Blast furnace accretions

The success of the process results from the fact that nowhere inside the furnace is heat extracted from the copper-saturated blast furnace buUion through a soUd surface. The problem of accretion formation (metal buUd-up), which has plagued many other attempts to estabUsh a copper dtossing operation of this type, does not arise. In the cooling launder, lead-rich matte and slag accumulate on the water-cooled plates, but these ate designed so that when they ate lifted from the buUion stream, the dross cracks off and is swept into the furnace via the cooled lead pot. 

As illustrated in Figure 5.2 accretions are a key feature of the lead blast furnace and must be adequately managed to maintain operation. Accretion formation is complex and as indicated above, results from reactions of volatile components such as zinc and lead sulfide as weU as fusion of the charge and solidification in cooler regions of the furnace. Analysis has been reported by Oldwright and Miller (1936), Ruddle (1957) and Polyvyannyi et al (1971). 

The lead blast furnace consists of a rectangular shaft with sidewalls made up from a series of water-cooled hollow steel jackets. Refiactory lining is unnecessary since an accretion layer forms on the inner surface to protect the steel from attack. 

It is usual to operate the blast furnace on a campaign basis, stockpiling feed nntil sufficient has been accumulated for a reasonable campaign mn of from one to fonr weeks. It is also possible to re-run blast furnace slags under different conditions, particularly if it is desired to concentrate antimony or other alloying elements into a small quantity of final bullion. This mode of operation provides considerable flexibility and allows the furnace to be cleaned between mns, rninimising operational difficulties due to accretion build-up and maximising the efficiency of the furnace (Pike, 1990). 

The furnace generally operated only for a few days and was then cleared of accretions and the walls were replastered with lute , a paste of clay and fine charcoal. The skill of the furnace operator was most important in regulating the air blast from the bellows and in the placement of the ore charge towards the front of the shaft so as to avoid the formation of a sintered mass or sow . Natural fluxing materials such as fluorspar were often also added to the charge depending on the nature of the ore. 

Most furnaces have a top open to the atmosphere with a central off-gas draft hood. The hood is under suction and considerable dilution air is sucked in down through the top of the furnace charge. The off-gas volume is consequently three to four times the blast air volume, depending on the hood suction applied and in some cases can be significantly higher. The purpose of the open top is to provide access for the regular cleaning of accretions from the furnace walls. It also facilitates the... 

One factor contributing to the formation of the central acaetion or deadman is the lack of adequate penetration of the blast. This can be corrected by increasing blast velocity through reduction in the diameter of the tuyeres and also by reducing the width of the furnace at the tuyere level. The Naoshima Smelter in Japan reported an effective removal of the central accretion by these measures, including reduction of furnace width from 1.66 to 1.42 m, and significantly increased furnace capacity (Moriya, 1989). 

Although the above calculations are simplistic they can be used to clearly illustrate the impact of key process parameters on furnace performance, such as the CO C02 ratio in exit gas, the Pb content of sinter, the blast rate and the oxygen content of blast air. There are other practical limits on these parameters to maintain practical operating temperatures at various regions in the furnace shaft, and to minimise the formation of accretions and other obstructions. 

The formation of accretions in the shaft was a particularly serious problem with the early design, aggravated by water leaks from the jackets forming the lower shaft. This problem became more significant as attempts were made to increase the blast intensity and productivity of the furnace. It was... 

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