Roasting process

Lead is obtained chiefly from galena (PbS) by a roasting process. Anglesite, cerussite, and minim are other common lead minerals. 

It is made from manganese dioxide ores through a reductive roasting process (31—33). 

Depending on the means of conversion of manganate(V) to (VI), the process may be classified as a roasting or Hquid-phase process (Fig. 8). The roasting process employs a soHd reaction mixture having a molar ratio between MnO and KOH in the range of 1 2 to 1 3. In contrast, the Hquid-phase route operates at a higher (>1 5) molar ratio between MnO and KOH. 

The roasting process, or variations of it, are most common. Liquid-phase processes are ia operation, however, both ia the United States and the former USSR. The former USSR is the only place where KMnO was produced by anodic oxidation of ferromanganese. Table 17 summarizes the various KMnO manufactuting faciUties worldwide as of this writing. 

Roasting Processes. The first step in roasting processes is the formation of K.MnO from Mn02 ore. This is promoted by high temperature and high KOH and low H2O concentration. The second step oxidizes Mn(V) to Mn(VI). A lower temperature and control of moisture in the air is used. 

This reaction is strongly exothermic and proceeds spontaneously from left to right for most common metallic sulfides under normal roasting conditions, ie, in air, because P q + Pq = - 20 kPa (0.2 atm) at temperatures ranging from 650 to 1000°C. The physical chemistry of the roasting process is more complex than indicated by equation 3 alone. Sulfur trioxide is also formed,... 

The sodium tungstate solution is filtered from the resulting slurry. Similarly, in the alkaH roasting process, the concentrate, either scheeHte or wolframite, is heated with sodium carbonate in a rotary kiln at 800°C and then leached using hot water to remove the sodium tungstate. Wolframite ores are also decomposed by reaction with a sodium hydroxide solution at 100°C. 

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. 

The use of infrared heat (gas-frred ceramics) or microwave energy to speed up the roasting process while providing a more even roast has also been patented (26,27). 

Rdst-posten, m. roasting charge, -probe, /. calcination assay or test, -produkt, n. = Rosterzeugnis. -prozess, m. roasting process, calcination process, -reduktionsarbeit, /. (Metal.) roast reduction process, rostrein, a. rust-free. 

These properties can be partly produced by fiber disgestion (steam explosion, steam pressure, thaw roasting process, water roasting process [8,9]), as well as by a direct application to the fiber. 

The roasting process mostly applied to sulfidic sources is carried out in appliances of different types with different design features. Without describing their features and constructional details, it may simply be mentioned here that some of the widely acclaimed roasting units are traveling grate furnaces, multiple hearth furnace, rotary kilns, and fluid-ized-bed roasters. 

The ammoniacal systems do not cause corrosion problems and this allows use of standard materials of construction. The process is mainly applicable to high iron limonitic laterites. The amount of acceptability of the transition ore (S-ore) is limited due to nickel loss caused by forsterite formation during the roasting process. 

The roasting process gives rise to the formation of S02 and As203, both of which are environmentally undesirable. For this reason, roasting, although still practised, is less favorably disposed as compared with the newer alternatives of pressure or biological oxidation. 

The aliphatic polyamines, putrescine, spermine, and spermidine, are present in green coffee beans, but they are all decomposed during the roasting process.17... 

The most volatile aliphatic compounds are largely lost at some stage of the roasting process. For example, acetaldehyde was among 12 volatile compounds evolved after an 8-min roast of coffee beans at 220°C.23 Waste... 

Although the aromatic polycyclic hydrocarbons are usually associated with tar and soot formation, in a firing or roasting process, compounds such as fluoranthrene and pyrene have also been found in green... 

The most abundant polycyclic hydrocarbon, 2,3-benzofluorene was seen at the 22 gg/Kg level in green coffee and was decreased to 12 gg/Kg in an Italian roasting process.33... 

After fermentation, cacao beans are dried and then transported to the factory for roasting. The ultimate purpose of roasting is to develop desirable bean flavor and aroma, as well as the necessary texture for later grinding. As shown in Table 4, the methylxanthine content of cacao beans was not found to change significantly during the roasting process.5-24 26... 

The sulfur trioxide reacts with lead oxide created during the roasting process (see above) to form lead sulfate... 

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