Wednesday, October 2, 2013

1300 Tonne Mixer Section (Steel Making Department).

1300 Tonne Mixer Section
(Steel Making Department Pakistan Steel).

The Hot metal (molten pig iron) is supplied to steel making department from different tappings of Blast Furnaces, stored in 1300 Tonne Mixer. 

1300 Tonne Mixer section is located in the charging aisle of LD-Converters of steel making department.

As the tapping schedule of Blast furnace does not match with the charging of steel making converters, Mixer acts as a reserve stock of molten pig iron at steel making department. Besides it equalizes the varying temperature and chemical composition of different tappings of Blast furnaces and helps the LD-Converter operators to set the operating parameters for steady pace of production cycle. 

There are two railway tracks for transportation and receiving the hot metal from Blast Furnaces of Iron Making Department  to Steel Making Department.

Mixer is made in cylindrical shape with the length to diameter ratio of around 1:3.
Mixer rests on special pads fastened on a strong reinforced-concrete foundation and when being tilted, it turns on its horizontal axes with the aid of rollers. The rollers are mounted in cast steel frames attached to the shell of the Mixer. A huge rocker arm and pinion system allows mixer tilt for pouring out hot metal.

As per project the safety lining of 1300 tonne Mixer is of heat insulating fireclay bricks which diminishes the heat loss to the surroundings. The roof is lined with heavy duty fireclay  bricks and inner working lining (900 mm thick) is made up of  two layers of Magnesite bricks.

The permanent pre- heating burners are provided at both the end faces and nozzle of the Mixer to keep the temperature of hot metal intact. As per project design, for Phase- ii expansion to 2.2 million tone, a premise adjacent to existing Mixer was left for installation of second 1300 Tonne Mixer.

Capital Repair of 1300 Tons Mixer.
The campaign lining life of Mixer is about 0.9 million tons hot metal holding cycle and after that Mixer is stopped for Capital repair for about two months.

 The main causes for stopping Mixer for relining are collapse of roof fire clay brick work, wearing of slag line and tap hole zone which need improvement. In order to enhance the lining life, safety and improve the service condition of hot metal Mixer, the conventional brickwork has been replaced with monolithic lining in developed countries.

Capital repair of Mixer is a labour and material intensive project. The refractory demolishing and relining work is carried out by R. R. Dept. Pakistan Steel. During capital repair of Mixer, besides the relining work, the electrical, mechanical, heat & power equipments and automation instruments pertaining to mixer operation are repaired and overhauled.

During capital repair of Mixer, the BF hot metal ladles are directly poured into hot metal charging ladle of Converter which increases the use of overhead charging cranes thereby interrupts the pace of production and affects the quality and yield of steel production.

 Hot metal mixer plays important role in achieving pace of production and quality in the Steel making shops which are operating without Torpedo vessels and computer aided dynamic models.

 Sulfur and silica are harmful components in Mixer slag. The unwanted Mixer Slag is skimmed off from Hot Metal Ladles before dispatching the hot metal to converter to prevent its passage into the LD- Converter.

As per demand of Converter operator, hot metal is poured out from 1300 tonne Mixer into a refractory lined ladle positioned under the Mixer spout on a weigh scale platform. The Mixer Section has arrangements for weighing hot metal, temperature measuring and taking samples for hot metal analysis.

Technical specifications of Hot metal for steel making. 

As per Pakistan Steel’s specifications, the average Physical  temperature of hot metal poured out of Mixer should not be less than 1320 ÂșC and the Chemical composition is kept controlled within the limits as under:

-          Carbon not less than 4%
-          Si   : 0.4 – 0.8%
-          Mn : 0.5 – 1.0%
-          P    : 0.2% max
-          S    : 0.04% max

On the basis of Physical temperature and hot metal analysis, the ratio of the amounts of liquid pig iron and scrap and the quantity of additives ( lime, dolomite,CaF2  etc) and amount of required O2 are calculated and adjusted by the Converter operator to achieve the targeted end point temperature and chemical composition for each batch (heat).

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