Roasting Operations

Jn segregation roasting the objective is to convert nonferrous metals, e.g., lateritic nickel ores, to volatile metal chlorides, which are then reduced on the surface of solid reductants (such as coke) that are added to the material charged to the roaster [15, 16]. The overall reaction scheme is 

The chlorine necessary for the formation of the hydrogen chloride intermediate may be obtained in a number of ways, for example, by the addition of a few weight percent of calcium chloride to the charge 

After roasting, the metal-covered coke particles may be removed by magnetic separation, flotation, or other mineral-processing techniques. Some further references may be found in the article by Iwasaki et al. [17]. 

Another recent and very topical example of complex solid-solid reactions proceeding through gaseous intermediates is the lime-concentrate pellet roasting process developed by Bartlett and Huang [18]. 

In this process, which has been proposed on the basis of laboratory scale studies, copper sulfide minerals, chalcocite, chalcopyrite, etc., are mixed with burned lime, pelletized, and then roasted at a temperature of about 500°C. The principal objective of the operation is to transform the copper 

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The chemical-grade ore, containing about 30% chromium, is dried, cmshed, and ground in ball mills until at least 90% of its particles are less than 75 ]lni. It is then mixed with an excess of soda ash and, optionally, with lime and leached residue from a previous roasting operation. In American and European practice, a variety of kiln mixes have been used. Some older mixes contain up to 57 parts of lime per 100 parts of ore. However, in the 1990s manufacturers use no more than 10 parts of lime per 100 parts of the ore, and some use no lime at all (77). The roasting may be performed in one, two, or three stages, and there maybe as much as three parts of leached residue per part of ore. These adaptations are responses to the variations in kiln roast and the capabihties of the furnaces used. 

Sodium chromate can be converted to the dichromate by a continuous process treating with sulfuric acid, carbon dioxide, or a combination of these two (Fig. 2). Evaporation of the sodium dichromate Hquor causes the precipitation of sodium sulfate and/or sodium bicarbonate, and these compounds are removed before the final sodium dichromate crystallization. The recovered sodium sulfate may be used for other purposes, and the sodium bicarbonate can replace some of the soda ash used for the roasting operation (76). The dichromate mother Hquor may be returned to the evaporators, used to adjust the pH of the leach, or marketed, usually as 69% sodium dichromate solution. 

When a potential is appHed across the ceU, the sodum and other cations are transported across the membrane to the catholyte compartment. Sodium hydroxide is formed in the catholyte compartment, because of the rise in pH caused by the reduction of water. Any polyvalent cations are precipitated and removed. The purified NaOH may be combined with the sodium bicarbonate from the sodium dichromate process to produce soda ash for the roasting operation. In the anolyte compartment, the pH falls because of the oxidation of water. The increase in acidity results in the formation of chromic acid. When an appropriate concentration of the acid is obtained, the Hquid from the anolyte is sent to the crystallizer, the crystals are removed, and the mother Hquor is recycled to the anolyte compartment of the ceU. The electrolysis is not allowed to completely convert sodium dichromate to chromic acid (76). Patents have been granted for more electrolytic membrane processes for chromic acid and dichromates manufacture (86). 

In the blast furnace reduction slag-making materials are also added together with a small amount of iron, the function of which is to reduce any sulphide which remains, to the product of the roasting operation to produce a sinter. The sinter is then reduced with coke in a vertical shaft blast furnace in which air is blown tluough tuyeres at the bottom of tire shaft. The temperature in the heartlr where metal is produced must be controlled to avoid the vaporization of any zinc oxide in the sinter. The products of tlris process are normally quite complex, and can be separated into four phases. Typical compositions of these are shown in Table 13.1. 

Walker, S.W. Jamieson, H.E., Lanzirotti, A., Andrade, C.F. 2005. Determining arsenic speciation in iron oxides derived from a gold-roasting operation Application of... 

Flow Measurements Measurement of flow rates of clean gases presents no problem. Flow measurement of gas streams containing solids is almost always avoided. The flow of solids is usually controlled but not measured except solids flows added to or taken from the system. Solids flows in the system are usually adjusted on an inferential basis (temperature, pressure level, catalyst activity, gas analysis, heat balance, etc.). In many roasting operations, the color of the calcine discharge material indicates whether the solids feed rate is too high or too low. 

Chemical modification of simple sugars during drying, baking, or roasting operations can either have a desirable or undesirable effect upon the organoleptic quality of the final product. We have become accustomed to the characteristic roasted or baked flavors of coffee, peanuts, popcorn, and freshly-baked bread. The color and flavor and aroma of caramel make it a useful additive for the food industry. On the other hand, the burnt flavor of overheated dry beans or soy milk reduces marketability of these products. 

The presence of carbonaceous material in the ore precipitates the gold in cyanide solutions, thus resulting in an increase of gold content in the cyanidation residue. For such complex gold-bearing ores, a roasting operation oxidizes the carbonaceous matter and thereby makes the ore more amenable to cyanidation. However, the alteration of the associated minerals presents new dissolution problems which must be solved. 

Many modern roasting operations now collect and use the sulfur dioxide present in roaster exit gases to produce sulfuric acid. Fluidized bed roasters are favored since they can give 15% sulfur dioxide concentrations, compared to the 3-6% range expected from earlier designs. 

Roasting operations which disengage deleterious or disagreeable fumes should be carried on in the open air or under a hood, and when the volatile matters are valuable they may be condensed as directed in DisHllation and Sublimation,... 

As we have seen earlier, sulfur dioxide causes severe problems if released into the atmosphere, and modern roasting operations collect this gas and use it in the manufacture of sulfuric acid. 

Zinc concentrates contain various levels of selenium. Part of this selenium is released in the off-gas from the roasting operation. Following treatment of the roaster off-gas at an acid plant, the ultimate fate of the selenium is the weak acid effluent from the acid plant. 

Dust-laden gases from a roasting plant may range from 5.5% to 12% SO2 after cleaning, depending on the type of roaster and the process employed. The rate of S02-conversion may be as much as 98% or more. The installations capacities differ according to the scale of the roasting operation, but installations up to 1,000 tpd are in use. 

This approach significantly improved the prodnctivity of both the roasting operation and the blast furnace, and lead smelters in the early 1900s had large numbers of converters producing blast furnace feed. 

The tonnages involved are illustrated in Tables 8.1 and 8.2, which indicate the annual production of copper, lead, zinc, and pyrites in the world and in the United States, respectively. The actual course of the roasting operations is illustrated by the following example using lead sulfide ... 

Depending on the metal compound, a reducing agent is used to reduce the metal oxide produced as a result of the roasting operation to produce the desired metal. Most metal oxides are reduced with carbon (coke). The oxide is reduced to metal with the resulting emission of carbon monoxide... 

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