TECHNOLOGY
Denver Introduces New Flotation Mechanism New recirculation pattern and improved aeration boost efficiencies and cut costs A new impeller mechanism for flotation machines is now being manufactured by Denver Equipment Co., of Denver, Colo. Denver says the mechanism, which it calls the D-R Denver flotation mechanism, betters flotation efficiency by improving aeration and by increasing the amount of recirculation in a flotation cell. As a consequence, users can expect benefits ranging from increased capacity to decreased requirements for power or flotation reagents. In flotation, mineral grains or chemical compounds in a slurry (pulp)
rise with bubbles of air. Attraction of the air and mineral is aided by suitable reagents. Unaffected (gangue) minerals settle to the bottom as tailings. Floated minerals gather in a froth where they can be removed for further processing. The better the contact between minerals in the pulp and air, the more efficient is the flotation. Denver's new flotation mechanism improves this contact by design changes which can best be illustrated by comparing it with the company's standard Sub-A mechanism.
RECIRCULATION. An outer pulp recirculation well directs large volumes of pulp from the upper zone of the cell to the denser lower zone. The large 360° opening above the impel40
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Both mechanisms have a horizontal impeller suspended from a vertical drive shaft. A standpipe surrounds the drive shaft and serves to conduct air downward. Beyond this basic similarity the two differ. A diffuser— a circular plate slotted radially on its bottom side—is connected to the standpipe of the Sub-A mechanism. The impeller spins right below the diffuser. The standpipe is punched with one, two, or four radial holes (recirculation ports) above the diffuser. Pulp enters recirculation ports, mixes with downflowing air, and is forced out be-
ler makes it possible for pulp to recirculate. Thus the amount of recirculating pulp and the volume of air introduced are evenly distributed throughout the cell during operation
tween the diffuser and impeller by the action of the impeller. The new D-R mechanism has no re circulation ports in the standpipe, and the diffuser is not connected to the standpipe. Instead, the lower part of the standpipe is surrounded by a pulp recirculation well, and the diffuser is connected to the bottom of the re circulation well. Between the standpipe and the recirculation well is an open, annular space through which recirculated pulp can flow downward. The standpipe ends as an open tube at the eye of the impeller. Several Effects. The new mecha nism has several effects, Denver says. Because the pulp recirculation well is open through 360°, it takes circulation from a higher level in the cell and circulates a greater volume of pulp. This moves a higher concentration of solids to the upper levels of the cell, and increases efficiency. Efficiency is also increased because aeration is more effective at the impeller's eye where air-mineral contact is better. The Sub-Α mechanism can be converted to the D-R mechanism easily. The D-R mechanism is already op erating in flotation machines recover ing copper, zinc, lead, molybdenum, iron, fluorspar, potash, phosphates, and coal. Such users can expect many benefits from the D-R mechanism, ac cording to Denver. The company says that improvements range from in creases in capacity or efficiency to decreases in power and reagent re quirements. Denver cites several examples how the D-R mechanism is used. A Flor ida phosphate beneficiation plant has been able to switch from an ore that was 2% + 3 5 mesh to an ore that is 12% + 3 5 mesh. At the same time, reagent consumption is down 2 5 % and tailings losses are down 50%. A Colorado molybdenum mill has been able to reduce impeller r.p.m. by 22%, cutting horsepower per cell by two. The mill estimates an annual saving in power cost of more than $8000. Recovery is now up 1% over prior performance. A 1200 ton-per-day Colorado leadzinc mill has achieved a 2 3 % drop in impeller r.p.m., saving 3 hp. per cell or an estimated $3500 per year. This mill also realized a tailing loss drop of 1%. In general, Denver Equipment points out, wear to the flotation mechanism is directly proportional to power consumption so the Colorado mills can expect added savings. Ε 3 5
SEMIWORKS. This is Union Carbide's semiworks production unit at Bound Brook, Ν J . The polymerization chamber at the far left is a 10-pass foil-coating unit which makes foil used in making Kemet brand capacitors
Aromatic Polymers Move into Specialties Union Carbide's parylene series is based on vapor-phase polymerization of p-xylylene Aromatic intermediates seem set for a heyday in specialty polymers. Union Carbide's commercial move with its parylene series of polymers based on p-xylene (C&EN, Feb. 22, page 35) follows a similar move by General Electric with polyphenylene oxide, made from 2,6-dimethylphenol (C&EN, Dec. 7, 1964, page 5 7 ) . Also, some firms have semicommercial polymers, such as polyimides made by reacting the dianhydride of an aro matic tetrabasic acid with an aromatic diamide. And there are some aro matic polymers still in the research stage—polybenzothiazoles and polyquinoxalines, for example. In common, these semicommercial polymers are very high priced, a factor which generally dictates a low sales volume. For example, polymers sell ing for about $2.00 per pound have annual sales of no more than 15 to 20 million lb. Carbide has a pilot plant at Bound Brook, N.J., to make the intermediate for its parylene. The intermediate, dip-xylylene, sells for $1000 per pound. However, depending on the accept ance of the polymers the company plans to build a 250,000 lb.-per-year plant, which will bring the price of the intermediate down to $100 per pound. If the company should then build a multimillion pound-per-year plant, the
intermediate would sell for about $2.00 per pound. Kits are available at $25 for in vestigation of properties. Carbide is also soliciting inquiries from fabri cating firms into new applications. The company does not want to go into the fabricating business and, therefore, will license its process as well as sell the intermediate. Synthesis. The new polymer series is based on vapor-phase polymeriza tion of p-xylylene. Carbide calls the series parylene. Basically, p-xylene is pyrolized at 950° C. in the presence of steam to yield a cyclic dimer, di-p-xylylene. The dimer is produced in about 15% yield, with a 60% efficiency. After separating the dimer by quenching with benzene or toluene, it is heated in a sublimation chamber and pyro lized in the vapor phase at about 550° C , yielding the monomer vapor, p-xylylene. This vapor is cooled to be low 50° C , and poly-p-xylylene, or parylene N, with a molecular weight of about 500,000, is formed. Variations are made by substituting up to three hydrogens on each benzene ring of the cyclic dimer. Any of the halogens, or alkyl, or phenyl groups can be used. For example, Carbide also makes what it calls parylene C: poly (monochloro-p-xylylene). MAR.
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Parylene is a linear thermoplastic free from cross-linking. It can be dissolved in a chlorinated biphenyl solution at about 520° F., and when cooled to about 320° F., the polymer crystallizes as a gel. When reheated to 520° F., it forms a free-flowing solution. Dr. W. F. Gorham, assistant direc tor of polymer research and devel opment of Carbide's plastics division, says that the structure of parylene is different from that of the poly-pxylylene reported in 1947 by Dr. M. Szwark, now at Syracuse University College of Forestry. Dr. Szwark pyrolized p-xylene directly, under re duced pressure at 800° to 1000° C. Dr. Gorham says that the resulting poly-p-xylylene is thought to be highly cross-linked. It can only be dissolved in solvents that are heated close to its crystalline melting point. At such temperatures, decomposition takes place. Carbide can make the polymer in powder form (by quenching the mon omer vapor in an aqueous solution), as a free film (by polymerizing it on a substrate and stripping it off), or as a coating. However, the polymer powder is hard to handle. Parylene Ν has a melting point (400° C.) above its thermal decomposition point (350° C.) and molding would be im possible. Parylene C's melting point is 270° C , and although molding wouldn't be impossible, properties of the molded material are poor. Carbide's main interest is coating substrates with a dielectric or a pro tective coating. One limiting factor, however, is that the substrate must resist exposure to relatively high vac uum. Parylene Ν is already being used as a dielectric in the Linde Division's Kemet brand ML-1 ca pacitors. The company coats alumi num foil with a parylene Ν by a vac uum-coating process. Cooled foil travels through a vacuum chamber (about 1 mm.) which is filled with monomer vapor. The vapor hitting the cooled surface polymerizes, form ing a coating on it. On cooled surfaces the deposition rate can be as high as 1.0 mil per minute. ML-1 capacitors have good capaci tance stability over a temperature range of - 8 0 ° to 170° C. They can be made to one fifth the size of polystyrene capacitors and 1/20 the size of Teflon capacitors of equivalent electrical rating. Carbide has also worked with pary 42
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lene as a protective coating. Solid chemicals, such as sodium hydroxide and sodium dichromate, can be en capsulated with parylene C. Pary lene C-coated polyethylene containers will contain liquids that readily pene trate uncoated polyethylene. How ever, parylene coatings are not effec tive in containing aromatics. Thermal endurance of parylene in air is not very good. For the shortterm use (1000 h r . ) , temperature for parylene Ν is 200° F., and for pary lene C 240° F. The corresponding long-term use (10 years) temperatures are 140° F. for parylene Ν and 175° F. for parylene C. Parylene performs better in the ab sence of air or in an inert atmosphere. Short-term use temperature for both parylenes Ν and C is 510° F.; longterm is about 430° F. for parylene Ν and somewhat higher for parylene C. C60
Freeze Purification Makes 99.99%-Purity Aluminum A new, low-cost process for produc ing aluminum of ultrahigh purity has been developed by Reynolds Metals Co. A freeze purification method, it consists of fractionally crystallizing part of a molten aluminum feed stream while strongly agitating the stream. Result, Reynolds says, is an aluminum ingot of uniform 99.95 to 99.99% purity at less than half the processing cost of that produced by the conven tional electrolytic refining process. Commercial-grade aluminum ranges from 99.5 to 99.8% purity, which is adequate for most applications. There is a substantial market, however, for ultrahigh-purity material which cur rently amounts to about 3 million lb. annually. This market includes cat alysts for the petroleum industry, foil for electronic parts, and jewelry. Alcoa, as well as Reynolds, is actively developing uses for ultrahighpurity aluminum. One use Alcoa has developed is automotive trim alloys. Today the price for ultrahigh-purity aluminum is about 44 cents per pound. It is usually made by an electrolytic process in a three-layer refining cell. Molten, commercial-grade aluminum is fed into the bottom of the cell, which contains a layer of liquid aluminumcopper alloy. This layer serves as the anode. The middle layer is electro lyte, usually sodium aluminum fluo ride and barium fluoride or chloride.
Ultrahigh-purity aluminum forms the top layer and also serves as the cathode. Efforts to develop a low-cost proc ess began at Reynold's reduction re search headquarters in Sheffield, Ala., nearly 10 years ago. Battelle Me morial Institute was also involved, Reynolds' development project direc tor John L. Dewey told the American Institute of Mining, Metallurgical, and Petroleum Engineers, meeting in Chi cago last month. Two Phases. In the Reynolds proc ess, molten commercial-grade alumi num is fed into a heated graphite mold equipped with a stirrer. Here the aluminum separates into a higher pu rity ingot, which is continuously drawn off the bottom of the mold and cooled with water, and a lesser purity molten phase which overflows from the mold. The stirrer is mounted so that it agitates the molten aluminum near the surface of the growing crystal of veryhigh-purity metal. The stirrer sweeps away impurities ejected from the freezing interface. Increasing speed of stirring tends to increase the purity. A furnace holds the mold and the molten aluminum at a constant temperature just above 660° C , the melting point of aluminum. The lower purity phase, which carries off the impurities rejected from the freez ing ingot, is returned for commercialgrade metal. Theoretically the degree of purity which can be obtained by freeze pu rification depends on the ratio of the solubility of a particular impurity in solid aluminum to its solubility in molten aluminum, Mr. Dewey ex plains. In practice, the degree of pu rity is limited by how fast the different impurities can diffuse from the grow ing crystal face. It is regulated by varying the production rate. Impurities removed by the new process include silicon, iron, copper, magnesium, zinc, and gallium. The major impurities in commercial-grade aluminum are silicon, iron, and copper, and these are the easiest to remove with the process, Mr. Dewey says. Reynolds has received a patent (U.S. 3,163,895) on the process and also holds several foreign patents. To date the process has been used to produce 8-in.- and 20-in.-diameter in gots in pilot plant and semicommercial units, respectively. Reynolds is now carrying out studies to determine if a sufficient market exists to justify regular plant use of the new process.
