AU739439B3 - Granulation of blast furnace slag - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A PETTY PATENT Name of Applicant: Address for Service: Invention Title: Australian Steel Mill Services Pty CULLEN CO., Patent Trade Mark Attorneys, 239 George Street, Brisbane, QId. 4000, Australia.

Granulation of Blast Furnace Slag Details of Associated Provisional Applications: PR2197 18.12.00 The following statement is a full description of this invention including the best method of performing it known to us: GRANULATION OF BLAST FURNACE SLAG FIELD OF THE INVENTION This invention is directed to a method and apparatus for granulating blast furnace slag which allows the granulation to be carried out at a site remote from the blast furnace, and where improvements can be made to the granulated product. The granulated product has improved characteristics which allows it to be used in value added applications such as a coarse sand replacement in concrete as well as its traditional use as a supplementary cementitous material in cement blends. The method and apparatus can reduce noxious gas emissions (such as hydrogen sulphide), and can reduce equipment wear.

BACKGROUND ART Wet granulation of slag, and of blast furnace slag is well-known. The granulation process typically involves rapid quenching of the molten slag through a water stream to cause the slag to granulate. The granulated product is dewatered and stockpiled. In a known process, molten slag is passed along a hot runner to a granulation head where the molten slag is quenched by a high-volume water stream. The quenched product passes into a granulation water tank and then into a dewatering drum. The dewatered granulated product travels along a belt conveyor to a stockpile. Water is recycled from the dewatering drum back to the granulation head.

This plant sits adjacent the blast furnace slag runners and receives molten slag direct from these runners. That is, the plant is a typical on-line granulator.

An advantage of the on-line granulator plant is that the slag handling requirement is minimised. However, a distinct disadvantage is that the plant must be constructed to handle the maximum slag flow from the furnace, must be constructed to suit any limitation of the furnace layout, and must be operated and maintained to suit the furnace operating and availability criteria.

For instance, a blast furnace has a variable slag flow along the slag runners. The slag flows may vary from a few tonnes per minute to many tonnes per minute. In order to attempt to obtain a relatively uniform granulated product, the quenching stage (where the molten slag is treated with high-volume water) must be able to cope with the various flow rates.

This requires large expenditure on equipment to ensure that water volumes and pressures can be quickly adjusted to accommodate the variable flow rates. This also results in the creation of large volumes of granulation water.

Excess water increases power consumption and maintenance costs especially for the dewatering and the recycle streams.

The properties of the granulated product can vary depending on the processing conditions. For instance, the density and the moisture content of the granulated product will vary especially if the process conditions cannot be kept constant. It is extremely difficult to keep the process conditions constant when the slag flows are variable. This results in a lower quality granulated product.

The furnace layout affects the granulation plant as the granulation plant must be built around the furnace components. This can make it difficult or expensive to build a granulation plant which is able to cope with the variable slag flow rates.

From time to time it may be necessary to shut down the granulation plant for maintenance and repair. However this is not possible during slag runs without aborting the slag cast. Thus, the granulation plant must be operated and maintained to suit the furnace operating and availability criteria.

Another difficulty with on-line granulating plants is in the control of noxious emissions, these being typically hydrogen sulphide and sulphur dioxide gas. These gases are formed at higher concentrations if the molten slag cannot be efficiently quenched, which can occur during peak flow conditions. To date, these gases are exhausted via a chimney stack with steam from the quenching process.

Occasionally, the molten slag is unsuitable for granulation and instead is directed to pits to produce a rock slag. Typically, if the iron content in the molten slag is too high, the slag is unsuitable for granulation and instead is directed to slag pits to produce rock slag. The rock slag is crushed and screened in a crushing and screening plant and magnetic separators are used to remove iron particles. The reason why this type of slag is unsuitable for granulation is that quenching of a high iron containing molten slag with water can cause an explosion. High iron slag granulate is also an unsaleable product for the major market of cement blend production. When the molten slag flows along the slag runners, it is inspected and high levels of iron in the slag can be detected by the colour of "sparks" coming from the slag flow. In modern furnaces, the slag runners are usually covered and there is only a small timeframe from when the iron in the slag is visible, to when the slag is quenched, for an operator to abort the slag flow to slag pits rather than the granulator. This results in a high risk, and if there is any doubt to the iron content in the slag, the slag is aborted to the slag pits. This can result in the loss of potentially valuable slag which would be suitable for granulation, but which has instead been aborted to the slag pits.

As described above, the quality of the granulated product depends on efficiently controlling the quenching process. One method to control the process is to ensure a uniform slag stream/water volume ratio. This can be achieved by maintaining a constant volume and shape of the slag stream over the nozzle plate (the plate through which high-volume water passes to quench the slag stream). This is extremely difficult, if not impossible with a variable slag flow. To compensate for this, conventional granulation plants can have variable water flow pressures and volumes to accommodate the variable slag flow. If the volume of molten slag is too large for the quenching process, slag can pass through the water jet without being properly granulated, and can adhere to the liners in the cold runner which is the discharge chute after the nozzle plate. This can severely affect the efficient working of the granulation plant.

After much research and experimentation, the present invention is directed to an apparatus and method which may overcome at least some of the abovementioned disadvantages and provide the producer with a useful or commercial choice.

This can be achieved by having the granulation plant at a location remote from the furnace. This frees the granulation plant from any limitation of the furnace layout, and allows the plant to be separate from the furnace operating and availability criteria. The molten slag from the furnace is initially poured into slag pots which are then transported to the remote granulation plant. This allows much greater control over slag flows in the granulation plant. This process also allows some settling of iron in the slag which allows slags containing iron which otherwise would have been diverted to slag pits, to now be suitable for granulation, as the settled iron can be left in the slag pot.

The granulation plant can be made more compact and does not require the plant and equipment required to handle the peak slag rate that occurs due to the variable slag flow. This allows a relatively compact granulation plant to use slag from a furnace which would otherwise require a much larger plant.

The variable flow rates from the furnace can be accommodated by using more slag pots. The granulation process can now be controlled much more effectively which provides a better quality granulated product which has higher value end uses, and noxious gas emissions can be reduced.

Slag pots are known and are able to accommodate up to sixty tons of molten slag from the slag runners. Slag pots can be used in blast furnace plants where on-line granulators are not present. The slag flows into the slag pots which are then transported to a slag pit for dumping.

In one form the invention resides in a method for granulating blast furnace slag comprising the steps of: pouring the blast furnace slag into a slag pot, (ii) maintaining the slag in the slag pot for a period of time sufficient to allow iron in the slag to at least partially settle to the bottom of the slag pot, (iii) tipping the slag pot in a controlled manner to allow a controlled amount of slag to flow from the lip of the slag pot, (iv) directing the slag flow from the lip of the slag pot through a granulation water stream to granulate the slag.

Preferably, the method utilises a temperature/heat sensor to determine the slag temperature to assure that the slag is in the appropriate temperature range for granulation, In another form, the invention resides in a granulated product formed by the above method.

In order to minimise iron contamination in the granulating process, the slag pot can be tipped such that an amount of slag remains in the slag pot, and is not poured or transferred to the granulating step. Suitably, the last or so of slag in the pot is not tipped out of the pot and into the granulating step. This portion typically contains an unacceptable high amount of iron, and also contains slag crust particles. This residual slag may be tipped at the skull pit for subsequent processing in the crushing and screening plant.

The slag is maintained in the slag pot for a period of time sufficient to allow iron in the slag to at least partially settle towards the bottom of the slag pot. Typically, a resident time of between 10 to 60 minutes is used which includes the filling, travelling, holding and tipping times. On the other hand, the slag should not cool to a temperature which is too low to result in efficient granulation, although some cooling is considered to be beneficial to the formation of a better quality granulated product.

It is found that improved properties of the granulated product can be obtained if the slag temperature at the granulator can be controlled. It is found that the slag temperature at the furnace taphole is higher than required to produce a good quality granulated product upon contact of the molten slag with water. A typical slag temperature at the furnace taphole is 1,510 degrees centigrade. However, it is found that a quality coarser granulated product can be obtained by granulating molten slag having a temperature of approximately 50-160 degrees lower than that of the furnace taphole. For instance, a slag temperature of approximately 1420 degrees centigrade is suitable at the granulator. Thus, the resident time of slag in the slag pots may be such to cool the slag to a temperature which is between 50-160 degrees lower than at the furnace taphole.

The slag pots are of considerable size and, when tilted, molten slag flows over the lip of the slag pot and typically onto a slag runner which directs the molten slag to the, and over the granulator nozzle plate.

The slag drop height from the lip of the slag pot to the end of the slag runner is approximately 4 m. This can result in the slag speed being such that it does not efficiently granulate at the nozzle plate, and instead can pass through the water stream and into the granulation tank without being fully granulated. Therefore, it is preferred that an intermediate slag runner/launder is incorporated to take the impact of the falling slag. It is preferred that the slag drop height from the intermediate runner to the water stream is 1.6 m or less.

Suitably, the shape of the slag flow at the granulator water spray is controlled to improve the properties of the granulated product. This now becomes possible with the method according to the invention where molten slag can be tipped from the slag pots at a controlled rate. This has not been possible with on-line granulating systems where the variation in slag flow rates makes it difficult if not impossible to control the shape of the slag flow at the water spray head. In normal granulation systems, the slag flow from the furnace slag runner is focused to fall from the end of the slag runner and pass through the granulation water stream. The water stream pattern is produced by a series of holes in a nozzle plate with water flow and water pressure being important factors in producing consistently high quality granulate.

If the shape of the slag flow can be controlled at the water spray head, the properties. of the resulting granulated product can also be better controlled. It is found that this can now be achieved with the method according to the invention. By controlling the width of the nozzle plate or focusing the slag stream over the nozzle plate the water volume required for granulation can be minimised. This optimises the physical size of the granulation equipment, including pumps and pipe work, hot water tank and dewatering drum.

The tipping rate of molten slag from the slag pots is preferably such to maximise the efficiency of the granulating process, and to minimise emission of noxious gases, and also to minimise the recirculating water flow rate. This will of course vary depending on the size of the granulation plant. It is found that a slag flow rate of two tons per minute is suitable for a granulation plant of conventional size. This flow rate is also commensurate with the average slag flow volume from a blast furnace. Any peak slag flow from the blast furnace can be accommodated by using more slag pots. At the slag flow rates of approximately two tons per minute, the generation of hydrogen sulphide emissions are lower than for higher slag rates.

The transfer of molten slag from the slag pot to the granulator can be by various means. For instance, the slag may be poured from the slag pot into a ladle, and a ladle can then be tilted at a controlled rate to control the slag flow. Alternatively, the ladle may have a bottom tapping arrangement to control the slag flow. The slag pot may be provided with means to enable a smooth slag stream to be produced when the pot is tipped. A tilting station may be provided to tilt the slag pot, which may be a conventional pot or a modified pot. Alternatively, the existing slag pot carrier may be modified to enable it to tilt the pot which may be a conventional pot or a modified pot.

Suitably, the pot tipping means allows the pot to be tipped at a controlled rate of, for instance, two tons per minute, for granulation of the molten slag, but also allows the pot to be tipped at a greater rate for instance the normal slag pot tipping rate for dumping the residue left behind in the pot.

The pot carrier may be fitted with communication means such as radio links to enable the carrier operator to start and stop the granulator plant from the carrier while transporting the slag to the granulator plant. At the granulator, the operator can position the pot carrier at a tipping station. The pot carrier can be switched to "controlled tipping mode", and the operator can leave the pot carrier unmanned. The operator can proceed to the control room or control pulpit for the granulator. From the control room, the operator can signal the pot carrier, via the radio link, to commence tipping. Suitably, the slag pot is initially tipped at a normal (higher) speed to bring the molten slag to the lip of the pot. Switch means such as limit switches on the carrier hydraulic cylinders may activate control valves to switch to the low displacement pump for controlled tipping, and a second set of limit switches can activate the valves for a return action, using the normal hydraulic pump.

A timer may be provided to cause the slag pot to return to the upright position if the second set of limits is not activated after a pre-set time. During the granulation process, all functions may be controlled by a computer. The operator may be present to monitor granulator data such as water pressure, water temperature, water levels and to control the radial stacker conveyor to the required stockpile area.

After the granulation tipping operation is complete, the operator can return to the pot carrier to collect another pot from the furnace. The pot carrier and the control room may be in radio contact with operators at the furnace. If slag flow rate at the furnace becomes excessive, the granulation process may be aborted and the slag pots returned to the furnace for filling.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a schematic plan view of a granulation plant for carrying out a preferred method of the present invention; Figure 2 is an elevational view of the plant of Figure 1; and Figure 3 is a detail of the plant of Figure 1.

Figures 1 and 2 illustrate a slag granulation plant (10) having a granulation unit (11) which includes granulation water tank 12 and a granulation stack 13. The granulation water tank 12 has an outlet 15. Inlet 14 is the stack outlet. A granulation nozzle plate 31 (this is more clearly illustrated in Figure 3) is located at inlet 14. The outlet 15 fluidly connects the granulation water tank 12 to a dewatering drum 16. A pipe 17 extends from dewatering drum 16 to the granulation stack 13.

The plant 10 also includes a shuttle conveyor 18 and transfer conveyor 19 for transferring granulated product to stockpiles Figure 3 shows a detail of the granulation unit 11.

In a preferred process of the present invention, four slag pots of about 56 tonnes capacity are sequentially filled upon tapping a blast furnace at a site remote from the granulation plant 10. The filling time for each pot varies between about 10 to about 60 minutes, depending upon the slag flow which varies between about I and about 6 tonnes per hour.

Filled slag pots 21 are transported to the granulation plant 10 by a pot carrier 22. It has been observed that, on occasion, the first slag from a cast has a lower temperature to begin with. It has been found that the granulate from these first slag pot yields a different glass content, reactivity and different densities. Results from the second and subsequent pots are relatively consistent. The differences between the first and second pots is summarised below.

1 Grading Ia Pots I 2 d Pots Thereafter mm 100 6.7 99 100 4.75 95 98 2.36 65 1.18 28 600 pm 11 16 425 6 8 300 4 6 150 2 2 1 1 Glass Content 89 92% 90 98% Moisture Content 2 4 4 7% Density 1.3 1.5 t/m 3 1.0 1.3 t/m 3 28 Day Relative Strength 90 100% 100 115% Granulation Temp 1380-14000C 1410-1420°C 28 Day Relative Strength is the ratio of the strength blend to 100% OPC of a 50/50 OPC/GGBS In order to obtain a consistent granulate product with optimum reactivity, the first pot may not be granulated. Alternatively, the granulate from the individual pots may be blended together.

However, if a coarse concrete sand product was established for granulated slag, the first pot material may be more suitable due to the higher density and therefore better grain morphology and shape.

In this case, the second and subsequent pots would give the same properties as the first pots if deliberately held for approximately an extra minutes before granulating.

The pot carrier 22 transports a slag pot 21 to the granulation unit 11.

Adjacent the tank is a retaining wall 30 which supports wheel stops 23. The pot carrier 22 is driven towards the tank 12 until the rear wheels 24 abut wheel stops 23.

The transport time from the blast furnace to the granulation plant is about 8 minutes. The slag exits the blast furnace at about 15100C and cools down to between about 1400 to about 1420oC by the time the pot has reached the granulation tank 12.

The slag pot 21 is tilted by the pot carrier 22 such that molten slag flows over the lip 25 of the pot 21. The slag pot 21 is tilted for about a minute period producing a slag flow of about 2 tonnes per minute. About 6 tonnes of slag may be retained in the bottom of each pot to minimise iron transfer to the granulate. The remaining slag is then taken to skull pits (not illustrated).

In an alternate process, the granulation plant 10 may include an automatic tipping station for tipping the slag pot. In this way, the pot carrier is released to return to the blast furnace to collect another slag pot. Generally, when pot carriers are used for the tilting operation, two pot carriers are required as there is insufficient time for the pot carrier to tip and return to the furnace before the next pot is filled.

The slag from the tipped pot flows onto a slag runner 28 which takes the impact of the falling slag and directs it towards the granulation nozzle plate 31 where the slag is quenched by a water stream before entering granulation tank 12.

The granulated product is dewatered in dewatering drum 16. Water flows into a hot water tank located in pump pit 27 for recycling to the quenching step.

The dewatered granulated product is then transferred to stockpiles Chemical and physical properties of the granulated product were assayed and compared with the properties of a conventional granulated product. The assay results are summarized in the following table.

Present Invention Prior art Temp of slag taphole 151000C 151000C Temp of slag 1420oC I 150000 granulation Density 1.2 t/m' 0.8 t/m 3 Moisture Content 7% (10 min) 20% (1 hour) 5%(11 day) 15% (4 weeks) *28 Day Relative Strength 100- 115% 100 -115% A comparison of the granulated product against a typical cement grade granulate specification is illustrated in the following table Present Invention Prior Art Typical Cement Grade Granulate Specification 1 Chemistry Total Iron as FeO 0.4% 1.0% 1.3% Si0 2 34% 34% A1 2 0 3 14% 14% 18.0% CaO 41% 41% MgO 6% 6% 10.0% 0.6% 0.6% P 0.005% 0.005%

K

2 0 0.3% 0.3% TiO 2 0.9% 0.9% MnO 0.4% 0.4% Hydraulic Index (CaO MgO A1 2 0 3 Si0 2 1.79 1.79 Grading passing passing passing mm 100 6.7 100 4.75 98 100 2.36 80 1.18 40 71 600 g±m 16 28 425 8 300 6 9 150 2 4 1 2 3. Chlorides <20ppm <20 ppm 250ppm 4. Glass Content 90 98% 97 99% 90% where practicable Moisture Content 4 10 15% 12% 6. Density 1.0 1.3 t/m 3 0.6 1.0 t/m 3 No set limit (reflected though in moisture content) 7. 28 Day Relative 100-115% 100-115% Strength The method of the invention has several advantages, which include: 1. The slag flow to the granulator can be controlled and is more consistent. There are no surges or peak flows as expected with on-line granulators. This allows the water/slag ratio to be much more consistent to provide a better quality granulate product.

2. As there are no unpredictable and uncontrollable slag flows, the granulator can be made more compact and does not require equipment or a construction to accommodate variable slag flow rates. Less water volume is required and there is less equipment wear.

3. By having molten slag remaining in the slag pot for a period of time (during filling, transportation and pouring), iron in the slag will settle towards the bottom of the slag pot A quantity of molten slag in the pot is not tipped into the granulator and instead is tipped into a skull pit. This minimises or can completely eliminate the possibility or risk of iron explosions caused by contact of the iron with water in the granulation step. Typically, the last of slag in the pot is kept separate from the granulation step.

4. It is found that when the molten slag is in the slag pot, a partial degassing of the slag occurs. This is due partially because of the cooling of the slag in the slag pot. The degassing beneficially reduces the effect of slag foaming reactions which occur during quenching of the slag with water. The slag foaming reaction occurs by the reaction of steam with the slag components. In normal on-line granulators, water cooling such as plate heat exchangers or cooling towers may be required to combat the slag foaming reaction, all of which involve significant capital and operating costs. With the method of the invention, the need for plate heat exchangers or cooling towers is reduced or eliminated.

The granulated product formed by the method of the invention has desirable properties making it suitable for value added uses. By controlling the quenching process, the granulate density can be improved. Lower density porous granulate particles contain water entrapped during the granvlation process and retain substantial amounts of water which are adhered to the pore surfaces. This lower density granulate retains its moisture and requires extra steps to drain excess water from the granulated product. Conversely, a higher density granulate product drains more quickly and can be despatched directly to an end use without requiring additional draining steps (which may comprise stockpiling the product for a period of time to reduce its moisture content). The granulated product formed by the method of the invention has a density of approximately 1.2 tonnes per cubic metre and a moisture content of 7% after 10 minutes and about 5% after one day. By contrast, a lower density granulated product may have a density of approximately 0.8 tonnes per cubic metre and may have about 15% moisture after one month. A low moisture content is critical if the granulated product is to be used as a cement substitute, by reducing yield losses during transportation, and to lower energy costs for drying, and to achieve a higher drier throughput.

The granulated product has a very low iron content. A low iron content allows for much more efficient use of stockpile space on site at furnace, allows all tonnages to be directed to high value added application (for instance cement usage) where low iron content is specified to avoid staining in the slag cement and can be used to dilute other granulated product having a higher iron content.

By having a lower moisture content, the granulated product is less likely to "set" into a solid mass over time. This allows the product to be stockpiled for longer and provides an improved shelf life. Granulated product having a higher moisture content tends to solidify more quickly into an unusable product.

The granulated product has a coarser particle size. It is believed that this may be due to the lower slag temperature at the granulator which may increase the viscosity of the molten slag resulting in the production of coarser granules. The coarser granules have a lower surface area to volume ratio making the granules less reactive and therefore less subject to aggregation.

The granulated product has a different grain morphology with a denser structure and a semi-rounded shape rather than an angular shape which is found with conventional granulation systems. The rounded shape and the lower porosity makes the granulated product much better for use as a coarse sand replacement in concrete. The production of the coarser granules in the granulation plants results in less dust emissions from the stockpiles.

It should be appreciated that various other changes and modifications may be made to the embodiment described without departing from the spirit and scope of the invention.

Claims (3)

1. A method for granulating blast furnace slag comprising the steps of: pouring the blast furnace slag into a slag pot, (ii) maintaining the slag in the slag pot for a period of time sufficient to allow iron in the slag to at least partially settle to the bottom of the slag pot, (iii) -tipping the slag pot in a controlled manner to allow a controlled amount of slag to flow from the lip of the slag pot, (iv) directing the slag flow from the lip of the slag pot through a granulation lo water stream to granulate the slag.

2. The method of claim 1, wherein the blast furnace slag is poured from a blast furnace tap hole and the temperature of the slag flowing from the lip of the slag pot is between about 50 to about 1600C less than the temperature of the slag at the tap hole.

3. The method of claim 1 wherein the flow of slag flowing from the slag pot is substantially constant throughout the tipping step. DATED this 21st dayof March 2001 Australian Steel Mill Services Pty By their Patent Attorneys CULLEN CO.