Ceramic Industry

SPECIAL SECTION/FIRING & DRYING: Construction & Repair Concepts

June 1, 2011
Colloidal silica-bonded refractory is often the only option for emergency repairs without shutting down hot equipment.

Figure 1. Separation gaps between brick joints allow gas flow and severe heat loss.

Industrial furnaces are necessary to perform the processes of sintering, transforming or melting the materials they are designed to produce. Many different types of industrial heat containment reactors exist, but, in general, most operate at temperatures as low as 500°F (260°C) and as high as 3200°F (1760°C).

Modern furnaces are designed to be more energy efficient than their predecessors. Though not every industrial plant has the latest and greatest furnace, every manufacturer wants to save energy. In today's competitive industrial world, optimizing the use of a furnace's capacity and energy is more crucial than ever before. The goal should be for the furnace to perform its task of producing a quality product with the least amount of energy and downtime.

Old furnace construction concepts and refractory materials entail pitfalls that can be avoided through newer, proven ways of saving time and energy with colloidal-bonded refractories. Furnace roofs and walls experience degradation over time due to the corrosive and erosive effects of the environment they are exposed to, as well as creep and thermal shock/thermal cycling damage. Furnaces that are constructed out of brick, blocks, and shapes suffer from built-in failure mechanisms and energy inefficiencies.

Gaps in brick joints, as well as corrosion and erosion.

Brick Joint Failure

Separation gaps between brick joints occur and allow gas flow and severe heat loss (see Figure 1). The brick joint is the first path for chemical attack. Industrial furnaces produce byproducts (or slags) and emit chemical gases that attack the joints of brick-lined furnaces. Severe damage can occur when a typical hot face brick lining's joints open up and expose the lower temperature-rated insulating brick or ceramic fiber-based lining to heat and corrosive gas.

Heat and heat-laden corrosive gases can even reach the steel shell of the furnace, causing warping and structural damage that will spread and cause more damage back toward the hot face brick. Once the damage starts, it is hard to keep it from spreading without shutting down the furnace. The wasted BTUs are bad enough, but with no way to repair without shutting down, the effect of heat loss in a section of a furnace can affect the quality of the final product. In order to repair or replace with brick, the operator is forced to shut down or take the furnace out of operation while tear-out and the subsequent repair or rebuild starts and ends days-or even weeks-later.

Repairing a deteriorated brick wall (left) with a new 70% Al2O3 colloidal silica-bonded hot face lining (right) eliminates joints.

Repair Options

Refractory monolithics provide the opportunity for fewer joints and have been used to repair brick linings for many years. Low-cement castables and phosphate-bonded, high-alumina rams, as well as sodium silicate-bonded mortars, can be used to patch joints or holes where brick have fallen out of a wall or roof. The problem with most of these products is that they all contain large amounts of low-melting-phase elements that lower hot strength. Low-cement castables also take many hours to dry.

Colloidal silica-bonded pumpables and shotcretes, on the other hand, can be used as a hot patch or over the coating material in an emergency repair where traditional monolithics are not feasible. Colloidal silica-bonded refractories often provide the best option for hot face linings because of their outstanding hot strength, creep and thermal shock resistance, as well as their resistance to chemical attack.

Figure 2. The emergency over-coating repair of a glass furnace crown where brick has fallen keeps the furnace in operation and provides immediate energy savings.

Colloidal silica-bonded refractories contain no cement, phos-acid or clay, and must use pure aggregate and fines in their matrix. In addition, colloidal silica-bonded pumpables and shotcretes feature outstanding installation characteristics that yield substantial savings in downtime for construction, reprofiles or repairs. Their installation rates are the highest in the industry, while their rebound rates for shotcretables are the lowest.

The majority (99%) of the moisture in a colloidal silica-bonded refractory is removed at 212°F (100°C), allowing the furnace to be up and running quickly. The fact that there is no chemically bonded water in the colloidal silica-bonded refractory allows it to be dried out quickly without steam spalling. Colloidal silica-bonded refractory can be pumped, shotcreted or grouted on, in or around a live furnace at very high temperatures (see Figure 2).

The ability of the nanometer-sized particles to penetrate the existing refractory substrate makes colloidal silica bonded-refractory ideal for patching and reprofiling furnace linings. The material can be continually reprofiled over itself for years.

The image on the left shows brick joints that have opened up, along with phosphate-bonded trowelable refractory peeling and spalling off. The image on the right shows a tunnel kiln's roof hot face brick failure, with opened gaps in the insulation brick layer. The bottom of this image shows the colloidal silica-bonded repair of the hot face roof.

An Effective Solution

The degradation of structures built using various designs with different material types and forming methods has been studied since the pyramids were built, and furnaces aren't immune to degradation over time. Furnace degradation rears its head in many forms: erosion, corrosion, cracks due to creep and stress fracture properties, thermal shock, and non-optimal design.

Monolithic colloidal silica-bonded refractory linings-combined with the proper anchoring and insulation-are a cost-effective way of saving time and energy. Colloidal silica-bonded refractory is often the only option for emergency repairs without shutting down hot heat containment equipment.

Bricks and shapes are energy- and capital-intensive to produce, and they require long lead times. Special shapes and specially formulated bricks are in stock in industrial user's warehouses everywhere, wasting capital. Alternatively, specially formulated colloidal silica-bonded refractories have short lead times and the same formulation can be used throughout a company's industrial furnace fleet, saving time and money. Colloidal silica-bonded refractories can be installed faster than brick and offer multiple repair and reprofiling opportunities that save time and energy.

For additional information, contact Magneco/Metrel Inc. at 223 Interstate Rd., Addison, IL 60101; call (630) 543-6660; fax (630) 543-1479; or visit www.magneco-metrel.com.