Calcination second stage

Table VI, Heat Balance of Model Calciner (Second Stage)...

In the second stage, the black ash process, the sodium sulfate was reduced to sodium sulfide and then converted to sodium carbonate by calcining with limestone and coal in a rotating kiln known as a black ash furnace or revolver ... 

In the second stage the monohydrate is calcined at 650 °C to produce hematite ... 

Experimental findings show that calcination reaction starts at every interior point of the reactant and after a certain time the reactant solid near the external surface is completely exhausted forming a bidisperse inert product layer. The period of reaction prior to the formation of the ash layer is designated as the first stage and the period following the formation of the ash layer as the second stage. 

Figure 7 shows a schematic diagram of the model calciner used for studying an economic continuous two-stage calcining system. The exhaust gas from first stage kiln is fed as fuel for second stage kiln. 

Table VI summarizes a typical heat balance in second stage of the above calcining system (Case A) in comparison with that of an alternative system (Case B) in which the exhaust gas from first stage kiln is not utilized for second stage calcination. The amount, composition and temperature of the exhaust gas from first stage kiln determine the auxiliary fuel requirements for second stage kiln.

The second phase in catalyst preparation by this process involves heating the precipitate. This takes place in two stages. In the first, the water of crystallization is lost and in the second, there is a loss of carbonate and other cinions as well as the dehydration of the metal hydroxides. The first occurs on heating to about 250°C, while the second stage requires calcination usually at temperatures between 300° and 600°C with 350°-400°C reported to be optimal. At these temperatures metal carbonates and hydroxides are converted to an intimate mixture of the component oxides (Eqn. 13.1). The oxides of the active metals are generally more easily reduced than are other metal compounds.3-5... 

In this connection, the lowermost cyclone is the most important one as the material from this cyclone is partly calcined, and some of the dust that escapes to the second stage from the bottom will recarbonatize and liberate heat. 

The second stage is a calcination of impregnated and dried sample at elevated temperatures to reduce Pt(TI) cations into Pt° with ammonia ligands ... 

Binary systems synthesized consisted of Cu/Fe, Ni/Fe, Cu/Al and Ni/Al and Cu/Cr for 4-10 wt percent Cu or Ni in the calcined mixed oxide. Anionic complexing agents acetic, citric and oxalic acids and EDTA were used in molar ratios of 1 1 with the initial copper or nickel. Two stage precipitations were used starting with an initial formation of aluminum, chromium or ferric hydroxide by addition of NaOH to an aqueous solution of A1 nitrate, Cr nitrate or Fe chloride. In the second stage aqueous solutions of Cu sulfate or Ni nitrate were mixed with the initial precipitate with or without the presence of a 1 1 mole ratio of selected anionic complexing agents to complete the precipitation. A second mode of coprecipitation used was to preadsorb oxalic acid on the initially precipitated AI, Cr or Fe hydroxide. 

It has been demonstrated that sequential precipitation in a moderately acid pH range for the binary metal systems Cu/Al, Cu/Cr, Cu/Fe and Nl/Fe in the presence of oxalic acid with aluminum or ferric hydroxide as the first stage and adsorption/precipitation of copper or nickel as the second stage provides metal oxides (after 250-350° C air calcine) with considerable enhancement in dispersion and in catalytic activity, notable for Cu/Al, for the room temperature decomposition of hydrogen peroxide and benzaldehyde oxidation by hydrogen peroxide. 

The main stages of coal combustion have different characteristic times in fluidized beds than in pulverized coal combustion. Approximate times are a few seconds for coal devolatilization, a few minutes for char burnout, several minutes for the calcination of limestone, and a few hours for the reaction of the calcined limestone with SO2. Hence, the carbon content of the bed is very low (up to 1% by weight) and the bed is 90% CaO in various stages of reaction to CaSO. About 10% of the bed s weight is made up of coal ash (91). This distribution of 90/10 limestone/coal ash is not a fixed ratio and is dependent on the ash content of the coal and its sulfur content. 

In that reduced cost was an objective, the possibilities of using glbbsite in similar experiments were examined [45]. Gibbslte was reacted with sulphuric acid solutions to produce aluininlijuii sulphate. This material was dried and calcined in two stages. In the first, the water of hydration was removed. In the second, aluminium sulphate was decomposed to form alumina. The alumina so produced was then heat treated In order to study the theriaal stability of the product. 

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