Blast Furnace Slag Granulation at the Cast House
Blast Furnace Slag Granulation at the Cast House
A blast furnace (BF) is a closed system into which iron-bearing materials (iron ore lump, sinter and/or pellets), fluxes (slag formers) and reducing agents (i.e. coke) are continuously fed from the top of the furnace shaft through a charging system. The products of reduction process which takes place inside the BF are hot metal (HM) and liquid slag. These products get accumulated in the BF hearth. The slag floats on the surface of the HM because of its lower density. The liquid products of HM and slag collected in the hearth are allowed to run out periodically from the taphole into a runner system where the HM and slag are separated by a system of weirs and dams. The HM is run off into HM ladles (either open top or torpedo) while the liquid slag is either run into large pits at the side of the furnace for air-cooling into an aggregate product, or through a slag granulation facility. Till 1970s, the BF slag was considered a waste product and was being dumped at a convenient place away from the BF.
The different processes of granulation of liquid slag at the cast house were developed during the period around early 1970s. These processes differ in the method of de-watering of the wet granulated slag. Today granulation of liquid BF slag is the well accepted technology and is being used in all the BFs around the world.
The amount of liquid slag depends very much on the charging material, for example, the grade of iron ore, the gangue content of iron ore, and the fluxes added to adjust the chemical composition of the liquid slag. Till around 1940s -1950s, slag generation in BF was around 980 kg/tHM. Presently, due to a better understanding of the slag formation mechanisms and of the overall BF process, it is now possible to control, optimize, and minimize slag generation in the BF. These days, the specific quantity of slag generated in the BF is in the range of 175 kg/tHM to 350 kg/tHM. The liquid slag is at around 1400 deg C to 1550 deg C temperature.
BF slag is a non-metallic by-product produced in the process of iron-making. It consists primarily of silicates, alumino-silicates and calcium-alumino-silicates. The liquid slag absorbs much of the sulphur (S) from the charge. The main objective of the slag granulation plant is the processing of liquid BF slag into valuable raw materials for the cement and construction industries. The parameters affecting the quality of the granulated slag include (i) chemistry of the liquid slag, (ii) temperature of the liquid slag, (iii) glass content of the granulated slag, and (iv) average distribution of the granulated slag particles. While the first two parameters relates to the BF operation, the last two parameters are related to the process of the slag granulation. The parameters of the process of the slag granulation include temperature, and pressure of the water used for granulation, as well as the flow and the granulation area available for the heat transfer.
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 BF burden and process, the applied granulation technology and its operating conditions and the storage and de-watering time etc. Fig 1 shows the complex dependency of slag technical properties.
Fig 1 Complex dependency of slag technical properties
The granulated slag has the appearance of concreting sand. It is glassy and fibrous in nature. There is no heat of crystallization in its formation and the material has a latent hydraulic property for forming solid hydration products just like cement. However, whereas cement is soluble in water facilitating the hydration process, the granulated slag is only soluble in alkaline solution. The alkaline condition can be produced by the addition of an activator or basic ‘catalyst’, such as lime. Typical properties of the granulated slag are given in Tab 1.
|Tab 1 Typical properties of granulated slag|
|2||Size||mm||Less than 10|
|12||Loss of ignition||%||Nil|
|14||Glass content||%||More than 90|
The product of the liquid slag mainly includes (i) air-cooled BF slag, and (ii) granulated BF slag.
Granulated BF slag is produced by quenching to a glassy state resulting into little or no crystallization to occurs This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinker like material. The physical structure and gradation of granulated slag depends on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production. From granulated slag, ground granulated BF slag (GGBS) can be produced. GGBS has cementitious properties and can be used as a partial replacement for or additive to Portland cement. The air-cooled BF slag is produced if the liquid slag is poured into beds and slowly cooled under ambient conditions. Air cooled slag has a crystalline structure. Air cooling produces a hard, lump slag, which is subsequently crushed and screened.
Coarse aggregate of BF slag for concrete mixing is produced by crushing air-cooled slag and then classifying through screens while fine aggregate is produced by lightly crushing granulated slag to control the grain size and then classifying. Fig 2 gives the production flow of BF slag.
Fig 2 Production flow of BF slag
Concept of slag granulation
The process of slag granulation involves pouring the liquid slag through a high pressure water spray in a granulation head, located in close proximity to the BF. Granulation process is the controlled quenching of the slag in cold water which does not give time for crystalline growth to take place. Large volume of water is needed (around 10 parts of water to 1 part of liquid slag being optimum). During this process of quenching, the liquid slag undergoes accelerated cooling under controlled water flow condition and gets converted into glassy sand with around 97 % of the solid granulated slag particles less than 3 mm and an average size of around 1 mm. The impact point of the liquid slag and the high pressure water is dependent on the slag flow, and its temperature as well as the slope and shape of the hot runner.
The heat exchange between the liquid slag and granulation water has to take place very quickly. The granulation water jets break up the slag stream into liquid slag lamellae which decompose initially into filaments and then into droplets. The best heat transfer occurs when the contact surface between the liquid slag and the water is at maximum, i.e., when slag has been converted into droplets and fully enclosed with water. The solidification time depends on the size of the slag droplets, the temperature difference between the liquid slag and the granulation water, and the contact environment between the slag and the water.
Depending on the granulation water temperature around the slag droplet, different heat transfer mechanisms take place. These are (i) heat removal only through steam release which is applicable if the granulation water temperature is equal to the water boiling temperature, (ii) heat removal through steam release and heat transfer into granulation water which is applicable to most cases, and (iii) heat removal without steam release but only through heat transfer into granulation water which is applicable if the granulation water is cold and allows an immediate condensation of the generated steam.
In general, boiling temperature is not reached when granulating with cold water, except for local spots due to transient high slag flows. Heat removal without steam release can take place if granulating with cold water and where good turbulence between the slag and the water allows optimum removal of heat. However, the most common situation is heat removal through steam release and heat transfer into granulation water. The concept of the slag granulation process is shown in Fig 3.
Fig 3 Concept of the slag granulation process
The granulation process of the liquid slag can be performed with hot or cold granulation water, allowing for two different water circuit layouts. The slag granulation plant designed for a hot water granulation circuit does not have a cooling tower. The granulation water, circulated in a closed loop, heats up close to boiling temperature. The heat removal from the liquid slag during hot water granulation is mainly through steam release. Cold make-up water is added to the system only to compensate for steam and moisture losses. The average water temperature in the circuit is around 90 deg C to 95 deg C. At the impact point, where the granulation water comes in contact with the liquid slag, water temperatures of around 95 deg C and even higher are to be expected.
In case of the granulation process of the liquid slag with the cold granulation water, the granulation process starts when the granulation water comes into contact with the liquid slag (Fig 3). The slag flow breaks up into lamellas and filaments, then into droplets. Only part of the slag is granulated on the way through the cold runner to the receiving hopper, but is likely to be completed after hitting the impact plate inside the receiving hopper and falling into the receiving hopper. With this design, only part of the water flow is directly used for the granulation process as part is used to cool the wear protection plates alongside of the cold runner front end.
The slag granulation plant designed for a closed cold water circuit is equipped with a cooling tower whose purpose is to keep the process (granulation) water at a constant cold temperature. Heat removal from the liquid slag in contact with cold granulation water takes place through heat transfer into the water and partly through steam release. Heat transfer through steam release varies depending on the granulation water temperature and the instantaneous slag flow. At low slag flows the heat transfer of the liquid slag takes place mainly through transfer into the cold water, whereas at high slag flows steam release takes place. A slag granulation plant with a cold water circuit has a higher potential for a fast heat removal compared to the slag granulation plant designed with a hot water circuit.
In the case of the cold runner design, the cold runner is installed as a continuation of the hot runner, with a built-in blowing box at the front end. The blowing box is fully embedded in the cold runner which is installed below the hot runner end spout. The cold runner serves the purpose of guiding the water-slag/sand mixture to the receiving hopper and is equipped with a wear-resistant lining as the granulated slag particles are very abrasive. The heat flux of the liquid slag needs some water spraying alongside of the cold runner at the front end.
The granulation basin located below the hot runner spout end consists of a water basin which can vary in size depending on the plant layout (Fig 3). The basin, filled with water to a defined level, permits water additional to the circuit water to be available for granulation. Thus granulation, being sustained by the turbulent water bath, takes place much faster when compared to the cold runner layout. The layout allows the design of water circuits with less water flow, but nevertheless having more water available for granulation, without compromising on safety. The basin can easily be protected against wear, which in the case of the cold runner, needs high maintenance. The basin layout has the potential to reduce the amount of push the slag into the granulation basin below the water level. The heat exchange between the slag droplets and the water is now not only given by the water jets from the blowing box, but also from the water surrounding and enclosing each droplet in the water basin. The water jets hitting the water surface inside the granulation basin contribute to creating turbulent conditions in the basin and help to promote a faster cooling effect of the slag droplets into granulated slag particles. Although this design has reduced water to slag ratio, more water volume is available for granulation, i.e., water volume in the basin and the water flow at the blowing box. The granulation process takes place faster and thus the solidification time is reduced
Slag granulation plant
The main components of a slag granulation plant consist of (i) hot runner, (ii) blowing box, (iii) granulation tank, (iv) stack and the condensation tower, (v) distributor and slow down boxes, (vi) de-watering equipment or facility, (vii) hot water tank, (viii) pumps, (ix) cooling tower, (xii) cold water tank, (xiii) buffer tank, (xiv) make-up water, (xv) conveyor belt, and (xvi) stock pile.
The slag granulation plant comprises (i) a granulation section, (ii) a de-watering section, and (iii) a storage section. The slag granulation section links the hot slag runner of the BF and the de-watering section. The schematic diagram of a slag granulation plant is shown in Fig 4.
Fig 4 Schematic diagram of a slag granulation plant
Granulation section – Here, the liquid BF slag is water quenched and solidified into small particles, eliminating the need for heavy crushing equipment. The liquid slag flows by runners from the BF to the granulation unit. From the runner, the liquid slag stream at around 1,500 deg C is poured into a high velocity water stream at the granulation spray head before ending up at around 50 deg 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 the BF tapping normally ranges upto 10 tons/minute. 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. 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 liquid 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, under normal circumstances, does not touch the bottom of the granulation chute. The spray heads generally contain detachable nozzle plates with ceramic inserts and spray headers which can be replaced quickly during short maintenance intervals.
The water quantities for the spray head are around 1,200 cum/hour for the lower bottom spray head, 600 cum/hour for upper bottom spray head, and 100 cum/hour for the side sprays. Under normal conditions, 1,800 cum/hour is used for granulation, but at the last part of the casting of the BF, when the slag volumes can increase, an additional 600 cum/hour can be added by activating additional spray heads and increasing the total flow to 2,400 cum/hour.
The requirements of the spray head are (i) simple and logical construction to reduce fabrication cost and simplify replacement, (ii) build-up of wear resistant materials, e.g., the use of ceramic insets for the nozzles and guides, (iii) easy to inspect and easy to replace, and (iv) 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 which includes an assembly of water spray nozzles. These sprays ensure that emissions are dissolved in the water. They are then partly neutralized by the CaO in the slag.
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 particles. This mixture is transferred by gravity to the de-watering section for separation of slag particles and water.
De-watering section – The granulation section adds water to the slag, which is required to be removed and recycled. De-watering is required to lower the moisture content to around 10 % to 12 %. The de-watered slag can be discharged directly into a truck or on to a material handling system for further transportation. Many de-watering systems have been used in the different type of slag granulation plants. These include gravel layered filtering bed, dehydrator, rotating de-watering wheel, de-watering drum, and static de-watering silos etc.
Storage section – The de-watered granulated slag is then conveyed to the granulated slag storage. Both silos and open storage methods are used for the storage of the granulated slag.
The slag granulation plant is normally a compact installation and suitable for applications with limited space. Granulation, de-watering and storage facilities are physically independent and can be installed in separate locations. The plant allows multiple combinations.
Process of cast house slag granulation
There are several processes for cast house slag granulation. Major slag granulation processes presently under operation are OCP (open cycle process) granulation system, Russian designed plants (Fig 5), RASA system, and INBA slag granulation plants (Fig 6) etc.
Fig 5 Russian cast house slag granulation system
The process of cast house slag granulation starts with separation of liquid slag from the HM. The liquid slag is led into the granulation chamber through a series of runners protected by lining and sand. Direct contact between pressurized water stream and liquid slag takes place in the granulation chamber. Due to rapid cooling and the impact of water pressure, the liquid slag is granulated into vitreous sand like particles which form a slurry mixture with water. This slurry mixture of water and granulated slag is transported to the de-watering section.
The simplest de-watering method is through gravel layered filtering bed. This filter bed is periodically back washed with water and air for removal of the choking by small particles of the slag. De-watering facilities of the RASA is composed of several filtering beds, which are made up of layers of different particle size pebble bed at the bottom. The most popular de-watering equipment is the rotating de-watering drum of INBA process. The granulated slag and water slurry is distributed evenly over the whole length of the drum. Axial vanes inside the drum continuously lift the granulated slag and deposit it onto a conveyor belt located above the distributor. The fine mesh on the exterior of the drum retains the granulated slag and allows the water to filter through. The granulated slag layer at the bottom of the drum creates a self filtering effect.
After de-watering the residual moisture of the granulated slag is around 10 % to 12 %. The de-watered water is cleaned, cooled and recycled. The dried slag is conveyed to the granulated slag storage for dispatch to the customers. Water vapours generated during the slag granulation are emitted through a stack normally made of stainless steel.
Fig 6 INBA process for slag granulation
Emissions during the slag granulation process
BF slag has S content of around 1 %. It can go upto 2 % depending on the S content of the coke and coal. The major S compound is calcium sulphide (CaS) and during granulation gaseous S compounds are generated and emitted. These consist mainly of hydrogen sulphide (H2S) and sulphur di-oxide (SO2) according to the simplified reaction equations (i) CaS + H2O = H2S + CaO, and (ii) CaS + 3/2 O2 = SO2 + CaO. These reactions occur mainly at temperatures above 1,100 deg C. As long as the slag droplet is liquid, CaS is sufficiently available to feed the slag/steam surface. The supply of S to the contact surface takes place through flow and diffusion. However, once the surface of the droplet hardens (skin), the transfer of S takes place only through diffusion. Since the coefficient for solid diffusion is much less than for liquids, further supply of S from the liquid to the surface is stopped. Only S contained in the skin reacts with the steam once a hard skin has been formed. As the steam is the product of H2O vapour and gaseous S compounds (H2S, SO2) in contact with the surrounding granulation water the S compounds go into solution according to the relevant partial pressures. The prevailing conditions like water temperature, pH value of water and solubility of H2S and SO2 define the amount of S compounds released through the steam and emitted to the atmosphere or bound with CaO contained in the water.
Benefits of slag granulation
The benefits of slag granulation process includes (i) it converts waste material to a valued useful product, (ii) it eliminates slag dumping and hence all the drawbacks associated with it, (iii) investment and operating costs are lower than the costs associated with the slag dumping, (iv) it is a reliable process, (v) reduces manpower when compared with slag dumping, (vi) the process can be fully automated, (vii) saves on land area needed for slag dumping, (viii) compact design of granulation plant needs only a small area, (ix) since it is installed adjacent to the cast house of the BF, it helps the BF operation because of continuous flow of data from the granulation plant.