CLIAMS:We claim:
1. A process for improving the fluidity of the residual Linz-Donawitz (LD) slag in the BOF slag pot left after the slag splashing in the LD converter formed during the Steel production to facilitate proper dumping of the slag in the slag yard, said process comprising the following steps:
a. pouring the left over slag after effective splashing into slag pot by turning the LD converter upside down for transporting the slag to the slag yard.
b. adding the desired quantity of quartzite sand during the pouring of the slag into the slag pot to minimize the basicity of the slag.
c. placing the required quantity of coke to the top of the slag surface in the slag pot to raise the temperature by burning of the coke; and
d. blowing the oxygen through the oxygen lance in the slag pot to burn the coke to provide the required temperature to melt the slag, resulting in opening of the slag crust for further blowing of oxygen into the slag beneath the top slag crust before dumping of the slag in slag yard.
2. A process as claimed in claim 1, wherein the quantity of quartzite sand used is in the range of 75-85 kg per ton of LD slag treated.
3. A process as claimed in claim 1, wherein the size of the quartzite sand used is in the range of 1-20 mm.
4. A process as claimed in claim 1, wherein the basicity of the slag minimizes from 4 to 2.64 after the addition of the quartzite sand.
5. A process as claimed in claim 1, wherein the coke used is about 55-65 kg per ton on top of the slag surface to ensure the fluidity of the slag by burning of the coke.
6. A process as claimed in claim 1, wherein the purity of oxygen blown into the slag for increasing the temperature and making it in fluid condition is about 99.5%.
7. A process as claimed in claim 1, wherein the oxygen is blown at pressure of 10 to 12 kg/cm2 in the slag pot.
8. A process as claimed in claim 1, wherein the oxygen is blown for at least 8 minutes in the slag pot.
9. A process as claimed in claim 1, wherein the slag formed is CaO.SiO2.FeO slag and dicalcium silicate slag.
10. The slag produced by the process as claimed in claim 1, having the composition of CaO,MgO,SiO2,FeO,Al2O3,P2O5,MnO in the slag before treatment after splashing and after treatment in the slag pot is 52,10,13,20.2,0.92,0.9,1.33 and 48.02,8.95,18.19,18.81,1.40,1.1,1.54 respectively.
11. The slag produced by the process as claimed in claim1, whenever used for the road constructions, bridge constructions and the like.
,TagSPECI:FIELD OF INVENTION:
The present invention relates to the process for treating the LD slag after it is drained from converter into the slag pot to facilitate proper dumping of the slag in slag yard. Due to high viscosity of the slag and low temperature, the slag which is drained from the converter stick to the walls of the slag pot and cleaning of the slag pot for next cycle gets affected. Slag fluidity is important for transportation of the slag from slag pot to the slag yard. The present invention relates to a process for improving the Linz-Donawitz (LD) slag fluidity in slag pot.

BACK GROUND OF INVENTION:

Refractory are vital inputs for the steel industry because of their huge requirements and high cost. Of the total consumption of refractory in a steel industry, more than 80% are consumed in Basic Oxygen Furnace (BOF) or (Linz-Donawitz) LD converter alone. In the overall economics of BOF steel making, refractory cost and consumption play an important role. Hence major thrust had been given for improving the life of refractory in the BOF shop as it plays an important role in the success of the steel plant.
The slag produced in BOF, a waste product of the steelmaking process, contains oxides that are refractory in nature which can be utilized for providing a protective coating on the working lining of BOF. The left over slag in BOF after tapping of steel, is blown with Nitrogen at about 12-14 Kg/cm2 pressure using the oxygen blowing lance. The slag splashes in all directions and entire lining of BOF is covered with the splashed slag. During the blowing of the next heat, this covered slag forms a protective coating and gets eroded first instead of affecting the working lining.
Slag splashing is practiced in many of the steel plants for improving the converter refractory life. After the slag splashing the slag becomes considerably viscous due to high basicity and low temperature. Because of these reasons, the slag after dumping in slag pot from converter sticks to the walls of the converter. The slag pot is a container used to transport the slag from converter to slag yard for further processing. If the slag from the slag pot does not flow out during dumping in slag yard, the main objective is defeated. After the slag from the slag pot is dumped in the slag yard the same slag pot is to be given for next cycle for collecting the slag from the LD converter. Hence it is very much essential to maintain the slag in fluid condition till the slag is dumped completely in slag yard for further processing.
Existing practices for complete slag dumping:
To ensure full dumping under the constraints given above, the following practices were tried at BOF shop:
a. Lime coating on the walls of the slag pot were tried so that the slag does not stick to the pot and gets dumped during dumping at slag yard. The dumping angle of dump car for slag pot dumping was about 90-100 degree. Due to viscosity/thickness of the slag, the slag pot does not get emptied at that dumping angle.
b. Lime and graphite coating were applied on the slag pot walls for ease of stripping but this also did not give desired results.
c. Some trials were conducted for slag dumping using overhead cranes so that the tilting angle can be increased up to 180 degrees. Even this did not give desired results.
d. Some trials were also conducted by poking the slag crust on the surface, and by hitting the bottom of the slag pot with a bloom piece to detach the solidified slag when the slag pot is in dumping position. But the efforts also did not provide the desired results.
The present process of the invention helps in maintaining the slag fluid till the slag pot is emptied in slag yard. This solves the problem of slag retention in the slag pot so that the slag pot can be utilized for next cycle efficiently.
Object of invention:
The object of the present invention is to improve the slag fluidity to the extent possible to ensure proper dumping of the slag from the slag pots.

SUMMARY OF THE INVENTION
The present invention relates to the improvement of the fluidity of the slag by addition of quartzite sand to slag in BOF slag pot to compensate the basicity increase occurred during conditioning of the slag for effective slag splashing. During conditioning of the slag Dolomite is added, which increases MgO content of slag. This increased MgO content in slag increases basicity of the slag than normal or actually required for steel making. This addition of quartzite sand in predetermined quantity is done in Slag pot as this addition cannot be performed in converter due to technical reasons.
More over temperature of the slag is reduced considerably after the slag splashing. This is due to the blowing of nitrogen for 3 to 5 minutes to affect the slag coating on LD Converter refractory. This is done in order to reduce refractory erosion during subsequent heats.
The present invention also relates to a process of blowing the oxygen in the slag pot through oxygen lance onto the surface of the BOF slag after which the thick slag is drained from the slag pot in the slag yard. During this treatment Oxygen reacts with FeO and generates sufficient heat required for increasing the temperature of the slag in pot. With the two treatments given to the slag, it becomes sufficiently fluid and ensures 100% dumping in slag yard without any difficulty.

Slag fluidity is important process parameter in steel making both for steel quality as well as slag pot maintenance. Slag splashing is practiced in many of the steel plants for improving the converter life. For better splashing effectiveness the fluid slag is saturated with MgO by adding Dolomite which makes the slag viscous. In slag splashing, left over slag after tapping the steel, is blown with Nitrogen at high pressure. Due to this, the temperature of the slag drops heavily & becomes cold. Time gap between deslagging and slag dumping in slag yard further aggravates the problem due to loss of temperature. Thus a process for improving slag fluidity by controlling the slag basicity and temperature in slag pot has been developed.
If the slag from the slag pot does not flow out during dumping in slag yard, the main objective is defeated. After the slag from the slag pot is dumped in the slag yard the same slag pot is used for the next cycle for collecting the slag from converter. Hence it is very much essential to maintain the slag in fluid condition till the slag is dumped completely in slag yard for further processing. The present process helps in maintaining the slag fluid till the slag pot is emptied in slag yard. This solves the problem of slag retention in the slag pot so that the slag pot can be utilized for next cycle efficiently.

DETAILED DESCRIPTION OF THE INVENTION

The Basic Oxygen Furnace (BOF) steel making process is the major modern process for the making of bulk steels. The BOF vessel weighs around 180 tons, and is mounted so that it may be rotated through 360° in the vertical plane. The vessels are lined with heat resistant refractory materials such as magnesite (MgCO3), tar or pitch bonded, dolomite, seawater magnesia etc.
Firstly the BOF vessel is tilted to allow the addition of the scrap metals, and then for the molten iron directly from the blast furnace. This is known as charging. There is usually 75% hot metal, and 25% scrap metals used during charging, and the balance between these amounts is used to regulate the temperature of the steel in the furnace. The vessel is then stood upright and the oxygen lance is lowered into it. Pure oxygen is blown onto the surface of the molten metal at a flow of 10 to 12 kg/cm2. The pure oxygen is used to remove impurities from the molten iron by oxidation. The main reactions taking place is the oxidation of carbon to form carbon dioxide, and carbon monoxide.
• C (s) + O2 (g) = CO2 (g)
• CO2 (g) + C(s) = 2CO(g)
• 2CO (g) + O2 (g) = 2CO2(g)
The gases formed by oxidation are removed via (induced draft) I.D. fan at the top of the hood which can be used as fuel elsewhere in the plant.
During the initial minutes of blowing, a flux of “burnt lime” (CaO) is added. Silicon and phosphorous are removed after adding the flux, as the slag is formed.
Manganese oxidizes, but remains in the iron, and the sulphur is dissolved in the slag.
4P + 5O2 = 2P2O5
• 6CaO + 2P2O5 =2Ca3(PO4)
• Si + O2 = SiO2
• CaO + SiO2 = CaSiO3
The oxygen blowing is continued for about 16 to 18 minutes till the carbon is oxidised, after which the vessel contents are analyzed to see whether enough of the impurities have been removed. The vessel is then tilted to tap off the steel that has been produced. During tapping, small quantities of Ferro alloys, Al and Petrocoke are added to the system to control the state of oxidation, and to meet the order for the correct grade of steel.
After the tapping of the steel is done, the slag is removed into a container by tilting the vessel. The slag can be used in the construction of roads. A BOF vessel can produce 140-150 tons of steel in every batch, which takes around 45 minutes to complete.

In the present invention, the Basic Oxygen Furnaces (BOF) have the capacity of 140-150 tons. The steel making process through the BOF route generates app. 90 to 100 kg LD Slag per ton of liquid steel. The typical composition of LD slag is as given in Table 1.

Table 1: Typical composition of LD slag
Compound CaO MgO SiO2 FeO Al2O3 P2O5 MnO Basicity
Percentage 48– 51 10 – 13 16 – 18 16– 24 < 1 1.5– 2.5 < 1.5 2.7– 3.2

Timely evacuation of LD slag plays a key role in BOF productivity. Proper maintenance of slag evacuation cycle is important.

Slag splashing in BOF is a maintenance practice of the wear out of the converter lining using the left over slag, modified with additives in the converter, after a heat is tapped. The modification of the slag is done depending on its fluidity and composition. The most common modifiers are calcined dolomite, lime and magnesite. Splashing is performed by blowing nitrogen through the oxygen lance of the converter on the modified slag. Some converters are reported to have separate lance, while in the other, the same oxygen blowing lance is used for splashing also. High pressure nitrogen at about 12-14 Kg/cm2 is blown with lance at pre- decided height depending on the identified area with converter in upright position for 3 to 5 minutes to throw the slag on the vessel refractory surface. The slag coats the refractory, cools and solidified on the vessel lining and creates a solid layer that serves as the consumable refractory layer for the subsequent heats. This layer reduces the wear rate of the original lining. The left over slag after coating of the vessel is dumped.
By employing slag splashing practice, BOF life could be improved considerably. This practice is in vogue for the last 10-12 years and the average BOF lining life has increased from around 930 in 2000-01 to 5123 during 2011-12. Slag splashing has become a process requirement now.
For effective slag splashing the slag should possess certain properties and composition. Slag should be sufficiently thick in order to stick to the walls of the BOF. The slag should be supersaturated with MgO content otherwise MgO will be taken from lining of the converter consisting of MgO-C bricks. It should also have high basicity as the converter lining is basic.
In order to achieve those properties slag conditioning is done before splashing, by addition of lime, dolomite etc for increasing the basicity. Immediately after blow, slag is sufficiently fluid at that high steel making temperatures. After splashing the slag becomes thick and cools down considerably.

OPERATING FACTORS AFFECTING SLAG SPLASHING:
EFFECT OF LANCE HEIGHT-: Raising the lance height increases the amount of splashing in the lower region while decreasing splash in upper areas. As the lance height is increased, the point of impact of the gas jets on the slag surface moves more towards the wall; leading to reduce splash at the top.
EFFECT OF NOZZLE ANGLE-: As the nozzle angle is increased, fewer splashes reach the
upper areas. This is because as the nozzle angle increases, the vertical component of the velocity imparted to the slag droplets decreases.12 degree lance tip is more suitable for splashing as compared to17.5 degree lance tip.
EFFECT OF FLOW RATE-: The amount of splash increases on all areas of the wall as the
gas flow rate increases although it is more pronounced in the lower areas of the wall. The higher momentum of the gas jets generates higher impact forces leading to increased splash.
EFFECT OF NUMBER OF NOZZLES-: As the number of nozzles used increases, the nozzles get closer to one another and interaction between adjacent jets become more prominent. As this interaction increases, the splash in central area decreases.
EFFECT OF VISCOSITY -: Amount of slag splash decreases everywhere as the viscosity
increases. The amount of reduction in splash is greatest along the centerline of each nozzle.
The overall splash reduction is caused by the higher forces needed to form droplets by
breaking a higher viscosity fluid.
EFFECT OF BATH DEPTH-: There is increase in splash in all regions with an increase in
bath depth. This is because as the bath depth is increased, the slag droplets have to travel to shorter distances to reach the top.
SELECTIVE SPLASHING-: For selective splashing it is important to know which factors
increase or decrease the splash in a particular desired region, allowing for selective coating of particular region.
EFFECT OF DOLOMITE ADDITION IN SLAG-:
Addition of Dolomite in slag improves its refractory properties. The slag is less corrosive as the slag is saturated with MgO and it does not take further MgO from refractory lining of the LD converter. FeO content of slag is corrosive in nature but by addition of Dolomite, MgO in dolomite ties up the slag FeO in an MgO - FeO solid solution. After addition of Dolomite in slag, the slag becomes less fluid in character due to thermo chemical reaction. It can be concluded that Dolomite is added to cool the slag, increase its viscosity and decrease the FeO content. It should also have high basicity as the converter lining is basic. In order to achieve those properties slag conditioning is done before splashing by addition of dolomite. Immediately after blow, slag is sufficiently fluid at that high steel making temperatures.

After splashing the left over slag is dumped into slag pot for transporting the slag to slag yard. The slag which is generated after splashing in BOFs is thick and causes a lot of inconvenience during dumping of slag in slag dump yard. It causes jamming of slag pots and hence availability of slag pots for de-slagging is affected. The retention of the material after a number of uses significantly reduces the effective capacity of the slag pot due to the buildup of successive layers of material within the slag pot. The removal of solidified material also often results in breakage or extensive damage to the slag pot.

The main factors responsible for maintaining the fluidity of BOF slag were identified by monitoring the time taken from blow finish to the dumping of slag at the slag dump yard, the temperatures and composition of slag at various stages of the dumping process. The effect of these factors were analysed for fluid BOF slag and viscous BOF slag. The data were collected at various stages of slag evacuation and the factors were analyzed for obtaining the significant factors which led to the formation of more viscous slag.

Time measured at each activity
The more the time taken for dumping, the slag would get cooled and become viscous. Hence the time taken for each activity was noted down in the process of slag evacuation and the activity was also analysed which was causing excessive delay in order to control the same.
The various activities for which time data were collected are: Time of blow finish, time at which tapping of liquid steel had started, time at which tapping of liquid steel had finished, time details of slag splashing, time of final de-slagging, time of dispatching the slag pot to slag yard, time of receipt of slag pot in slag yard, and the time of start of slag dumping.

The activity times were categorized into three groups as given under:
1. Process Times – The time gaps between blow finish to tap start =10 minutes,
tap finish to splash start =5 minutes, and splash finish to final de-slagging to slag pot= 5 minutes
2. Dumping Time – The time gap between final de-slagging to dispatch from slag pot to slag dump yard for dumping=40 minutes

Temperature measured at different stages
The temperature of the slag affects the slag fluidity, the lesser the temperature the more viscous the slag. The data pertaining to temperature of slag at various stages were collected in order to compare the relationship of time delay with slag temperature.
Nitrogen was blown at about 12-14 kg/cm2 pressure for about 3-5 minutes during slag splashing which causes temperature loss. Invariably some time delay was taking place in
sending the slag pot to slag dump yard after final de-slagging. To take these into account the required temperatures were measured using optical pyrometers and the measurements were taken from a fixed standard distance for all the heats for which these details were collected.
The temperature of slag were measured at four stages. Two optical pyrometers were used simultaneously for collection of the data. One pyrometer was used to measure the temperature of the slag when de-slagging was being carried out before the tapping (during measurement of opening temperature of the steel and collection of bath slag sample).
Next the temperature after splashing of the slag and final de-slagging was measured.
Regular practice is to keep the slag pot beneath the BOF till next heat is charged in the
BOF and then dispatch the pot to slag yard from the BOF shop. Temperature of the slag surface in the slag pot was measured just before dispatching the pot to slag dump yard from BOF shop. The second pyrometer was used to measure the temperature of the slag during dumping in slag dump yard.

Composition of the slag
The composition of bath slag was analyzed through X-ray Fluorescence Spectrometer.
The collected data were recorded in the developed format.
Initially it was decided to carry out the slag analysis at three stages and the temperature were measured:
1. Bath slag-1600°C to 1700°C
2. After splashing-1300°C -1400°C
3. After Dumping-1000°C -1100°C

The composition analysis at three stages is required in view of the additions of lime and calcined dolomite before splashing to condition the slag, which leads to changes in composition from the bath slag. Slag sample was taken after dumping for identifying the presence of any undissolved lime and found that the free lime content is less than 2%. The physical observation is not possible in the hot semi viscous state. The slag compositions after splashing are not taken as it is expected to have the same composition as the sample after dumping.

Analysis of data
The data collected was normalized and then empirical analysis of the data was carried
out. The data was divided into two categories based on the fluidity of the slag at dumping. The data related to fluid slag formed one category while data related to viscous slag formed the second category. As no means was available to measure the viscosity of slag directly, the type of dumping, partial dumping or full dumping was considered as a categorical measure of fluidity.

From the data analysis it was found that Slag was becoming thick due to two reasons. They are High basicity & Low temperature.
Hence to attack both the parameters to make the LD slag fluid the following actions were taken
According to the present invention, Quartzite sand is added in pre-calculated quantity to minimize the basicity .The quartzite sand requirement is found to be around 70-90 kg per tonne of slag treated) to the LD slag in the slag pot after draining of the slag in slag pot from converter.
According to an embodiment, coke is placed on the top of the slag surface. Due to burning of coke and when oxygen is blown which provides required temperature to melt the slag and provides opening for further blowing of oxygen into the slag beneath the top slag crust.
According to further embodiment, oxygen is blown for about 8 minutes in the slag pot just before dumping of the slag in slag yard to increase the temperature.

According to further embodiment, the slag after treatment is collected and analyzed for its composition. It is observed that the basicity has been reduced considerably. After the treatment the slag is sufficiently thin and it has produced desired results. The average slag composition before and after treatment is as follows:

CaO MgO SiO2 FeO Al2O3 P2O5 MnO Basicity
Before splashing 47.57 9.21 15.02 23.64 0.95 0.88 1.16 3.17
After splashing 52.0 10.00 13 20.2 0.92 0.9 1.33 4.0
After experiment 48.02 8.95 18.19 18.81 1.40 1.1 1.54 2.64

The invention is further illustrated hereinafter by means of example without limiting the scope of the invention.
Example 1:
The slag produced in the LD converter is used in the slag splashing for improving the converter refractory life. In slag splashing, left over slag in converter after tapping the steel, is blown with Nitrogen at high pressure. Due to this the temperature of the slag drops heavily and becomes cold. Addition of the dolomite to increase the refractory property of the slag makes the slag less fluid and thus viscocity increases. The high viscocity and low temperature of the slag creates problem in dumping of the slag at the slag dump yard as it sticks to the walls of the slag pot. The present process thus helps in maintaining the slag fluid till the slag is emptied. For minimizing the basicity of the slag, quartzite sand in a amount of 78 kg per ton of the LD slag is added during pouring of the LD slag into the slag pot. The size of the quartzite sand used is 10mm. Then 63 kg coke is placed to the top of the slag surface in the slag pot to raise the temperature by burning of the coke. The oxygen is then blown at a pressure of 10 kg/sq.cm in the slag pot for about 7.5 minutes through the oxygen lance in the slag pot to burn the coke to provides the required temperature to melt the slag, resulting in opening of the slag crust for further blowing of oxygen into the slag beneath the top slag crust and thus facilitating the slag for proper dumping in the slag yard by maintaining the slag fluidity by minimizing the basicity to about 2.64 and increasing the temperature.

Example 2:
The slag produced in the BOF or LD converter is used in the slag splashing for improving the converter refractory life. In slag splashing, left over slag in converter after tapping the steel, is blown with Nitrogen at high pressure. Due to this the temperature of the slag drops heavily and becomes cold. Addition of the dolomite to increase the refractory property of the slag makes the slag less fluid and thus viscocity increases. The high viscocity and low temperature of the slag creates problem in dumping of the slag at the slag dump yard as it sticks to the walls of the slag pot. The process thus helps in maintaining the slag fluid till the slag is emptied. For minimizing the basicity of the slag, quartzite sand in an amount of 82 kg per ton of the LD slag is added during pouring of the LD slag into the slag pot. The size of the quartzite sand used is 20mm. Then 57 kg per ton coke is placed to the top of the slag surface in the slag pot to raise the temperature by burning of the coke. The oxygen is then blown at a pressure of 12 kg/sq.cm in the slag pot for about 8.2 minutes through the oxygen lance in the slag pot to burn the coke to provide the required temperature to melt the slag, resulting in opening of the slag crust for further blowing of oxygen into the slag beneath the top slag crust and thus facilitating the slag for proper dumping in the slag yard by maintaining the slag fluidity by minimizing the basicity to about 2.64 and increasing the temperature.