Slag Monitoring and Detection System (SMDS)

The Need

Steel making in modern integrated steel plants is performed in large-scale refractory-lined vessels known as Basic Oxygen Furnaces ( BOF ). During the processing, a layer of heterogeneous liquid, non-metallic material called slag is produced. Being lighter than the liquid steel, it floats over steel.

 

Steel is removed from the vessel through a side opening, known as the tap hole. The liquid steel is poured into a transfer vessel (ladle). An operator manually controls the entire pouring. Judgment about the slag content in the tap stream is based on visual observation. The steel stream is very bright because its temperature is about 1650deg C. (~3000deg F). It is nearly impossible for an observer to directly observe any details, let alone brightness variations of the stream. Even using a furnace glass filter in the scene, visual contrast is quite small and it requires considerable training for an observer to distinguish slag from steel by their brightness differences; slag being brighter than steel, because it has a higher emissivity. Routine observations have been made and found that the system is subject to the person-to-person variations and similar problems associated with any observer-based judgment, e.g.: concentration and awareness level, reaction time, etc. Hence, some slag inevitably carries over into the ladle.

 

Minimizing the outflow of converter slag into ladle is one of the fundamental preconditions for improved efficiency in the further treatment of steel. Presence of the converter slag in ladle could raise the following problems:

 

  1. Alloy and conditioner additions are hindered by presence of a slag layer.
  2. A high level of FeO and MnO (slag constituents) result in high Oxygen content of steel leading to increase processing time and treatment costs.
  3. Phosphorus and silicon reversion occurs.
  4. Ladle de-sulpherisation is hampered.
  5. Poor yield.
  6. High inclusion formation.
  7. Increased risk of caster nozzle clogging.
  8. High wear of ladle refractory
  9. Increased consumption of Aluminum.
  10. Steel quality deterioration.

 

The Concept

A new concept of detecting percentage of slag in the steel ladle at BOF has been introduced in Tata Steel Limited. This concept is based on differentiating between different metals based on the emissivity of these metals. This system works with Infrared (IR) camera that deals with the thermal image of the object. SMDS is capable of identifying the steel-slag transition under varying operating conditions.

 

Schematic of a slag monitoring and detection system

 

SMDS works in the online, real time mode. The image of the metal stream is captured by an infrared digital camera, which is connected to an image processing hardware. The thermal image of the tapping stream is displayed on a video monitor for the operator. The stream image is continuously analyzed to determine the percentage of slag. Once the slag percentage in the image is detected, and using the permitted slag percentage value, logical algorithms are executed. A sub window shows the extent of slag in digits and on a bar graph. Several user accessible data are also displayed via interactive screens. An alarm is generated at a preset value of slag percentage in the stream for the vessel operator.

 

The theory behind the detection system is shown in the graph where there is a significant difference in emissivity of the slag and steel at higher wavelength. The IR camera used for slag detection system detects this difference to calculate the slag percentage in the molten steel.

 

steel pouring

"Steel Pouring" in progress

slag pouring

"Slag Pouring" in progress

 

Features

  1. Its a non-contact type method and independent of operator involvement, thus safer compared to other techniques.
  2. Adjustable threshold is available which the operator can set.
  3. Automatic control of vessel tilting.
  4. Accurate detection independent of charge weight.
  5. Improved yield.
  6. Steel purity increases.
  7. Provide tap termination consistency.
  8. Reduce the number of slag related non-conformance heats.
  9. Double wall enclosure cooling system using Vortex cooler.
  10. It uses a low maintenance solid-state camera.
  11. Images can be stored and analyzed at a later date
  12. Effective arrangement for transferring the signal from the infra red camera to the system.

 

Benefits :

SMDS provides the following benefits :

  1. Automatic detection of slag %.
  2. Eliminates manual intervention by application of computer vision architecture to decision-making.
  3. Increases yield, quality and productivity.

Slag Monitoring and Detection System works in real mode and analyses the molten stream of metal to detect the slag percentage in it. Implementation of SMS system increases the productivity, performance and purity of steel. It is designed to give ease to the operator and improve the quality of product. SMDS is developed for simulation and real time working mode. Customization as per the local environment and local situation is required and will be done at the local commissioning site only.