Yang Fluidization, Solids Handling, and Processing

516 Fluidization, Solids Handling, and Processing

Of prime importance is the initial distribution of solids at the top of the apparatus. Figure 14 shows the bullet-head solids distributor designed for this purpose. Solids fed from a nearly point source falls on a bullet-shaped target from which they bounce off to land at some distance below, on a fallbreaker baffle which either straightens the particles into essentially vertical paths or simply redistributes them.

Figure 15 illustrates the derivation of a criterion for assessing the lateral distribution of solids by baffles. Solid particles fed at a point source are required to be distributed as uniformly as possible throughout the circular area of radius R. The amount of solids fed is designated M, which, when uniformly distributed will give an average population density of

w¯ = M/πR2

For any circular band of width dr and located at distance r from the center O, the actual density would be w, which differs from the average density w¯. Thus the difference in the amount of solids collected in this band as compared to the average density w¯ is

dm = 2π r dr |w - w¯ |

The overall difference from uniform distribution for the entire circle R is, therefore, the integrated value of dm

M = òdm = 2π òOR w − w r dr

When normalized against the total solids added M, this gives a nonuniformity index defined as

4.4Pilot Plant Demonstration

A few examples will be given on the countercurrent dilute-phase G/S heat exchanger.

Ore Preheating. Figure 17 (Kwauk and Tai, 1964) shows the inside contour of a brick-lined 15-tpd pilot plant sulfatizing roaster for cupriferous iron ore. It consists of an upper section, i.d. 850 mm, heated by combustion of producer gas and provided with baffles, in which the cupriferous ore, crushed to 0–2 mm, was heated in dilute-phase through a fall of 12 meters, to the reaction temperature of 500–550oC. The preheated ore was fed pneumatically via an angle-of-repose valve into a lower section where it was sulfatized in a dense-phase fluid bed, i.d. 500 mm, with a gas containing 6–7% SO2 produced by roasting pyrite concentrate in a separate auxiliary roaster at higher temperatures. About 60% of the copper and cobalt in the ore was rendered soluble in 0.3% sulfuric acid, while sulfatization of iron could be held below 1%. By ore preheating, the pyrite consumption was as low as 67 kg/t of the cupriferous ore, while for autogenous roasting by admixture with the cold cupriferous ore, it would have been as high as 330 kg/t.

An alternative process for winning copper from cupriferous iron ore was segregation roasting, in which the hot ore was mixed with small amounts of NaCl and powdered coal, in order to transport the copper content of the ore via the gaseous copper chlorides which were, thereby, reduced to metallic copper on the surface of the coal particles, followed by ore dressing for concentrating the metal thus formed. Figure 18 shows a segregation pilot plant, in which cupriferous iron ore was preheated in a dense fluid bed by direct injection of powdered coal, and the sensible heat of the hot flue gas was recovered by the incoming ore falling in dilute phase. Ore was introduced at the top of the roaster by a rotary feeder to a number of radially positioned bullet-head distributors located above two tiers of fallbreaker baffles. With a final ore preheat temperature of 850oC, it was possible to keep the coal consumption to about 65 kg/t of ore, at a roaster top temperature of around 250oC for the exit gas.

Semi-Conveying Magnetizing Roasting. Figure 19 shows diagrammatically the “two-phase magnetizing roaster” for low-grade iron ores— dilute-phase ore preheating in an upper section with an i.d. of 1,050 mm and dense-phase reduction with producer gas in a lower section of i.d. 825 mm. The roasting consisted of a mild reduction to convert the iron values to

(H2 + CO). The major portion of the fines present in the ore feed was removed by pneumatic classification in a zigzag tube at the top of the vertical pneumatic transport feed pipe, whereby the remaining coarse portion was fed, as usual, by gravity at the top of the dilute-phase preheating section while the fines were carried by the same transport air downwardly again to the combustion zone, where the high temperature reducing flame converted these minute particles almost instantly to the required magnetite state. The upflowing dilute emulsion of fines travelled to the top of the roaster, delivering its heat to the falling particles of larger diameters, and the fines, now already reduced, were finally collected in an external cyclone. Thus, part of the ore, that is, the fines, was reduced during upward conveying. Inasmuch as the upflowing heat capacity was augmented by the presence of these fines, the amount of excess combustion air could be reduced, thus increasing the roaster capacity.

Heat Recovery from Both Hot Calcine and Hot Flue Gas. Figure 20 shows a roaster with two sections for dilute-phase heat recovery from both the hot calcine at the bottom and the hot flue gas at the top. With this design, low-grade cinnabar ore containing as little as 0.06% Hg and crushed to particle sizes as big a 0–12 mm, has been successfully roasted at around 800oC for mercury extraction with a coal ratio of 90 kg/t of ore. This design has recently been tested on a pilot plant scale to roast low-grade pyrite, containing 13% sulfur on the average, and therefore, hardly autogenous, to produce a gas containing over 10% SO2 for use in sulfuric acid manufacture.

5.0FAST FLUIDIZATION

One disadvantage of dilute raining particles is its very diluteness, signifying low volumetric utilization of equipment. Particle population may be concentrated by recycling solids to the bottom of a fluidized system. Such system can operate at relatively high gas velocities, and is, therefore, known as “fast fluidization” (Reh, 1970, 1971, 1972, 1985; Squires, 1975a, 1975b, 1975c, 1985; Kwauk, 1994). Solids suited for fast fluidization aggregate into strands (also called clusters, swarms, etc.) which form and disband in rapid succession, thus ensuring good gas/solid contact. The design of fast fluid bed reactors calls for a physical model totally different from that for bubbling fluidization.