Purging is carried out in ladle by the injection of inert gas mostly Argon which facilitates rinsing of liquid steel. Argon (Ar) gas is preferred for rinsing owing to its inert nature apart from its low solubility in steel. The gas moves from the bottom of the ladle to the top slag and expands due to heating and decrease of the ferro-static pressure during its course of ascend.
The rinsing of liquid steel is meant to carry out major functions like homogenizing temperature, composition, and promoting slag metal interaction for refining to take place. The rinsing of liquid steel also helps in the floatation of non-metallic inclusions. Argon gas bubbling at moderate gas bubbling rates, e.g. less than 0.6 N cum/minute was successful in achieving homogenized temperature and composition.
Ar gas is introduced in the teeming ladle through a bottom purge plug. However, when the plug is out of commission or the plug is jammed, a deeply inserted refractory lance from the top into the molten steel bath brings about the rinsing process. Normally rinsing operation is performed by introducing argon gas through the porous plug arrangement located in the bottom of the teeming ladle and the top lance mechanism serves as a backup means for liquid steel bath rinsing. But rinsing with a top lance has shown the decreased free open performance of the ladle at the caster. The top refractory lance can be ‘T’, ‘Y’, or straight bore type.
The liquid steel which gets stratified in the teeming ladle due to the additions of the ferroalloys and the carburizer in the teeming ladle at the time of tapping of the steel gets agitated by the purging of Ar gas. Ar gas purging helps to generate enough bath turbulence to facilitate rapid thermal homogenization.
Some shops also utilize electromagnetic induction stirring in the ladles. These have been found to have better stirring homogeneity, the ability to reverse the direction of the stirring forces which is useful for alloy additions, and stirring without breaking slag cover and exposing the steel to the ambient oxidizing atmosphere. Although the use of electromagnetic stirring is beneficial from the metallurgical point of view but these benefits are offset by the high capital cost, including ladles equipped with stainless steel panels which comprise at least one- third of the ladle shell.
Stirring with argon has a substantial effect on the mixing rate for chemical additions. The most influential factors include gas purging rate, heat size, degree of superheat, amount of carried over slag or synthetic slag or ladle covering compound, amount of mixing needed for chemical additions.
The intensity and the effectiveness of the Ar gas purging in the teeming ladle is determined from visual observations by experienced operators by looking at the bubbling or slag-metal bath turbulence.
A gentle Ar rinsing promotes floatation of nonmetallic inclusions since the high-pressure bubbles of the Ar gas acts as the carrying agents which take the non metallic inclusions towards the surface of the slag. The effectiveness of Ar rinsing also determines the caster performance. Non metallic inclusions which unless removed may interrupt casting operations by means of nozzle clogging(Eg- formation of alumina inclusions) and also interfere in clean steel productions. Thus, clean steel production and good castability is directly dependent on a consistent rinsing practice at the secondary metallurgy section.
The gas supply connection to the teeming ladle can be done either manually or automatically with an auto coupler system made when teeming ladle placed on the transfer car at the charging position of the rinsing station.
The stirring efficiency of the ladle is effected by
- Ladle jam and improper plug cleaning.
- Channeling of argon gas resulting in lower than expected rinse rate.
- Leakage in the argon supply system.
- Existence of variable back pressure due to changing plug conditions.
Disadvantages associated with the failure of proper argon rinsing are poor castability, higher oxygen ppm, improper deoxidation, reduced ladle free opening performance, inadequate inclusions removal or hindering the production of clean steel, excessive argon flow and refractory wear at the slag line.
The fundamentals of Ar rinsing are based on mass transport control which is dictated by convection current in the system. Homogenization of bath temperature and composition by gas bubbling is brought about by the dissipation of the buoyant energy of the injected gas. A convection current is generated because of the buoyancy of the inert gas introduced in the high-temperature system (around 1600 deg C). The slag metal reaction rates are controlled predominantly by mass transfer of the reactants and products across the slag metal interface. In argon rinsed steel bath systems, the slag metal interfacial area is affected by the degree of agitation in the steel bath which is determined by the rinsing power.
Pluschkell derived a relationship for the thermodynamic relationship which describes the effective stirring power and the equation is given by:
Where
e is the rinsing power in W/ton
V is the gas flow rate in N Cum/minute
T is the bath temperature in Kelvin
M is the bath weight in ton
H is the depth of gas injection in meters
Po is the gas pressure at the bath surface in atm
The mixing time t is the time taken to achieve 95 % homogenization.
Mazumdar and Guthrie derived a relationship expressing the mixing time, t (s), in terms of the stirring power, e (W/ton), ladle diameter, D (m), and depth of injection, H (m) and is given by
It can be seen from the graph that a 200-tonne heat will be homogenized in 2.0–2.5 minutes at an argon flow rate of 0.2 Nm3/min.
Plug location
Placing the bottom plug off-center, e.g. at mid-radius, has been successful in decreasing the mixing time. Eccentrically located bottom plugs help in the circulation of metal throughout the bath, thereby avoiding dead zone formation. Whereas a centrally located stirring plug generates a toroidal loop of metal flow in the upper part of the bath while a dead zone is created in the lower part, which is considered to be a reason for the longer mixing times.
Eccentrically located plug impedes the mass transfer between metal and slag whereas centrally located plug results in increased slag metal emulsification with increasing gas flow rate. The eccentrically located plug creates a slag free zone, known as the eye, close to the teeming ladle wall which affects the detachment of slag particles from the main slag phase and decreased emulsification. Therefore, the final choice of location of the plug, therefore, appears to be determined by which aspect of stirring is more important for a given operation.
Porous plug assembly
The porous plug assembly consists of a housing block, sleeve, porous plug, plastic, shell, C clamp, Tie-down system. Sleeves provide stability to the plug and the air setting plastic provides good strength and minimal shrinkage in service to prevent any joints or looseness in the material. Mortar applied on the sides of the plug provides a horizontal allowance for the expansion of the plug but is susceptible to steel/slag penetration.
Different types of porous plugs
The figure shows the standard shapes of 6 types of porous plugs. Standard shapes of isotropic plugs: (a) and (b); component plugs: sliced (c), concentric (d); and capillary plugs: conical (e) rectangular (f).
Porous plugs have either a conical or a rectangular shape. The conically shaped plug provides more ease to change when the plug wears out. Rectangular plugs are advantageous when the life of the plug is the same as that of the lining. Production of the element in two or three components stacked with metal can improve the performance of isotropic plug. The advantage of the capillary plug is that the plug can be made of the same refractory as that of the lining brick which results in better hot compression strength, erosion resistance, and service life. However, they are more prone to infiltration by liquid steel when argon gas pressure is lost.
Online Plug change
The argon plug can be changed offline or online hot although the preference is given for changing hot to avoid cooling down of the ladle lining with associated thermal shock and additional heating required to bring the ladle back in service.
The following things are to be ensured for the online repair method:
· Ensuring the ladle bottom is clean of all the slag/metal at the bottom.
· Having a design that allows access to the plug for repair.
· Either pushing the plug out from the front to the back with a large piece of machinery and a good operator or digging the plug out from the backside with a jackhammer or other handheld device.
· After this the cleaning of the block to the original surface is necessary to get a clean fit.
· A plug/sleeve combination that has been pre mortared to the correct mortar joint thickness is to be installed in place and everything is to be reconnected.
© Metal world insight
