Sintering machine feeding

Zinc concentrate sinter mix Micropelletized sinter machine feed... 

A typical up-draft sinter machine (Fig. 2) has an endless belt of malleable iron pallets with grate bottoms upon which the charge is evenly spread. Beneath the pallets, wind boxes produce an up-draft of air through the charge. At the feed end, an ignition box starts the roasting. The combustion products, mostly SO2 and SO, are collected, usually for sulfuric acid production (see Sulfuric acid and sulfur trioxide). 

The ISP evolved to fill a very special niche in nonferrous metallurgy because of its capability of treating Lead-zinc concentrates which may also contain appredable amounts of copper. The concentrate is normally-oxidized in a sintering machine to produce a feed for the blast furnace where the zinc oxide is reduced with coke. Some effort has been underway to develop a hot briquetting operation to produce a suitable feed without sintering. Other efforts to improve the economic competitiveness of the process include air preheat and the use of an oxygen-enriched blast to reduce coke consumption. 

Fig. 9.26 shows schematically the operating principle of such a travelling grate sintering machine. First, recirculated, fired (= sintered) fines from the sinter screens are deposited on the grate as a hearth layer. Then, feed, consisting of a blend of fine ore... 

Solid particles that are entrained in the combustion air, settle in bins, which are part of the main suction duct and fine dust is removed in a dust collector. These solids are transported to the burden preparation plant and ultimately fed back to the sintering machine. Fig. 6.8-13 already includes two improvements. First, oversized pieces from the sinter crusher are recrushed in closed loop with a screen (not shown) and all screen fines are recirculated to provide a hearth layer, which protects the grate from excessive temperatures. Second, to achieve uniform bed depth across the grate, feed is placed onto the insulating hearth layer with a swinging conveyor and leveled with a roll feeder. 

These studies, associated with pilot-plant trials, indicated that Sirosmelt technology, because of its flexibility, versatility and simplicity, was the most suitable process to replace the conventional sinter machine-blast furnace. The process has been optimized since the start-up of the furnace based on increased industrial experience and pilot plant trials at the Metaleurop Research Centre. Improved automation and expert systems are necessary to stabilize the process because of the short residence times involved. The bath smelting process operates with a large range of primary and secondary feed materials, and carries out different metallurgical operations in the same reactor.The exhaust gases produced are processed in a sulfuric acid plant, and final SO2 conversion efficiencies of more than 99.5 % are achieved. All the emissions have been drastically reduced CO2 emissions have decreased by 60 %. 

Reduced levels of return sinter will cause a decrease in bed permeabihty, reducing possible gas flow and hence, sinter machine capacity. On the other hand gas permeability is enhanced by increased levels of return sinter, but fuel value of the charge is reduced and peak bed temperature is lowered. This leads to poorer quality sinter with increased fines, which report to returns and reduce sinter bed permeability, thus reducing gas flow and machine capacity. The product sinter in this case is also weak and readily breaks up in the shaft of the blast furnace, reducing permeability and blast furnace throughput. Hence, there is an optimum level of recycle sinter and of fuel to maximise sinter machine capacity. This normally corresponds to a sulfide sulfur content of sinter feed of six to seven per cent and a minimum recycle ratio of 1 1 (recycle net sinter production), or 50 per cent of total sinter production. 

Measurement of the moisture content of sinter feed with feedback to direct water additions at various mixing points is thus an essential part of good sinter machine control. Various devices are used for online moisture measurement, including infrared devices and electrical conductivity probes. The latter are most common and can simply be two trailing electrodes in contact with the feed material on a conveyor belt. These devices are not highly reliable and need careful and regular calibration since they are affected by the presence of soluble salts, which can change the electrical conductivity of the free interstitial water in the sinter bed. 

In general the combustion of most of the sulfur tends to take place towards the feed end of the sinter machine, and the concentration of SO2 in gas is much lower towards the tip end of the machine. A typical gas concentration profile is shown in Figure 4.5 for a machine with ten windboxes. Average SO2 levels as shown are about half the maximum and need to be above five per cent for sulfuric add production by the standard contact plant technology. 

Coarse product sinter from the primary screen is sent directly to surge bins from where it is proportioned and mixed with lump coke to provide feed to the blast furnace. Provision is made to crush and return product sinter to make up the required recycle ratio if necessary. Under steady state operation of the sinter machine, control of the amount of product sinter returned is normally through the level of material in the returns sinter surge bin, which is drawn off at a steady rate to satisfy feed blend requirements. 

Sinter returns. This is usually the minus in. (0.6 cm) or minus f in. (1 cm) material separated from the sintered product and recycled to the feed end of the machine. An optimum value of sinter returns rate exists to maximize bed permeability. In current practice 20% to 30% of the machine discharge is recycled. 

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