Roaster

Rhenium does not occur free in nature or as a compound in a distinct mineral species. It is, however, widely spread throughout the earth s crust to the extent of about 0.001 ppm. Commercial rhenium in the U.S. today is obtained from molybdenum roaster-flue dusts obtained from copper-sulfide ores mined in the vicinity of Miami, Arizona, and elsewhere in Arizona and Utah. 

Only two companies produce specialty malts in roasters or specialty kilns in North America Breiss Malting Co. (Chilton, Wisconsin) and Extractos y Maltas (Mexico City). Other malting companies produce high dried malts in conventional kilns which are used by brewers for color or flavor purposes. Specialty malts represent less than 2% of malt sold in North America. 

Spain, Tmbia nea Oviedo 1972 spray chamber as primary roaster, plate reactor as secondary stage continuous electrolysis of filtered electrolyte, continuous crystallization 2,000 112... 

India, Goa and Bombay two plants built ca 1950 batch-type open-hearth roasters ceUs are operated discontinuously, the KMnO is crystallized separately in agitated tanks 1,200 114... 

The raw potassium manganate(VI) from the secondary roaster or the Hquid-phase oxidizer contains a fair amount of insoluble material such as unreacted MnO and ore gangue. In most continuous processes, these insolubles are removed by sedimentation using thickeners or filtration and are disposed of as waste. 

Sodium alum occurs naturally as the mineral mendo2ite. Commercially, it is produced by the addition of a sodium sulfate solution to aluminum sulfate. Small amounts of potassium sulfate, sodium siUcate, and soda ash can be added to improve product handling and performance. After adjustment of the ratio of aluminum sulfate to sodium sulfate, water is evaporated to give a hard cake ia the cooling pans. This cake is further heated ia roasters and ground to a fineness of 99% through a 100-mesh (- 150 fiva) sieve. 

The absorption of sulfur dioxide in alkaline (even weakly alkaline) aqueous solutions affords sulfites, bisulfites, and metabisulfites. The chemistry of the interaction of sulfur dioxide with alkaline substances, either in solution, slurry, or soHd form, is also of great technological importance in connection with air pollution control and sulfur recovery (25,227,235—241). Even weak bases such as 2inc oxide absorb sulfur dioxide. A slurry of 2inc oxide in a smelter can be used to remove sulfur dioxide and the resultant product can be recycled to the roaster (242). 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

In a usual roaster gas at equiUbrium, the sulfate decomposes at ca 860°C, but it is difficult to avoid some sulfatization during cooling and indeed some plants require a degree of sulfation to maintain sulfate balance. 

Another important reaction accounts for sulfur trioxide in the roaster off-gases and is a component of reaction 2. 

Reaction 3 also occurs on cooling since the concentration of SO is very low at roaster temperatures of 950°C and approaches zero at 1000°C. Another important reaction that occurs during roasting is the formation of zinc ferrite, Zn0-Fe2 03 above 650°C (see Ferrites). Zinc ores contain 5—12% iron. Zinc ferrite forms soHd solutions with other spinels, such as Fe0-Fe203, and therefore the zinc—iron compositions formed are of indefinite stoichiometry. Ferritic zinc is difficult to solubilize in hydrometaHurgical leaching but several recovery processes are discussed below. 

For environmental and economic reasons, the eady practice of roasting zinc sulfide and discharging the sulfur dioxide to the atmosphere gave way to plants where the sulfur dioxide is converted to sulfuric acid. Desulfurization takes place while the ore particles are suspended in hot gases. Called flash-and fluid-bed roasters, these processes are described below. Some plants use combinations of roasters and sintering for desulfurization. 

Multiple-Hea.rth Roasters. The circular types consist of a series of hearths arranged vertically in such a way that the ore entering the top is rabbled and dropped down from hearth to hearth, until it is completely oxidized. The hearths are usually stationary and the plows revolve, such as in the Wedge, Herreshoff, Ord, Skinner, and other roasters (21). In other furnaces, the hearths revolve and the rabbles are fixed, eg, the deSpirlet and its modification, the Barrier. 

A conventional circular-wedge roaster consists of a brick-lined steel shell with hearths arched gendy upward from the periphery to a central shaft. The brick hearths may number from 8 to 16 and are ca 1 m apart. The central steel shaft (ca 1.2 m in diameter) revolves at 1 rpm or less carrying two rabble arms per hearth. These rabbles, cooled with air or water, plow the ore from the outside to the center of the hearth where it is dropped to the next hearth for plowing in the opposite direction. The calcine thus proceeds to the bottom where it is dropped into a conveyor. The sulfide sulfur at this point is ca 3.5% (22). 

New flash roasters dry on the bottom hearth the ore is introduced in two opposed burners for increased turbulence (24). Such roasters with combustion chambers of 8—9 m high are capable of dead roasting (sulfide removal to <0.5%) over 300 t of zinc concentrates per day with 10% sulfur dioxide in the off-gas. 

The flash roaster is flexible ia handling various flotation concentrates and reaching the degree of desulfurization desired, ie, 0.5—3.0% sulfate sulfur. Waste heat is easily recovered. However, grinding and rabbling must be done mechanically. 

A similar FluoSoHds roaster handles 155 t/d of ziac coaceatrates through a roaster which is 1 m higher than standard (28). 

The New Jersey Zinc Company patented a fluidized-peUet roaster which was instaUed in several zinc plants. CaUed a fluid-column roaster, it resembles a shaft furnace and can handle 370 t of concentrate per day. This roaster can be operated at 1080—1100°C to eliminate 90% of the cadmium and 92% of the lead. The fluid-column roaster has the same advantages as the MHO roaster the pelletizing cost is a disadvantage for both systems. 

St. Joe Minerals Corporation uses a fluid-bed roaster to finish the roasting at 950°C of material that has been deleaded in a modified multiple-hearth furnace operated with insufficient oxidation (34). First, sulfur is reduced from 31 to 22% and lead from 0.5 to 0.013%. Somewhat aggregated, the product is hammer-milled before final roasting. Half of the calcined product is bed overflow and special hot cyclones before the boiler remove the other half total sulfur is ca 1.5%. Boiler and precipitator dusts are higher in sulfur, lead, etc, and are separated. 

The ziac concentrate is first roasted ia a fluid-bed roaster to convert the ziac sulfide to the oxide and a small amount of sulfate. Normally, roasting is carried out with an excess of oxygen below 1000°C so that comparatively Htfle cadmium is eliminated from the calciaed material ia this operation (3). Siace the advent of the Imperial Smelting Ziac Furnace, the preliminary roasting processes for ziac and ziac-lead concentrates result ia cadmium recovery as precipitates from solution or as cadmium—lead fume, respectively, as shown ia Figure 1. 

Roasting . Roasting has been largely abandoned ia modem copper smelters, ia which this function is combiaed with the smelting furnace. In older systems, the multiple-hearth roaster is a brick-lined tower having horizontal brick hearths. The concentrate is iatroduced at the top hearth, where rotating arms with rabble blades turn it over and move it to holes ia the hearth. The concentrate is transferred successively to lower hearths and finally... 

In duidized-bed roasters (19), the concentrate is suspended in an upward-moving air stream. The vessel is a refractory-lined steel shell with air entering through holes in a refractory-lined plate at the base. The sulfur dioxide concentration of the exit gas is 10—15%. 

The fumes from the roaster are passed through a train of water-spray scmbbers and an electrostatic precipitator. In the scmbber, selenium dioxide [7446-08-4] reacts with sulfur dioxide (eq. 37) to produce elemental selenium, which is purified to provide a commercial product. 

The roaster product is then charged to the Dorn furnace where it is melted and the resulting metal is fire-refined to eliminate the arsenates, selenates, antimonates, teUurates, and residual copper. 

FIG. 17-26 Single -stage FliioSolids roaster or dryer. Doir-Oliver, Inc. )... 

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