فایل ورد کامل مروری بر کاربرد سرباره و لجن های به دست آمده از صنایع فولاد سازی

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تعداد صفحات این فایل: ۲۵ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:An overview of utilization of slag and sludge from steel industries~~en~~

Abstract

Significant quantities of sludge and slag are generated as waste material or byproduct every day from steel industries. They usually contain considerable quantities of valuable metals and materials. It is generally possible to recover some values by physical or chemical mineral processing techniques such as crushing, grinding, classification, hydrocyclone, magnetic separation, flotation, leaching or roasting. Transforming these solid wastes from one form to another to be reused either by the same production unit or by different industrial installation are very much essential not only for conserving metals and mineral resources but also for protecting the environment. This paper analyzes the characterization, beneficiation and utilization aspects of blast furnace flue dust, blast furnace sludge, LD sludge and LD slag generated at modern steel plants.

۱ Introduction

Integrated steel plants utilize mostly five materials such as raw materials, air, water, fuel and power to produce steel. During the production of steel, 2–۴ t of wastes are being generated per tonne of steel produced. The various solid wastes in the form of slags and sludges that are emerged from steel plants are blast furnace slag, blast furnace flue dust and sludge, Linz–Donawitz (LD) converter slag, LD sludge, LD dust, mill scale, mill sludge, acetylene sludge, etc. The solids and liquid wastes generated from a steel plant are shown in Table 1. The composition of these materials vary widely depending on the source of generation but usually contains some useful resources such as iron, carbon, calcium, zinc, lead, etc., which can be recovered and reused in a judicious manner. Besides that, some slag and sludge contain a notable amount of heavy metals and release of them to earth may cause some environmental problems. The harmful components like As, Cd and Hg, etc. are highly objectionable from the pollution point of view. So, the simple disposals of these wastes are becoming less attractive which not only occupies plenty of land but also increases the disposal costs. Therefore, it is desirable to recover the valuables and utilize these wastes. However, detailed scientific investigation is necessary to establish the scope of the problems associated with recovering the valuable materials from different slags and sludge and the proposed utilizations are to be carefully evaluated.

Steel plant slags mainly include blast furnace slag and steel melting slag (open hearth or LD process slag). Various efforts have been made on the utilization of blast furnace and steel slags. These are competitive raw materials for industrial mineral industry. A comparison of the qualities between air-cooled blast furnace slag and natural sand, LD slag and natural sand, LD slag and gravel or crushed stones shows that they are equivalent. The special processing of slag sometimes produces products originating from the rock and mineral industries. Therefore, lot of studies has been made for the concrete, plastering and environmental technology industries (Kolb and Leipold, 1993). Today blast furnace slag has found a wide application in cement production and marketed as Port land blast furnace slag cement (PBFS). Since blast furnace slag contains little iron and high calcium oxide content with similar composition of clinker, it can be safely used along with the clinker and gypsum for production of cement.

LD slag is a byproduct of steel industry, which comes from pig iron refining processing using LD converters. Because of its physical, chemical and mineralogical properties, it can be used as a substitute for aggregates in civil engineering projects. LD slag has the useful components like CaO, MgO with high basicities (CaO/SiO2) of above 3.0. LD slag therefore has high fluxing capacity and is being charged in the blast furnace due to easy melt and better utilization of calcium values. In the European countries, 30% of such slags are recycled into the blast furnace. However, the most harmful components in the LD slag is P&S which are to be removed before use either in sintering plant or blast furnace. LD slag, used in blast furnace at Bhilai Steel Plant, India was therefore discontinued due to sulphur and phosphorus content (Sharma et al., 1993). The slag is however not suitable in cement making due to the presence of high percentage of iron oxide. In many instances it is usually subjected to metal recovery before its application in steel and iron industries.

Most of the materials of sludge and dusts from steel industries are recycled through sinter making in more advanced countries. The recycled wastes also have some effect on sinter quality, strength and productivity. The recycling is generally controlled depending on the analysis of the waste material. In the direct use of the steel plant wastes, a briquetting plant was commissioned at National Steel Corporation, Great Lake to recycle 300,000 metric tonnes of waste materials such as BOF sludge, BF dust and sludge, LD sludge, mill scale and other materials (Landow et al., 1997). A number of technologies have been developed to allow better utilization of Fe-bearing steel plant fines in primary operations. Methods ranging from simple agglomeration techniques to new hot metal production processes for treating the dust materials are cited which are useful in pretreating the waste materials to make them more amenable to recycling in iron and steel making operations. These processes involve thermal, hydrometallurgical and physical beneficiation methods.

The process byproduct of mill scale from the rolling process containing >70% Fe is generally recycled into the sintering plant. Generally, 70–۱۰۰% mill scale containing high iron is being recycled through either briquetting or sintering route with out any difficulties. In some cases, de-oiling of the material is required. Rolling mill sludge is the fine particles, which takes the oil portion along with the rolling cooling water. Recycling of these particles are challenging due to very high oil content. The reduction of oily mill scale sludge along with blast furnace flue dust in laboratory experiments and in a pilot plant rotary kiln has indicated that it is possible to reduce oily mill scale sludge to sponge iron in the rotary kiln (Eriksson et al., 1999).

The extent of generation, characterization and utility of various sludge and slags with a review of number of methods for the recovery of valuables or recycle in the plant from such materials have been summarized.

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