Slag splashing technique in converter operation
Slag splashing technique in converter operation
The erosion of refractory lining of a converter has a major contribution for the low lining life. Erosion occurs because of chemical erosion due to attack of slag and molten metal on the refractory of the converter at the high operating temperatures and because of thermal shocks as well as due to mechanical wear. Slag splashing technique has been developed to counter this erosion and produce a freeze lining.
Today slag splashing has become a powerful tool not only for increasing of the lining life of the converter but for increasing of the converter availability and maximizing of production besides reducing of the refractory and gunning costs.
Slag splashing technique was first developed in 1970 but was not put to large scale use. The Indiana Harbour plant of LTV steel was first to report success in 1992 with respect to improvement in the lining life by the use of this technique. Slowly this technique was used in the other steel melting shops of the world. Inland no. 4 BOF shop has reported a lining life of plus 60,000 heats.
The slag splashing steps are as follows
- At the end of the previous heat the liquid steel is tapped in steel teeming ladle and molten slag remains in the converter.
- The converter operator visually inspects the slag condition to determine the quantity of slag conditioner to be added.
- The converter operator visually inspects the converter lining to determine if any specific area of the lining needs special attention.
- The molten slag is conditioned with respect to its temperature, FeO and MgO contents by the addition of a conditioner in required quantity.
- The converter is rocked for slag coating of the charge pad and tapping pad.
- The oxygen lance is lowered to a predetermined level and the nitrogen flow is started.
- Nitrogen gas at high pressure is blown through the lance over the conditioned molten slag for it to splash over the lining of the converter. The splashed layer gets deposited on the lining surface
- The height of the lance is changed by the converter operator to get slag coating on entire converter or it is kept in fixed position to slag coat a particular area.
- The converter operator determines the time of nitrogen blowing which is around 2-4 minutes.
- After the nitrogen gas blowing, the gas flow is stopped and lance is lifted.
- The remaining slag is dumped to avoid excess build of converter bottom and converter is ready for next heat.
A schematic diagram of slag splashing is shown in Fig 1
Fig 1 Schematic diagram of slag splashing
The splashed layer on the refractory lining acts as a working lining layer in subsequent heat/heats and hence protects the original refractory.
The nitrogen supply parameters namely pressure and flow rate, general slag condition and consistency of operation are the three major factors for the success of slag splashing. The flow rate and the pressure of nitrogen depend on the design of the oxygen lance used for oxygen blowing in the converter.
Slag quality is important to ensure the success of slag splashing technique. A slag with low FeO content and with high MgO content and having a creamy to gummy consistency is desirable for slag splashing. Slag FeO level of around 13% and MgO levels of 8% to 14% are recommended as an acceptable range for slag splashing.
Flux charges are to be adjusted so as to reach at least the MgO saturation point of the slag. A high MgO content of the slag ensures that the thermal properties of the slag are similar to those of refractory material. The improvement in slag condition is achieved by the addition of calcined dolomite or calcined magnesite after tapping. In case of high FeO slag, coke is added to reduce the level of FeO in the slag before slag splashing.
Achievement and maintenance of optimum slag conditions is one of the key to success in using the tool of slag splashing.
Factors affecting slag splashing
The following factors affect the slag splashing.
- High velocity nitrogen gas results in both agitation and the formation of crater. When the crater depth reaches a critical value slag droplets are ejected. This is due to the high shear forces generated by the high velocity jet. The velocity and the angle of the ejected slag is dependent on the jet characteristics (momentum, flow rate, height, angle and nozzle) and the nature of the cavity.
- The amount and the location of the slag coating is dependent on the mass and the size of the slag globules, its velocity and trajectory angle and the pattern of gas flow in the converter. The heat transfer affects the adhesion of the slag on refractory
- The amount of slag splashed increases with the increase in the gas flow rate.
- The erosion of protective splashed slag layer is dependent on the viscosity of the liquid slag and hence on the temperature inside the converter and the MgO content of the liquid slag. Lower temperature and higher MgO reduces the erosion.
- The amount of slag splashing increases as the viscosity of the slag decreases.
- The lance height has a marked effect on the slag splashing process since the lance height affects the shape of the cavity and the wave form. Decreasing the lance height results in a larger recirculation zone in the cavity and hence less slag washing and ejection. Increase in the lance height increases the amount of slag splashed up to a maximum value beyond which it decreases. Hence for good slag splashing it is necessary to have optimum lance height.
- With the increase in the number of nozzles in the lance results into lesser amount of slag splashed in the central region of the nozzle and with more uniform slag splashing.
- If the lance is inclined with respect to the vertical then it generates larger shearing forces, alters the shape of the cavity, increases the slag transferred during ejection mechanism, reduces the vertical component of the velocity but increases its horizontal component. This results into higher amount of slag hitting the wall and lower amount of slag going to the upper area of the converter.
- Increase in bath depth results in a higher mass flux of the slag in all the region of the converter but it is more in the central region.
- The required amount of slag splashing for satisfactory coverage is dependent on the size of the converter. It increases with size.
- A higher superheat of the slag (difference between tapping temperature and the liquidus temperature of the final slag) results into thinner slag and faster melt back of the protective slag layer.
- For good slag splashing it is necessary that there is a good balance between the low melting and the high melting components in the slag. The low melting phase is rich in FeO, acts as a binder and contains most of the sulphur present while the high melting phase provides the necessary protection to the refractories and contains around 10-15% FeO, enhanced value of MgO and a CaO/SiO2 ratio of 2.5.
- The amount of slag splashing decreases with the increase in slag density
- The effect of various parameters is shown in the Table 1.
Table 1 Effects of various parameters on the slag splashing. Upward arrow shows an increase while the downward arrow shows a decrease. No. Of arrows shows the magnitude of the effect
Normally same lance tip is used for both oxygen blowing during steel making and for nitrogen blowing during slag splashing. These lance tips are normally designed to match the converter shape and melting requirements. If the lance tip port angle is narrow then there is a possibility that slag splashing may cause build up at the cone and mouth. It will also cause big build up on the lance surface. If the lance tip port angle is wide then the slag will only be splashed onto the lower part of the furnace. Hence for successful result during slag splashing it is necessary to have an optimum tip design.
Slag splashing is to be carried only after ensuring there is no steel in the converter otherwise heavy skulling will result. Also the control of the lance height is an important parameter for the success of slag splashing.
Advantage of slag splashing
- Increase in the lining life of the converter.
- Yield improvement due to lesser slopping because of increase in the converter volume.
- Lesser consumption of flux because of dissolution of basic slag during the steel making process.
- The melting of the low melting phase of the slag lining results in the rapid formation of a basic slag and the rapid dissolution of the CaO from slag coating by SiO2 in converter slag. This leads to rapid de- phosphorisation.
- Slag splashing helps in recycling of the steel making slag.