1 Silica Crown – Corrosion & Innovative Countermeasures By GT Lim et al

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Silica Crown – Corrosion & Innovative Countermeasures

By

GT Lim et al

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Silica Crown Corrosion

The used of high purity silica brick for the crown lining of the conventional fired furnaces had been well established over the years due to its proven resistant to corrosion.

The main causes of corrosion are;

Poor furnace operation and / or maintenace

Advance firing system by injecting oxygen or oxy-fuel firing;

Open joints.

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Pic 1: CRT Panel Crown – effect of preferential corrosion

Pic 2: CRT Panel Crown – effect of Oxy- fuel firing

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Joints between refractories, rather than refractories themselves, are the areas most vulnerable to corrosion.

Open joints in the crown can result from any or all of the following;

The actual process of making the refractories itself;

How well matched the brick contact surfaces are;

How thin the mortar joints can be;

How well the expansion joints are sealed during installation;

How well the heat up schedule is carried out;

How well the expansion joints are sealed after heat up.

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Silica Crown Corrosion - Refractories

Joints poss as the initiation of ratholing as follow;

The flatness of bricks’ surfaces is another concern as shown below;

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Silica Crown Corrosion - Dimension

Refractories’ dimensions tolerance poss another threat to the ratholing as shown below;

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Silica Crown Corrosion - Countermeasures

The effects of corrosion can be minimised by the followings;;

The correct pressing and firing of the silica shapes;

The use of a silica quality with no residual quartz;

The use of a special, larger crown wedge – JUMBO WEDGE;

The use of an enhance corrosion resistant silica in critical areas or with Oxy-fuel firing – HEPSIL VOF2®;

To use of a specialised expansion joint sealing system – LOKnGO®

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Silica Brick Manufacture – Pressing

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Fig 1: Vesuvius’s edge pressing on non-critical face compared to conventional side pressing on critical face

end/edge pressing flat/side pressing

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Fig 1a : Implications of pressing method on dimensional tolerances

8 end/edge pressing flat/side pressing

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Fig 1b: Implication of pressing method on surface flatness;

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Fig 2: the pressing mechanism also play an important role;

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The impact pressing mechanism enables the green brick to have equal compacting effects and make the brick stronger due to the high pressure of the impulses and vibrations helping a good placement of the grains

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The combined effect of the edge pressing method specially developed by Vesuvius, as per Fig 1, and the use of an impact press, as per Fig 2, results in consistent accurate wedge dimensions, without any grinding or finishing, which ensures the following;

Excellent surface flatness ( +/- 0.8mm warpage);

Excellent taper tolerance ( +/- 0.5mm);

Minimal mortar joint thickness since even the dipping method can be employed, if necessary.

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Silica Brick Manufacture – Residual Quartz

It is a well established fact that green bricks made from a too pure quality raw material will not achieve complete transformation of the quartz during firing.

The crystallographic shape of the quartz raw material as illustrated here;

This shape is transformed through 22 complex stages into cristobalite and tridymite crystals during firing.

Any untransformed crystals remain in the brick matrix as residual quartz.

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If the original quartz crystal is not fully transformed during firing, the expansion of the finished brick will not have been completed and the brick structure will not have become fully stabilised.

This incomplete transformation will be completed in the crown lining as a result of furnace heat up, creating open joints and possible corrosion.

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Consequences of the presence of residual quartz after

installation and heat up


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Consequences of using silica bricks containing residual quartz

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To ensure no residual quartz in the final product, Vesuvius has identified, from their extensive experience, that the following aspects of the firing process are critical.

The use of a ring kiln;

An extended firing cycle -- 3 to 4 weeks

The use of pulse burners to ensure accurate temperature control and to minimise thermal shock;

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Countermeasure 1 -- JUMBO WEDGE

The JUMBO WEDGE system is based on the minimising of joints within any given crown compared to conventional European and Imperial standard wedges.

The average taper of each Jumbo Wedge is 110.5 mm, resulting in fewer radial joints when compared to existing standard wedges and a constant wedge weight per wedge throughout any given crown.

The combination of longer bricks and a wider taper results in a

reduction of up to 35% in the number of wedges/m2 of crown surface, and a similar reduction in the number of joints.

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Vesuvius can redesign your existing crown to utilised Jumbo Wedges without any additional charge;

The combination of thinner and fewer joints ensures that a Jumbo Wedge crown minimises possible joint attack and subsequence « RATHOLING »

In addition, the use of larger Jumbo Wedges allows quicker installation which can save 1 - 2 days in comparison to conventional smaller wedges.

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Countermeasure 2 – HEPSIL VOF2®

Hepsil VOF2® represents a new generation of silica crown refractories, initially developed to resist the more corrosive operating conditions present in Oxy-fuel fired furnaces.

Thereafter, it was determined that the enhanced properties of this new product could also be used successfully in areas of high preferential corrosion in conventionally fired furnaces

Through the combination of the careful selection of premium raw materials with sophisticated pressing method and an enhanced firing cycle, it has been possible to optimise physical properties, as follow;

Enhanced Bulk Density (1.88 g/cm3 normally 1.80-1.83)

Reduced Apparent Porosity (18% normally 20 – 21%)

Increase Cold Crushing Strength (60 N/mm2 normally 30-33)

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To illustrate the advantages of this new product, Vesuvius developed a specialised simulation test to compare and contrast Hepsil VOF2® against other silica products in an Oxy-fuel firing environment, and discovered the following;

The silica brands tested exhibited widely varying degrees of corrosion resistance;

No correlation was found between corrosion resistance and flux factor;

Corrosion resistance depends primarily on textural properties such as porosity(1), pore size distribution(2) and grain size distribution(3);

The corrosion resistance of HEPSIL VOF2® is twice as good as that of its nearest rival

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Countermeasure 3 – LOKnGO®

Correct joint sealing is essential for successful crown lining performance.

Even using the correct product and installation conventional sealing methods do not eliminate the weakness of the expansion joint area nor its vulnerability to corrosion.

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Certain of these conventional sealing methods are illustrated

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The Vesuvius LOKnGO® shape is perhaps the first ever brick shape specifically designed for this particular application.

The special LOKnGO® shape is intended to provide the maximum contact surface for whatever crown radius is involved.

LOKnGO® Is designed to be installed easily, efficiently and effectively even in an uncomfortable working environment.

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Sealing Piece

Expansion joint

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This advantages of the special LOKnGO® shape are as follow;

It is independent of the radius and thickness of the crown;

It is independent of the shape or size of the expansion joint;

It can be manufactured in a variety of different refractories materials;

It is simply inserted in the joints and tapped home to provide a firm support onto which the sealing material can be rammed after the heating up;

It is light and provides good protection from fumes and radiant heat from the furnace while the sealing patch is being rammed into place.

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Countermeasure 4 -- Hot Repair

In the event of ratholing or localised crown corrosion, one option is to undertake the hot patch repair

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The convention method of plugging silica crown holes is to use suspended fused silica bricks in conjunction with silica or zircon patch on the cold side of the crown.

While this method can satisfy immediate requirements, it does not allow patching the hot side of the crown where it is most needed.

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Vesuvius markets the HGF-International patented method of hot repair utilising a suspended “UMBRELLA BRICK” which is inserted into the crown hole, after which it unfolds like an umbrella, creates a horizontal base on the hot side of the silica crown, ready to receive patching mix to completely fill the hole around and on top of the bricks as shown below;

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Patch

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If the hole is too big for a single brick, several can be used as follow, to effectively sea the entire hole;

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The shape as shown below;

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The Umbrella Brick can only be used for crown lining thicknesses up to 400mm, after which a newly developed patch block must be used as follow;

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Hanging hook

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The design principle is similar to the Umbrella Brick in ensuring that a firm base is provided for the effective installation of the patch mix;

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Acknowledgement

I would like to thank the Directors of Vesuvius for their permission to present this paper as well as my colleagues in Vesuvius Glass in their detail contributions.

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Documents

1 Silica Crown – Corrosion & Innovative Countermeasures By GT Lim et al