Recommendations for commissioning and operating practice montaje horno

Once the assembly is complete the ladle must be moved as quickly as possible to the burner for pre-heating to prevent hydration of the lining.

a) Pre-heating of the lining.
Initially, the temperature must be increased at 100C/h for a period of 10 hours. The vertical burner must have enough capacity to maintain this temperature gradient. Initially, for the first three hours, the top of the burner can be left 200 mm away from the crown of the ladle so that there will be surplus air inside it and to reduce fume formation. For the first 7 hours the ladle must remain upright, as up until that point sufficient heat stress will not be achieved to prevent collapse if it falls. At this moment it can be placed in a horizontal burner. After the first 10 hours, the lining reaches 1000C and must be kept at this temperature for a minimum period of 3 hours to pick up enough heat in view of the high specific heat (1 kj/KgC at 1000C) and density of the material.

The total duration of a heating cycle may be 22 hours, although it evidently depends on the quantity of product to be preheated and on the power of the burner. Longer pre-heating times may cause oxidisation of the carbon present in the binders or graphite of the bricks causing weakening that may cause heat cracking between the decarburised area and the rest of the bricks or excessive wear in the first pourings.

b) Maintenance of the lining.
Carbon dioxide and water vapour are produced during the burner fuel combustion process. The latter can hydrate Dolomite at temperatures below 700C. Therefore, when a casting ladle is withdrawn and is kept hot under the burner while the production conditions are reached to start the pouring cycle, it should never fall below 750C. The ladle may be maintained at this temperature for the necessary time (normally less than 12 hours) and the temperature increased to 1000C in the 8 hours prior to the cycle being restarted.

c) Reheating of the lining.
For long shutdown periods (more than 12 hours) the ladle should be cooled.

To prevent hydration, it is a good idea to add lime to the inside for it to pick up the air in the inside of the ladle so that the latter will not act on the lining, and close the ladle with a lid, sealing the joints between the lid and top retaining plate. This sealing operation can also be conducted with lime.

The reheating will be restarted once the lime has been removed, from ambient temperature atmosphere to 1000C in 7 hours, although a minimum 3-hour heat absorption period will be required.

d) Spalling or heat cracks.
One of the problems of dolomitic linings are that they tend to lose part of the thickness of the hot face, between 10 and 20 mm. This phenomenon occurs in two situations:

1.- In new linings that have been pre-heated too quickly, reducing the pre-heating cycle time; the decarburised area detaches as it is not yet well-anchored to the area with carbon. The reason for the spalling is the lack of binder by decarburisation.

2.- In used linings submitted to a major heat shock; in this case the sintered hot face detaches due to the temperature it was submitted to during working. The reason for the spalling is the different thermo-mechanical behaviour between both parts.

To avoid this problem the pre-heating, maintenance and reheating curves must be observed.

e) ladle work cycle
Working with continuous cycles, i.e. increasing the number of heats per day per ladle and reducing intermediate stops gives the best results in dolomitic linings for the following reasons:

1.- After the first 4/5 heats, the lining has picked up all the heat possible and has greater thermal inertia due to its new heating capacity, and temperature drops are small throughout the cycle, thus eliminating the risk of spalling.

2.- The lining acts as a source of heat and this makes it possible to use lower furnace tapping temperatures as well as greater regularity in the temperatures of the ladle-furnace. This makes for energy saving in the furnace and in the refining process.

3.- The linings are less exposed to hydration as they spend less time at low temperatures, and therefore none of the lining is lost in calcium hydroxide powder. 4.- Steel filtrations are minimised as material contractions that may cause joints to open are avoided.

In order to make the most of these advantages, the number of ladles in rotation should be reduced, thus guaranteeing a good work cycle.
Belen Industriagunea, 1 - 20150 Aduna (Guipúzcoa) - Tel: +34 943.69.60.99 - Fax: +34 943.69.17.77 - Email: - Kelsen 2011

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