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Blast furnace slag granulation plant technologyA modern blast furnace can produce more than 1Mt/yr of slag. Slag granulation using water sprays is now an established technology. It not only simplifies blast furnace casting floor operations, but also provides an economic route for the production of a valuable raw material for a host of industries.

The blast furnace produces large amounts of liquid slag, which can be processed into valuable raw materials for

the cement and other industries. For example, in cement production, granulated slag can replace up to 70% of the more costly Portland clinker. The slag volume depends on the quality of the raw materials used in the BF and may range from 200 to 500kg/THM. Typical production levels of a modern blast furnace are 8,000-12,000THM/day so the annual slag production could amount to more than 1Mt.

Efficient processing of blast furnace slag into a product suitable for further use is accomplished by a slag granulation plant. This paper summarises modern slag granulation plant technologies, which have been successfully demonstrated at industrial plants for more than 20 years.

SLAG GRANULATION PLANTThe main objective of the slag granulation plant is processing liquid blast furnace slag into valuable raw materials for the cement and construction industries. The value of the slag depends on its mineralogical, chemical, physical and mechanical properties, such as basicity, glass content, structure and moisture content. These technical properties are related to the blast furnace burden and process, the applied granulation technology and operating conditions, storage and dewatering time, etc, and Figure 1 illustrates its complexity.

Authors: R van Laar, E Dupon, J Barel and M KamerlingDanieli Corus BV

r Fig 1 Complex dependency of slag technical properties

r Fig 2 Schematic of a slag granulation plant

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granulation, but at the last part of the BF cast, when the slag volumes might increase, an additional 900m3/hr can be added by activating additional spray heads and increasing the total flow to 2,400m3/hr.

The requirements of the spray head are:` Simple and logical construction to reduce fabrication

cost and simplify replacement` Buildup of wear resistant materials, eg, the use of

ceramic insets for the nozzles and guides` Easy to inspect and easy to replace` Easy access and easy to clean.

During this quenching process, water is evaporated and SOx compounds are released. These emissions can be eliminated by the application of a condensing tower that includes an assembly of water spray nozzles. These sprays ensure that emissions are dissolved in the water. They are then partly neutralised by the CaO in the slag. Views of the condensing tower system are shown in Figure 4.

The entire granulation tank is lined with wear resistant refractory as it is exposed to a highly turbulent and erosive mixture of water and slag sand. This mixture is transferred

The slag granulation plant comprises a granulation section, a dewatering section and a storage section, as illustrated in Figure 2.

Granulation section Here, the liquid blast furnace slag is water quenched and solidified into small pieces, eliminating the need for heavy crushing equipment. The liquid slag flows by runners from the blast furnace to the granulation unit. From the runner, the liquid slag stream at around 1,500°C is poured into a high velocity water stream at the granulation spray head before ending up at around 50°C in the granulation tank. This rapid solidification followed by breaking up of the material into small pieces is controlled by the excess of water used. Due to the high temperature of the liquid slag, the water is partly evaporated and subsequently condensed in a condensing tower located above the granulation tank.

The production of slag during a blast furnace tapping normally ranges up to 10t/min. To cope with this variation, the water stream to the granulation unit can be controlled by an energy balance calculation to ensure efficient and economic performance. The control element is a valve, regulating part of the water flow to the granulation spray head.

The granulation spray head is the technological heart of the granulator and is where the water and slag are mixed intensively, ensuring fast and efficient granulation (see Figure 3). The spray head is designed to produce a specific flow pattern of water for optimum mixing and is located to direct high-pressure water jets into the free falling molten slag stream.

The main volume of water is directed by the jets to form a fast moving water trough in the bottom of the granulation chute. The purpose of this water layer, in addition to granulating the slag, is to protect the granulation runner and to carry away the granulated mix. Additionally, a number of jets are positioned on the sides of the slag stream to ensure all slag is granulated into small pieces as quickly as possible, to supply additional cooling, and also to propel the slag/water mix in the chute. The configuration of the water jets is such that the liquid slag will, under normal circumstances, not touch the bottom of the granulation chute.

The spray heads contain detachable nozzle plates with ceramic inserts and spray headers that can be replaced quickly during short maintenance intervals.

The water quantities for the spray head are approximately as follows:` Lower bottom spray head 1,200m3/hr` Upper bottom spray head 600m3/hr` Side sprays 100m3/hr

Under normal circumstances, 1,800m3/hr is used for

r Fig 3 The granulator spray head and transport chute

r Fig 4 Condensing tower and spray nozzles

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by gravity to the dewatering section for separation of slag sand and water.

As an alternative to wet slag granulation and subsequent dewatering, dry slag granulation technology has been pursued for many years but has not yet been demonstrated to be economic on an industrial scale as these economics depend strongly on the heat recovery efficiency. It is acknowledged that heat recovery is difficult to justify economically with water granulation systems because of the intermittent nature of the process and the relative low temperature of the water after dewatering.

Dewatering section The granulation section adds water to the slag, which must be removed and recycled. Dewatering is required to lower the moisture content to ~15%. Additional dewatering to 12% can be achieved, if required, by stockpiling the granulated sand on a natural draining stockpile and letting it drain for another 24 hours prior to further transportation. Alternatively, silo dewatering down to 12% can be achieved in 12 hours or less. In that case, the dewatered slag can be discharged directly into a truck or on to a material handling system for further transportation.

Many dewatering systems have been used in the ironmaking industry. This paper will describe two of them: rotating dewatering wheel and static dewatering silos.

Two slag granulation plant arrangements are illustrated in Figure 5. Both systems are identical with regard to the granulation section and recycling of granulation water.

Rotating dewatering wheels These wheels have been operating for more than 30 years in our industry. The main advantage of this system relates to the fact that it is compact and requires a lower investment compared to static dewatering silos. The rotating dewatering wheel is installed in a hopper, which is filled with the mixture of slag sand and

r Fig 5 Slag granulation plant and dewatering system arrangements

r Fig 7 Dewatering silos

r Fig 6 Dewatering wheels

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the produced slag is possible. High temperature resistant rubber is recommended for the belt material, as occasional high slag temperatures can occur.

Normally, the granulated slag can be stored in a vertical sand hopper with a clamshell lock-hopper discharge device. However, in mild climates without major frost problems, it can also be stored in simple stockpiles and reclaimed by front loaders or shovelled into trucks.

The size of the stockpiles could be made sufficiently large, such that reclaiming can be done in day shifts only.

Some measures must be taken at the conveyor discharge end, such as by rubber flaps, to prevent the discharged slag sand blowing away via wind or turbulence. The advantage of storing the granulated slag on the ground is that it naturally dewaters the slag pile over time. Since the slag has very strong chemically binding properties, there are hardly any contaminants in the drained water.

CONCLUSIONSEfficient blast furnace slag granulation contributes to low cost hot metal.

Advantages of these systems can be summarised as:` Simple plant configuration with few moving parts` No heavy crushing equipment required` Specially designed spray head produces uniform grain

size (up to 3mm)` Adjustable spray head produces slag sand of >98%

glass content` Condensation tower eliminates H2S and SO2 emissions` Efficient cooling tower eliminates sand deposits and

clogging` High availability (>98%) and low maintenance due to

the above. MS

granulation water. The slag sand is removed and dewatered by ‘buckets’ containing screens, then the dewatered slag sand is discharged to a conveyor belt (see Figure 6).

Rotating dewatering wheels are compact and efficient, but this design results in minor amounts of slag carry-over into the recycling stream, which could cause erosion of the recycling water, cooling tower plant and piping system unless these are protected by a lining system. Also this carry-over could settle in stagnant areas of the recycling water and cooling tower plant sections and piping system, requiring regular maintenance to mitigate this contamination. Finally, it is generally acknowledged that any rotating equipment requires more maintenance compared to static equipment, hence static dewatering silos introduce important advantages of minimum maintenance and high reliability.

Static dewatering silos These were developed more than 30 years ago and have been successfully operated at many European and Asian plants. The higher investment cost and footprint area are balanced by higher availability and lower operation costs. Examples are shown in Figure 7.

Three silos are required. As one is filled with the mixture of water and slag sand, the second is being drained, and the third is discharged by a vibrating screen to trucks or a conveyor belt. The silos also act as a temporary storage unit. The water/slag mixture is charged to one of the silos by a ‘revolving’ charging unit at the top of the silos (see Figure 8). A fourth discharge line to enable bypassing of the silos when required, is also included.

The silos include a vertical conical assembly of metallic screens. These are flushed each time the silo has been drained and ensure that the screens remain clean and efficient. The lifetime of the screens and dewatering plant is more than 20 years.

The recycling water temperature is reduced to less than 50°C by a series of cooling towers whose design is optimised to eliminate settlement of any fines. This lower temperature also minimises sulphur emissions.

This proven granulation process ensures economic granulation and produces valuable and consistent properties of the slag, as characterised by:` Glass content of the slag sand >98%` Water content at discharge <12%` Size of slag sand: 98% <3mm` Plant availability >98%` H2S emission <5mg/Nm3³` SO2 emission <50mg/Nm3³

Storage section The dewatered granulated material can be transported by conveyor belt using conventional bulk handling technology and can be equipped with a belt weighing system, such that a continuous measurement of

r Fig 8 Dewatering silo charging unit