The Roasting Process

The ZnS will form zinc oxide, ZnO, whereas practically all of the iron is combined with zinc to give zinc ferrite or franklinite (ZnFe O, cubic). Hence, the iron reacts with contributes up to about 2 to 15 wt.% of the zinc content (ca. 0.6 tonne of iron per tonne of zinc). 

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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. 

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 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). 

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... 

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... 

Cooked glandless cottonseed protein quality was superior to that of raw and roasted cottonseed. The roasting process adversely affected protein quality. Supplementation of roasted cottonseed with 0, 0.2, 0.4, 0.6 and 0.8% L-lysine indicated a peak PER response at 0.45%. The adverse effect of roasting on the protein quality of cottonseed was overcome by addition of L-lysine which made the protein... 

However, coffee beverages are complex mixtures of several hundred chemicals that either occur naturally or are later induced in coffee by the roasting process in the form of nicotinic acid or melanoidins. 

Niacin is present in foods mainly as coenzyme NAD and NADP, which are hydrolyzed in the intestine, and it is adsorbed as nicotinamide or nicotinic acid. The free forms, nicotinamide and nicotinic acid, only allowed to be added in fortified foods [403], occur naturally in limited amounts. Instead, niacin occurs as nicotynil ester bonded to polysaccharides, peptides, and glycopeptides. In general, niacin is widespread in foodstuffs (cereals, seeds, meat, and fish). High concentrations are present in roasted coffee beans as a primarily product of the roasting process [417]. 

The use of zinc blende and mixed zinc ores as a convenient source of sulphur dioxide for sulphuric acid manufacture has been advocated. The residue from the roasting is often more valuable than in the case of pyrites, but the roasting process is more troublesome and the presence of fluorspar in the blende exerts a deteriorating influence on the lead chambers.2... 

When heated to 215° C., realgar decreases in weight owing to oxidation to arsenious oxide and sulphur dioxide 12 the reaction becomes more rapid with rise in temperature, and the sulphide finally inflames and bums with a bluish flame.13 The heat of the reaction representing the roasting process has been given as14... 

Reduction by carbon. Naturally occurring oxide ores or those produced by the roasting process are frequently reduced by carbon at elevated temperatures. Depending largely on the temperature employed, an... 

The mercury vapor is condensed to liquid mercury, and the sulfur dioxide escapes as a gas. Mercuric oxide is not formed since it is unstable at the temperatures employed in the roasting process. 

Coffee represents one of the major beverages consumed in the world. Like cocoa the coffee bean must under—go a fermentation step before the roasting process can develop the fine coffee aroma so cherished by man. Early studies supported the Maillard pathway as the significant producer of volatiles. 

In the theory of heat transfer, H2 is known as the Fourier number, Fo. Now, the roasting process is presented in a two-dimensional framework ... 

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