What should we monitor during the melting?

– Do not raise the temperature beyond what is strictly necessary.

– To control the melting process.

– Keep the metal for the shortest possible time.

– Maintain the shortest possible time the furnace in a sloping position or spillage with molten metal.

– Perform the necessary strain and return the furnace to its work position.

– If it is possible, empty the furnace completely, without leaving feet of baths.

– Not desulfurize in the furnace with a silicon lining.

– Remenber that in the crown:

– Minor degree of sintering.

– Impacts of the metallic load.

– Regrowth of slag.

– Avoid loading dirty and rusty scrap.

– To verify that the returns are exempt from moulding sand.

– To make sure that no scraps contain aluminium.

– To avoid the additions of ferrosilicon with high aluminium content.

– Abrade the furnace after every load.

– Not work with full power with low levels of bath.

– Assure that the whole load penetrates in the broth before covering the furnace.

–  To verify that there no anomalous vertical expansions in the lining.

 

What should we monitor at the start of melting?

– To verify correct melting of the metallic load realized in the sintered one.

– Indispensable to have maintained for 1 hour, 50ºC above the nominal temperature of strain.

– Check thtaa the vent-walls is in good condition and there is no crack formation. If any apply a plastic mass in the zone as INSEPLAST 70 PF.

– To verify that quartzite expansion has been corrected

– Do not recarburate with low feet of baths in the nascent linings.

– In the first strain and successive strains, empty the furnace and verify the superficial aspect of the lining. It must be homogeneous and exempt from cracks.

– To assure that in the deslagging practice, the permanency time of the slag coagulant is strictly necessary.

 

Why do the walls at the bottom of the furnaces wear away more?

When melting with the foot of bath the most frequent wear takes place in the bottom part of the furnace. This type of wear it is known as elephant leg. By adding a new load for melting and putting power, a part of this one warms the metal of the bottom doing that this raises its temperature favouring the natural chemical wear of the silicon lining.

What can happen?

Refractory wall gets lost and it must be controlled with inspections.

What to do?

If it is possible, avoid working with foot of bath.

Why do vertical cracks appear in the cooling?

The formation of cracks is inherent in the nature of the raw material quartzite when the silicon lining are done. They are contractions and volume changes involving breakage of the lining.

What can happen?

The vertical cracks, normally they close.

What to do?

– Ideally working in 3 shifts and the furnace does not cool.

– To observe the closing of the cracks putting the furnace back on track.

Why slag may appear adhered to the lining?

The slag adhered to the silicon lining carries an inefficient running of the furnace. It can be low or high melting point.

Low melting point. When the furnace works at low temperatures silicon oxide is formed and it sticks to the walls of the lining as:

Si metal + O2 > SiO2 slag

High melting point. Slag difficult to eliminate are formed because these have higher melting point than the own lining:

2 FeO oxidized load + Si metal > 2 Fe + Si O2 slag.

Al metal >Al2O3 + SiO2 lining >Al2O3 SiO2 mullitic slag.

What can happen?

In any of two cases the furnace loses capacity, increases the risk of cracks forming, presents and irregular melting with distinct areas of magnetic behaviour , in short the life of the furnace decreases.

What to do?

It is indispensable to analyse the reason of this regrowth.

With low point of melting slag, raise the metal level and proceed to raise the temperature up to eliminate it. With high point of melting slag, it is indispensable to analyse the load and used additives. Often high concentrations of Si appear in FeSi causing of this problem.

 

Why do horizontal cracks appear in the lining?

There is no single answer. There are several possible reasons that we can number as being responsible for the appearance of these cracks:

–  Lining  compaction inappropriate.

– Segregation of the lining.

– Loss of thin through the coil.

– Hanging of the lining from the pan.

What can happen?

Most likely metal infiltration and as a result the furnace should be demolished immediately.

What to do?

Analyse how it has been possible that it has been formed together with the supplier of the lining.

Why does Zn appear against the mica during the demolition of the lining?

The presence of Zn in the demolition is associated with its presence in the used load and with the incorrect melting of the same one in the furnace of induction.

The Zn  volatilizes at 919 ºC going from solid form (of galvanic sheets) to gas. In gas form it penetrates in the pores of the lining and having found its isotherm of solidification at 419 ºC solid form happens again.

What happen?

If coils are in poor condition Zn can be resposible for arches between turns.

What to do?

– Improve the densification of the lining.

– Proceed to change the way fo providing the metallic load. The galvanized package should be introduced once the furnace has enough metal and temperature.

 

 

What is the difference between acid and boric oxide?

Boric acid is the hydrated form (it contains water) of boric oxide (anhydrous form or free water). Boric acid H3BO3 at low temperature (less than 150ºC) decomposes to boric oxide (B2O3) and water (H2O) representing approximately 44% by weight.

This means thta when we have 10 kg of boric acid in a lining it will descompose releasing 4,4 kg of water.

This fact should be taken into account when the suitable percentage and the definition of the curve of sintered are chosen.

From the point of view of thermal behaviour the major dispersion of boric acid makes that the lining presents a better light mechanical resistance than the similar percentage in boric oxide.

Why do small black balls appear in the lining when it is demolished?

Carbon monoxide produced of the oxidation of the carbon of the metal spreads across the porous zones of the silicon lining where it dissociates itself depositing coal.

2C metal + O2 (air) >2 CO (gas) > C + O2

What can happen?

It does not have effect on the refractoriness of the mass. However, in old or poorly conditioned coils it can cause arches between turns.

What to do?

– Avoid overheating.

– Do not put the graphite into the empty furnace.

– Increase the density of the area by improving material quality and / or vibration.