November ISSO
Industrial 1111 astes The glass-grinding industry has engineering experience on processes which can abate stream pollution bg Harold R. Murdock HE process of grinding and polishing plate glass a t the Creighton, Pa., plant of Pittsburgh Plate Glass Company, produces several thousand gallons per minute of a water suspension of powdered glass, silica sand, emory, rouge, plaster of Paris, and smaller amounts of powdered iron. Until June 20, 1949, this industrial waste was purged without treatment into the Allegheny River, where the abrasive settled and was periodically dredged from the river, loaded into barges, and hauled to disposal dumps. State authorities requested the company to find other means for disposal of this abrasive waste which amounts to several hundred tons daily. Rust Engineering Company was retained by the glass company to design and construct a disposal system which would eliminate this pollution of the Allegheny River. I n collaboration with the Pittsburgh Plate Glass Company engineers, a plan was approved and placed in operation on June 20,1949. As the first step in the new disposal method, a large thickener was installed t o receive the combined waste waters from the entire plant operation. The clear overflow water from this thickener has a turbidity less than 100 p.p.m. and is approved for direct addition to the river. The thick slurry of abrasive leaving the thickener has a concentration of about one-third solids. It is pumped through a 4-inch heavy steel pipe line, over a 320-foot high hill, and then on to a natural settling basin 1 mile from the plant. This basin is a natural recession in the terrain, closed in by an earthen dam. The total waterhead for pumping is about 530 feet. The clear water overflow from this settling basin runs t o a small creek and eventually finds its way into the Allegheny River. Rust ,Engineering Company’s biggest problem in designing this waste disposal plant was t o pump economically 230 gallons per minute of a thick abrasive sludge over a hill and on tq a basin 1 mile away. The components of the abrasive waste would rapidly erode impellers and other exposed parts of ordinary pumps, and the sludge would leak back through the clearance between the rotating impeller and the stationary side plate, then into the stuffing box where it would quickly wear the shaft. Rust called upon Dravo Corporation in the design and construction of the pumps t o do this job. It was found that rubber withstood the wear of abrasives four t o six times longer than the best metals, so Hydroseal rubber-lined slurry pumps were used. Into the clearances between the rotating impeller apd the stationary side plate of these pumps, means were provided to introduce a small amount of abrasive-free water at a static pressure slightly higher than the pump
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discharge. This procedure made it impossible for the abrasive waste to enter the stuffing box. Two of these pumps were installed in series a t the thickener, and two each, also in series, in two booster stations along the pipe line. All six pumps were identical with V-belt drives and motors, permitting a minimum of spare parts requirement. In anticipation of mechanical or electrical troubles a duplicate complete stand-by pumping system was installed t o function automatically in case of failure. I n the event of a complete power failure which would make the stand-by unit inoperable, air-operated dumping valves functioned to discharge the sludge into emergency lagoons near the booster stations. . This design has been found to be simple and practical in operation. Stream pollution has been eliminated satisfactorily. Ophthalmic abrasives
Another interesting and well engineered process for handling spent abrasives is found a t the Southbridge, Mass., plant of American Optical Company. This company produces large quantities of eorundum abrasives, some of which are sold t o other industries, but the greater portion of the production is consumed in grinding optical lenses in their own plant. Natural corundum, mined in Africa, is used because it has the property of grinding into hexagonal shaped particles and never splinters. Every particle has the same six cutting points which permit fast abrading during use. This raw material is first ground by the wet method in ball mills. The trouble with this method is that the original crushing process produces B complete assortment of all the smaller sized particles whereas a uniform size particle is best for precision grinding. Until a few years ago, classification was performed by using a great array of tanks and a tremendous amount of time and labor to settle the ungraded material, decant off the supernatant slurry, and dry the settled residue, over and over again, with each finer grade taking longer t o settle. Although the best care and skill were used in this gravity separation method the sifie difference between undersized and oversized particles prevented obtaining the perfect corundum abrasive. The larger sized particles scratched te Iens surface so that a prolonged polishing step to remove the scratches was necessary. Everything possible appears to have been done to perfect the gravity method. The American Optical Company desired perfection in ophthalmic abrasives and discussed their problem with outside engineers experienced in particle-size fractionation of many materials. This resulted in the abandonment of the gravity method and adoption of a controlled (Continued on page 118 A ) centrifugal force method. 111A
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The equipment specifications called for a rigid mechanical control together with a sensitive electronic control to ensure the highest degree of particle size uniformity. The Bird centrifugal filter was the answer, following extensive research made in the Bird Research and Development Center. These centrifuges which American Optical Company has dubbed the “whirling dervish” are in principle a revolving cone which can be spun fast or slow by a variable motor drive. On passing a suspension of an insoluble substance continuously through the revolving cone and controlling the rotation speed and other factors, a particle size uniform within 4 to 5 microns, in the case of American Optical Company’s material, can be produced with ease. At the American Optical plant today, new corundum, ground in ball mills, is added to all the residual grinding waste waters from the lens grinding operations. This amounts to a total of 150 to 200 gallons per minute. The suspension carries from 1to 5% of solids and is regulated with a density control device in order to produce a uniform percentage of solids in the feed to the Bird centrifugal clarifier. If the level in the tank drons to a point where the uniform volume of feed is affected, the feed is shut off automatically until the tank refills. The first centrifugal accepts only those particles of 5 microns or larger. This cut leaves the unit as a slurry carrying about 70 to 75% solids. Particles under 5 microns in size, together with dirt, oil, and other extraneous matter, are carried away in the centrifuge rejects to waste lagoons. The acceptable 5-micron particle slurry is diluted to a 20y0 solid suspension under automatic density and tank level controls. This product then goes through a series of Bird centrifugal classifiers where in each unit a specific predetermined particle size is removed. In the first unit the particles over 25 microns in size are removed. These are returned to the ball mill for further grinding. The effluent from this unit then passes through five whirling dervishes in series. The precise degree of centrifugal force is applied in each unit to produce a definite grade of corundum particle size. Accurate performance is maintained simply by controlling the volume with feed tank level control and pumps of regulated capacity. The solids discharged from each classifier represent the finished product. The water which leaves the last classifier is virtually clear since the unusable fines were already removed in the first classifier. Considerable savings of water and manpower and a distinct improvement in classified uniformity of the graded abrasive at a lower production cost are outstanding advantages from the process. American Optical reported a distinct drop in lens “casGalties” when abrasives graded by this continuous centrifugal process were first used in their toric lens-grinding operation. These examples show what can be done in overcoming waste Droblems when a technical approach through capable engineers is used. Waste disposal problems involve specializations which are not always common to the experience of the industry creating the waste.
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