Carbon Industry Calcium Silicate Insulation Products

Hard silica-calcium composite brick for anode baking furnaces. Resistant to thermal cycling and fluoride vapour. Mingfa, Laizhou, Shandong, China. Founded 1991.

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Insulation Needs in Carbon Baking Furnaces

Anode baking furnaces for the aluminium smelting industry operate at 1100-1250°C in the firing zone. Flue gas temperatures in the combustion channels reach 1200-1300°C. The standard ring furnace design — whether open-top or closed-top — circulates sections through preheating, firing, and cooling zones over a 14-28 day cycle. Each furnace section experiences 50-100 thermal cycles per year.

This thermal cycling regime differentiates carbon baking from other high-temperature industrial processes. A glass furnace holds relatively steady temperature over an 8-12 year campaign. A carbon baking furnace section cycles from ambient to 1250°C and back to ambient every 14-28 days for 15-25 years. The insulation material must maintain dimensional stability and thermal performance through thousands of such cycles.

A second requirement specific to carbon baking is resistance to fluoride vapour attack. Pre-baked anodes contain approximately 0.5-2% fluoride compounds (primarily cryolite and aluminium fluoride) recycled from the smelting process via anode butt returns. During baking, these fluorides partially volatilise. Sodium fluoride and hydrogen fluoride vapour can chemically attack certain insulation materials — particularly aluminosilicate ceramic fibre products, which undergo devitrification and embrittlement in fluoride-containing atmospheres at these temperatures.

Calcium silicate insulation, being based on CaO-SiO₂ chemistry rather than Al₂O₃-SiO₂ chemistry, shows markedly better resistance to fluoride degradation. The calcium component forms stable CaF₂ on the exposed surface, which passivates the material against further attack.

Hard Silica-Calcium Composite Brick for Carbon

Mingfa's silica-calcium composite brick for carbon baking applications is supplied in density grades of 400-600 kg/m³, with compressive strength of 4 MPa minimum. The temperature rating is 1000°C for continuous service. This suits flue wall backup and headwall insulation positions, where the refractory hot face (typically 230mm fireclay or high-alumina brick) absorbs the peak 1250°C flue gas temperature and the composite brick sees approximately 800-950°C on its hot face.

The brick is manufactured through the same autoclaved hydrothermal process used for all Mingfa calcium silicate products. Quartz and lime react under saturated steam at approximately 190°C and 12 bar pressure to form xonotlite (6CaO·6SiO₂·H₂O). Additional siliceous reinforcement — finely ground quartz of controlled particle size — increases the compressive strength and abrasion resistance beyond that of standard calcium silicate board.

The composite brick's porosity of 77-85% creates the insulation performance. Pore sizes are predominantly in the 0.1-5.0 micron range, as measured by mercury intrusion porosimetry. This pore size distribution is critical: pores in this range are small enough to suppress convective heat transfer within the material (the mean free path of gas molecules at 1000°C is approximately 0.3 microns) but large enough that the material does not behave as a nanoporous desiccant.

Property	Value	Test Method
Density (dry)	400-600 kg/m³	GB/T 5480
Compressive strength	≥4.0 MPa	GB/T 5486
Thermal conductivity at 100°C mean	≤0.10 W/m·K	GB/T 10294
Thermal conductivity at 400°C mean	≤0.14 W/m·K	GB/T 10294
Maximum service temperature	1000°C	Continuous rating
Linear shrinkage at 1000°C (12h)	≤1.5%	GB/T 5486
Loss on ignition	≤9%	GB/T 5486
pH value	7-10	—

Product Properties Relevant to Carbon Baking

Thermal cycling stability: Samples of Mingfa composite brick were subjected to 50 thermal cycles from ambient to 1000°C in laboratory testing conducted at the company's Laizhou quality control centre. Each cycle comprised 2-hour heat-up, 4-hour hold at 1000°C, and 6-hour ambient cooling — an accelerated profile intentionally more severe than a typical ring furnace cycle. Linear shrinkage after 50 cycles measured 1.2%, within the ≤1.5% specification limit. Compressive strength retained 92% of the initial value. These results confirm that the xonotlite crystal structure remains stable through repeated cycling — xonotlite does not undergo the dehydration-to-wollastonite conversion (which produces approximately 10% linear shrinkage) until temperatures exceed 1100°C in dry conditions.

Fluoride resistance: In comparative exposure testing using NaF vapour at 950°C for 200 hours, calcium silicate composite brick showed 3% mass change versus 8% mass loss for aluminosilicate ceramic fibre blanket of equivalent temperature rating. The ceramic fibre showed visible surface embrittlement and dusting; the calcium silicate brick remained physically intact.

Non-wetting to pitch volatiles: During anode baking, coal tar pitch binder (typically 13-16% of green anode mass) pyrolyses and releases volatile hydrocarbons. If insulation absorbs these condensates, carbon deposition within the insulation pores can create localised hot spots — carbon is thermally conductive, and its accumulation progressively degrades insulation performance. Calcium silicate boards with surface skins (formed during autoclaving) resist pitch condensate absorption. The pore structure is closed at the surface and the material chemistry (basic CaO component) does not catalyse carbon deposition.

Flue Wall Insulation Configuration

The standard flue wall lining configuration in carbon baking furnaces, from hot face outward, is: refractory brick (230mm) + silica-calcium composite brick (65mm) + calcium silicate board (50mm).

The 230mm refractory brick layer — typically fireclay (42-45% Al₂O₃) or high-alumina (55-60% Al₂O₃) — faces the flue gas directly at 1200-1300°C. The material selection depends on the fuel type: natural gas firing permits fireclay; oil firing with higher flame temperature requires high-alumina brick in the combustion zone.

The 65mm composite brick layer serves dual roles. First, it provides moderate thermal resistance, reducing the temperature at the outer calcium silicate board face to below 650°C — within the calcium silicate board's continuous rating. Second, it provides mechanical support for the flue wall. In open-top ring furnaces, the flue wall is a free-standing structure 4-6 metres tall and only 230-300mm thick. Without lateral support from insulation, these walls can bow under thermal expansion forces, narrowing the flue gas passage or, worse, cracking and allowing flue gas bypass.

The 50mm calcium silicate board provides the largest thermal resistance contribution in the composite wall, owing to its lower density (230-270 kg/m³) and correspondingly lower thermal conductivity. The total wall U-value for this configuration is approximately 0.45 W/m²·K.

Headwall insulation follows the same principle but with thicker composite brick (80mm) because headwalls are exposed to direct radiation from the burner flame and experience higher thermal flux.

Energy and Process Benefits

Temperature uniformity between flues: In a ring furnace without sidewall insulation, flue wall heat loss creates a temperature gradient between the central flues (hotter) and edge flues (cooler). This non-uniformity translates directly into anode quality variation — under-baked anodes on the edges, over-baked anodes in the centre. With properly installed composite brick + board insulation, inter-flue temperature variation can be held to ±5°C, versus ±15°C in an uninsulated furnace. The resulting improvement in anode quality consistency reduces anode butt rejection rate at the smelter.

Furnace hall ambient temperature: In tropical and subtropical aluminium smelter locations (Middle East, India, Southeast Asia, South America), uninsulated furnace hall temperatures reach 45-55°C near the firing zone. This creates challenging — and in some jurisdictions regulated — working conditions. Insulated flue walls and headwalls reduce hall temperature to 30-35°C in the same ambient conditions. This is a measurable occupational health benefit and, in some locations, extends the effective working hours per shift under local labour regulations.

Fuel saving: The reduction in heat loss through flue walls and headwalls delivers 8-12% fuel saving, as measured at several carbon plant retrofit projects where before-and-after data was collected. For a 150,000 tonne-per-year anode plant consuming 2.5 GJ per tonne of baked anode, an 8% saving represents approximately 30,000 GJ per year — equivalent to about 750,000 Nm³ of natural gas.

Operational experience from a Middle East ring furnace retrofit (2020): A 54-section open-top ring furnace producing anodes for a Gulf-region aluminium smelter installed Mingfa composite brick during a section-by-section rebuild programme conducted over 18 months without interrupting production. The plant reported: average fuel consumption reduction of 9.3% (measured by natural gas meter for the furnace firing system, comparing 12-month pre- and post-retrofit periods); inter-flue temperature variation reduced from ±14°C to ±6°C; anode property consistency improvement — baked anode density standard deviation reduced from 0.012 to 0.008 g/cm³, electrical resistivity standard deviation reduced from 3.2 to 2.1 micro-ohm-metres; and furnace hall ambient temperature reduced by approximately 8°C during summer months. The plant calculated a payback period of 14 months based on fuel saving alone, excluding the value of improved anode quality consistency.

Ordering for Carbon Plant Applications

To prepare a quotation, Mingfa engineering requires the following information:

Furnace type — open-top or closed-top ring furnace
Number of sections per fire and total fire count
Flue wall construction drawings showing refractory type and dimensions
Peak firing temperature and typical baking cycle duration
Fuel type (natural gas, heavy fuel oil, or other)
Anode size and production capacity (tonnes per year)
Any known issues with current insulation — hot spots, wall bowing, excessive hall temperature

Mingfa provides thermal calculations and insulation layout drawings as part of the quotation package. Standard board dimensions: 600mm x 1000mm, 600mm x 1200mm. Standard brick dimensions: 230mm x 114mm x 65mm (matching refractory brick format for common installation). Custom dimensions are manufactured for specific furnace designs; the autoclave process permits forming to non-standard dimensions without tooling cost.

For greenfield carbon plant projects, Mingfa can coordinate with the furnace design engineering company to integrate the insulation specification into the overall furnace design. For brownfield retrofits, Mingfa engineers can visit the plant to measure existing flue wall dimensions and assess current insulation condition before preparing the retrofit specification.