JOURNAL OF CHEMICAL EDUCATION
PRODUCING AN ALLOYING ELEMENT OF HIGH PURITY w
E ~ ~ ~ O Lmanganese V T I Cis relatively new. ~t first became commercially available in 1939 when the plant of the Electro Manganese Corporation began operations at Knoxville, Tennessee. In the war years its use was restricted by the War Production Board t o certain essential uses, such as high-tensile bronzes, and it has only become generally available since the removal of government restrictions. Because of its high purity (99.9 per cent plus), electrolytic manganese is finding favor as an additional agent in both ferrous and nonferrous melting, and in the preparation of pure manganese salts and catalysts in the chemical industry. Prior to the development of this process, manganese was added to other materials for alloying purposes in the form of ferro-manganesc.
W. I.HAMMERQUIST Electro Manganese Corporation, Knoxville, Tennessee
The Knoxville plant is currently producing an average of 10,200 lb. a day of pure manganese metal, and has a construction program nearing completion which will increase this production by about 50 per cent. This plant continues to be the sole commercial producer of pure manganese. Nickel alloys have been of great value in the production of electrolytic manganese, and for many purposes have proved irreplaceable. The first step in the process is the reduction roasting of dioxide ore to make the manganese content soluble in cell acid. This is accomplished in two rotary-tube electrically heated furnaces. These furnaces use nickel in some important applications. One of these is the tubes, which must withstand a high degree of heat, as well as some abrasion.
JULY. 1948
It was found that ordinary steels burned out in a very short time, scaling heavily on the outside. This scale would fall off and short-circuit the electrical heating elements. After consideration of many alloy steels, Misco Metal, with a composition of 35 per cent nickel and 15 per cent chromium, was selected. Michigan Steel Castings Company furnished this alloy in centrifugally cast tubes 15 feet long .and with 12-inch inside diameter. These have proved very satisfactory, having so far given service in one furnace of about 16,000 hours and in another of about 9000 hours, with no sign of failure. These tubes pass through electrically heated fireboxes, in which the heating elements are of a resistance type having a composition of 60 per cent nickel, 16 per cent chromium, 24 per cent iron, and 0.1 per cent carbon. These are furnished ,by Hevi Duty Electric Company. In the next stage the cooled reduced ore is fed to wood-stave leaching tanks equipped with motordriven impellers. Anolyte is fed into these digesters, together with some make-up sulfuric acid. Three types of impellers have been tested-lead-covered mild steel, rubber-covered mild steel, and type 316 chromium-nickel-molybdenum stainless steel. Despite their higher initial cost, the longer life and negligible maintenance cost of the type 316 impellers make them the most economical. Eventually, all leach tanks will be equipped with this type of impeller. Stainless steel wear plates are also used in the leach tanks. Leach slurries are settled in lead-lined Dorr thickeners equipped with large wooden impellers, which are assembled with type 316 stainless steel bolts and washers. These bolts show no evidence of corrosion after several years of continuous service. Thickened muds from the Dorr thickeners are filtered and washed on Moore vacuum filters. The essent(ia1parts of this filter, which come into contact wit,h the electrolyte, are made of wood, canvas, and type 316 stainless steel, containing 18 ger cent chromium, 12 per cent nickel, and 2 per cent molybdenum. Xany centrifugal pumps are required to move electrolyte through the various steps of the process. Durimet B (35 per cent nickel, 12 per cent chromium, 45 per cent iron, 5 per cent silicon), produced by The Duriron Company, has been selected as the most satisfactory pump alloy to resist corrosion by manganese electrolytes. Electrolytes are transferred through lead pipe or rubber hose. Where rubber hose is used, connections are made by means of cast Durimet B fittings. Valves in these lines are made either from Durimet B or type 316 stainless steel.
Probably the most important, certainly the heaviest, use of stainless steel in the production of electrolytic manganese is in the cathodes, or starting sheets, upon which the manganese is deposited. Cold rolled steel, which was originally used, corroded badly at the liquidsurface line and contaminated the electrolyte with iron. When manganese was deposited on these cathodes, it was always difficult and usually impossible to st.rip it off, and the cathodes were so roughened in the stripping process that they could not be used a second time. Use of stainless chromium-nickel steel cathodes was pioneered and brought to its final development a t the Knoxville plant with the active cooperation of stainless producers, especially the American Rolling Mill Compaw. Type 316 stainless steel was finally selected as the most suitable material because it had the necessary corrosion resistance combined with the proper temper to withstand the flexing operation to remove the manganese deposit without permanent deformation. It also requires a minimum of surface preparation between deposition cycles. Cathodes of this material withstand years of rough service. Passage of electric current through the cells causes the temperafure of the electrolyte to rise, and the cells would become too hot to permit the deposition of manganese if some means of controlling temperature were not provided. This is accomplished by circulating the portion of the cell electrolyte in contact with the anode through redwood cooling towers, assembled with stainless steel bolts and nails. The weirs which control flow and distribution of electrolyte in the towers are also made of stainless steel. The system contains over 300,000 gallons of a highly corrosive electrolyte which moves through at a fairly rapid rate. Once the raw leach liquors are purified, it is necessary to make sure that they are not contaminated by the materials with which they come in contact. Nickel is one of the metals who; presence in the electrolyte, even in amounts as low as 0.0004 gram per liter (0.000052 ounce per gallon), will inhibit the deposition of manganese; yet the purified electrolyte is allowed to come into contact with these high nickel alloys. This rather remarkable paradox is of course an eloquent testimonial to the corrosion resistance of the materials. This is another practical example of how a continuous trouhle-free operation can be assured and production costs cut through the use of relatively high-priced heat and corrosion-resistant alloys in a process where deterioration from such hazards must be aroided to the maximum degree.
ERRATUM In the review of "The Theory of Valency and the Struoture oi Chemical Compounds" which appears on page 180 of our March, 1948, issue the author should have been given as Priyadaranjan Riy, rather than Pandit R&y.
