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Oxygen Blowing Lance and Lance Tips in Converter steelmaking

• satyendra
• April 12, 2013
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• CD nozzle, lance tips, Laval nozzle, mach number, multi hole lances, Nozzle, oxygen lances, single hole lances, Supersonic jet,

Oxygen Blowing Lance and Lance Tips in Converter Steelmaking

In the steelmaking by basic oxygen furnace (BOF), the removal of impurities such as carbon, silicon, manganese, and phosphorus etc. is carried out by oxidizing them with high purity oxygen (O2).  For this purpose, a water-cooled lance is used for the injection of a high velocity stream of O2 onto the molten bath. The velocity or momentum of the O2 jet results in the penetration of the slag and metal to promote oxidation reactions over a relatively small area. The most important part of the lance is the nozzle. The functions of the nozzle are (i) supply and distribution of O2, (ii) production of a gaseous jet, (iii) induce bath agitation, and (iv) produce metal droplets. The jet velocity and penetration characteristics are functions of the nozzle design.

Lance nozzle has a convergent-divergent (CD) design. The CD nozzle is also known as laval nozzle and is characterized by a flow passage whose cross sectional area decreases in the direction of the flow, attains a minimum cross section area and then increases further in flow direction. The minimum cross section area of the flow passage is called the throat of the nozzle. The CD nozzle design helps in accelerating the gas velocity to the supersonic velocities. The gas flow jet is sub-divided into potential core, supersonic, and subsonic regions. Within the core region, the velocity is constant. At the end of supersonic region the velocity becomes Mach 1. Downstream the velocity is sub sonic. The jet interacts with the converter environment and produces a region of turbulent mixing. The entrainment process increases the mass flow rate and the jet diameter and decreases the mean axial velocity as the distance from the nozzle exit increases. The impact force on the slag melt surface is reduced with increasing lance height. The jet length and the spreading angle are affected by the gas temperature and pressure as well condition of metal slag mix in the converter.

Lance nozzles are designed for a certain flow rate of O2, normally measured in N cum/min, resulting in a certain exit velocity (Mach number), with the required jet profile and force to penetrate the slag layer and react with the steel bath in the depression area. Supersonic jets are produced with the CD nozzles (Fig 1). A reservoir of stagnant O2 is maintained at a pressure P1.  The O2 accelerates in the converging section up to sonic velocity (Mach number 1) in the cylindrical throat zone. The O2 then expands in the diverging section. The expansion decreases the temperature, density, and pressure of the O2 and the velocity increases to supersonic levels (Mach number higher than 1). As the O2 jet leaves the nozzle into the converter, at a pressure P2, it spreads and decays. A supersonic core remains for a certain distance from the nozzle. Supersonic jets spread at an angle of around 10 degrees to 14 degrees.

Both the proper design of the nozzle and proper operation are necessary for getting efficiently the desired steelmaking reactions and to maximize the life of the nozzle. If a nozzle is overblown, which means that the jet of O2 is not fully expanded at the time it leaves the nozzle, shock waves develop as the jet expands outside of the nozzle. Useful energy is lost in these shock waves, and an overblown jet impacts the molten bath with less force than an ideally expanded jet.

Nozzles are under blown when the jet expands to a pressure equal to the surrounding pressure and then stops expanding before it leaves the nozzle. In this case, the O2 flow separates from the internal nozzle surface. Hot gases from the converter then burn back or erode the nozzle exit area. This erosion not only decreases the nozzle life, but also results in a loss of jet force, leading to a soft blowing condition. Under blowing and over blowing conditions are also shown in Fig 1.

Oxygen lance is subjected to the heating load in the converter from radiation, convection, and conduction. It is subjected to continuous corrosion by high temperature slag and splashing. Further during the converter blowing   molten slag particles gets solidified on the lance surface and sticks to the lance. These slag particles impact the transfer of heat to the lance.

Fig1 Mechanics of supersonic jet formation and overblown and underblown conditions at the nozzle tip

Type of lance tips

The lance tips can have either (i) single hole, or (ii) multiple holes. In a single hole lance tip the design of the laval type lance tip nozzle is simple. It comprises of contracting section, throat opening, and expanding section. At the throat section, which is at the inter section of the contracting and expanding sections, the cross section area is smallest. Throat opening diameter is the critical diameter and the area at the throat is the critical area. Presently single hole lance tips are no more under use.

Multi holes lance tips are most popular in converter steelmaking these days. In case of these lances, the lance tips can contain from three number laval type nozzles to nine number laval type nozzles. These lances are at angle to centre line of the lance. Four holes to six holes lances are more popular. These lances produce multiple strand supersonic jet at the nozzle outlet. Metallurgical process performance of these lance tips is very superior. However manufacturing of these lance tips is more complex. Normally components of these lance tips are manufactured separately and then assembled and welded. In this way the manufacturing is easy, dimensions are accurate and the operating performance is good. However for longer life of these nozzles, the high temperature of steels is to be avoided, there is to be good slagging and low position blowing is to be avoided. The important parameters in the design of multi holes lance tips are (i) O2 flow rate, (i) theoretical pressure of O2, (iii) Mach number of nozzle outlet, (iv) nozzle angle and spacing, (v)  expanding section and length of expanding section, (vi) throat opening section dimensions, and (vii) dimensions of contracting section

In the 1950s when the top blown converter size was limited to 50 t maximum then a single hole lance nozzle was being used for the blowing of the O2. With the passage of time the converter size went on increasing. This has necessitated increase of number of holes in the lance nozzle for better distribution of O2 over a larger surface of the bath in the converter. Presently 5 holes to 6 holes lances are very common. Even 8 holes lances are under use.

Oxygen is blown in the converter through a water cooled lance nozzle at high pressure and at supersonic velocity (Mach higher than 1). Supersonic jet of O2 from the nozzle helps higher entrainment of O2 in the molten metal. During the blow, a three phase dispersion consisting of slag / metal droplets / gas bubbles is formed. Fig 2 shows the comparison of the jet impact area on the bath for a three holes lance tip and a single hole lance tip.

Fig 2 Comparison of the jet impact areas for a three holes and a single hole lance tips.

The requirements of lance tips include (i) withstand ing of very high thermal and mechanical loads, (ii)  necessity of efficient cooling, (iii) to have high reliability and efficiency, (iv) easy to install, (v) need to have a precision engineered design to have the specific flow of O2, (vi) to have minimum manufacturing tolerances and not to have any manufacturing defect, and (vii) to have purity of material and grain structure which is required to provide an optimum of heat transfer and good surface strength.

The lance barrel is a series of concentric pipes, an outer pipe, an intermediate pipe and the central pipe for the O2. The material for the lance pipe is seamless pipe of low carbon steel and thickness to withstand the pressure requirements in the lance. The lance is to be structurally sturdy. Lance is to be designed to compensate for thermal expansion and contraction. The outer pipe of the lance is exposed to the high temperatures in the furnace. As its temperature increase it expands and the overall lance construction internally is constructed with O-ring seals and various joints, but can accommodate the thermal expansion and contraction while in service. The lance also is to have a stress-free design and it is to be built with such quality so that it is able to withstand the normal operating conditions existing in the converter.

Fig 3 displays the major components of the O2 lance. These include O2 inlet fittings, the O2 outlet (lance tip, which is made of a high thermal conductivity cast copper (Cu) design with precisely machined nozzles to achieve the desired flow rate and jet parameters. Cooling water is necessary in these lances to keep them from burning up in the converter. The lance is to ensure required flow of circulating cooling water.

Fig 3 Typical details of O2 lance and lance tips

The oxygen flow rate in the O2 pipe is not to be high since the pressure loss is approximately proportional to the square of the speed of the flow. Normally the O2 flow rate in the pipe is to be lower than 55 m/sec and the velocity of gas in the pipe line in the range of 0.1 Mach to 0.2 Mach. The typical design parameters of lance pipe for different converter sizes are in Tab 1

 Factors affecting performance of BOF lance

There are a number of factors which affect the performance and the efficiency of the lance. The performance of the lance depends on the conditions in the BOF. The hot metal (HM) silicon content is very important since this affects the amount of slag which is formed, the amount of slag which is to be penetrated by the jet.  The HM silicon also controls the amount of steel slopping in the BOF. The lance operating height is also very important and is to be included in the nozzle design calculations. If the lance is too low in the BOF, it is exposed to extremely high temperatures and the heat transfer from the cooling water is not likely to be sufficient to keep the face of the lance from melting or being burned away prematurely. If the lance is too high, the thrust of the jet becomes less efficient and the refining time needed is longer, and more O2 is required to achieve the necessary decarburization and steel temperature. The O2 flow rate is a design parameter which is sometimes limited by the O2 supply system, and/or emissions concerns. The exit velocity of the Mach number is also a factor which is used in the design of the lances. Generally the higher the Mach number the more forceful is the jet.

Important considerations are also to be given to the number of nozzles and the nozzle angle. When BOFs were originally developed, the first ones were operated with a single nozzle blowing directly down unto the bath. This caused a lot of slopping and molten material was ejected straight up the mouth of the vessel. Lance tip with three nozzles slightly angled were developed to minimize slopping, resulting in a high process yield. Currently many BOFs are operating with 4, 5, or 6-nozzle configurations. Fig. 4 shows the effect of increasing the number of nozzles and nozzle exit angle on the impact area in the bath. As the nozzle angle is increased more of the lateral force component, rather than a vertical force component develops, contributing to more stirring and agitation in the bath. However, if the lateral component of the jet becomes excessive, higher refractory wear is likely to occur.

Fig 4 Comparison of lance tip parameters

Factors affecting lance tip life

Long life of lance nozzle tip is beneficial for the economical operation of the BOF. However, in normal BOF working practice many individual parameters have their influence on the process, Example of these parameters are HM chemistry, slagging practice, lime quality, lance pattern, dynamic or static lance control, restrictions in the O2 supply pressure, and the shape and the volume of the converter. These also differ widely from plant to plant so it is only possible to give general recommendations based on a more or less ideal working practice in order to describe the general relationship between BOF process parameters and lance nozzle life.

The most vulnerable part of a lance nozzle is the so called lance head crown, which is exposed to temperatures of above 2,000 deg C when in use. Thus, the lance head crown is to be made of Cu of conductivity which is close to about 100 % conductivity. Normally, only forged copper can provide such a high conductivity. Due to casting restrictions, cast Cu lance nozzles typically have a minimum electrical conductivity of around 90 %. Electrical conductivity is directly proportional to thermal conductivity.

Lance tip life varies from shop to shop, depending on the various operating practices. A typical life of the lance is 200 heats, although there are some shops where upto 400 heats of lance tip life are being achieved. There are also steel melting shops which are not been able to achieve even 100 heats. Cooling water is critical for maintaining high lance tip life. The flow rate is to be maintained at the design rate. The cooling water outlet temperature is not to exceed 60 deg C to 65 deg C. The quality of water quality is also an important parameter. If the water is contaminated with oxides or dirt, deposits normally form inside the lance pipes and nozzle, resulting in a negative effect on heat transfer and this reduces the life of the lance tip.  Operating height is critical for achieving the penetration of the O2 jet in the liquid bath. However, if the lance height is too low, there is the possibility of erosion or melting of the face of the lance nozzle.

The under blowing of BOF converter results into erosion of the nozzle exit and failure of the lance nozzle takes place. Excessive skull buildup on the lance nozzle is required to be mechanically removed or burned off. Both of these practices can cause damage to the lance.