very damaging to various metals. For example, if you use common household bleach to disinfect your metalworking fluid, most of the metals it touches will corrode. In one case, even the pump in a parts washer was destroyed. Some common control agents may introduce heavy metals to your solution and cause disposal problems later. Almost any agent can damage yellow metal alloys, and you will have to take measures to protect brass nozzles and fittings. Also, consider operator safety when putting control agents into your system.

Be involved on a regular basis with fluid management, and work closely with the manufacturer of your process compounds. Consider alternate sources of process compounds, and if necessary, use the services of a consulting biologist with experience in metalworking fluids.