Summary of Silicon Carbide (SiC) Manufacturing Process

Silicon carbide abrasives are typically produced using quartz and petroleum coke as primary raw materials. In the preparatory stage, these materials undergo mechanical processing to achieve the desired particle size before being chemically proportioned into furnace charge. To regulate the permeability of the furnace charge, an appropriate amount of sawdust is added during mixing. For the production of green silicon carbide, a certain quantity of salt is also incorporated into the furnace charge.

The furnace charge is loaded into a batch-type resistance furnace, which features end walls at both ends with graphite electrodes positioned near the center. The furnace core body connects the two electrodes, surrounded by reactive furnace charge materials, while insulating materials encase the outer perimeter. During operation, electrical power heats the furnace core to temperatures between 2600-2700°C. Heat transfers from the core surface to the charge materials, which, upon exceeding 1450°C, undergo chemical reactions to form silicon carbide while releasing carbon monoxide.

As the process continues, the high-temperature zone expands, progressively forming more silicon carbide crystals. These crystals evaporate, migrate, and grow within the furnace, eventually coalescing into a cylindrical crystallized mass. The inner walls of this mass experience temperatures exceeding 2600°C, causing decomposition that releases silicon, which then recombines with carbon to form new silicon carbide.

The electrical power distribution varies across three operational phases:

1.Initial phase: Primarily used for heating furnace charge

2.Intermediate phase: Increased proportion for silicon carbide formation

3.Final phase: Dominated by thermal losses

Optimal power-time relationships are developed to maximize thermal efficiency, with typical operation durations around 24 hours for large-scale furnaces to facilitate workflow coordination.

During operation, secondary reactions occur involving various impurities and salts, causing material displacement and volume reduction. The produced carbon monoxide escapes as atmospheric pollutant. Post-power shutdown, residual reactions persist for 3-4 hours due to thermal inertia, though at significantly reduced intensity. As surface temperatures decline, incomplete combustion of carbon monoxide becomes more pronounced, necessitating continued occupational safety measures.

The post-furnace materials from outer to inner layers consist of the following components:

(1) ‌Unreacted charge material‌

Portions of the charge that fail to reach reaction temperature during smelting remain inert, serving solely as insulation. This zone is termed the insulation band. The composition and utilization methods differ significantly from the reaction zone. Certain processes involve loading fresh charge into specific insulation band areas during furnace loading, which is retrieved post-smelting and blended into reaction charge as calcined material. Alternatively, unreacted insulation band material can undergo regeneration treatment by adding coke and sawdust for reuse as exhausted charge.

(2) ‌Oxidized silicon carbide layer‌

This semi-reacted layer primarily contains unreacted carbon and silica (20-50% already converted to SiC). The altered morphology of these components distinguishes them from exhausted charge. The silica-carbon mixture forms amorphous gray-yellow aggregates with loose cohesion, pulverizing easily under pressure—unlike exhausted charge where silica retains original granularity.

(3) ‌Bonding layer‌

A compact transitional zone between the oxidized layer and amorphous zone, containing 5-10% metal oxides (Fe, Al, Ca, Mg). The phase composition includes unreacted silica/carbon (40-60% SiC) and silicate compounds. Differentiation from adjacent layers becomes challenging unless impurities are abundant, particularly in black SiC furnaces.

(4) ‌Amorphous zone‌

Dominantly cubic β-SiC (70-90% SiC) with residual carbon/silica (2-5% metal oxides). The friable material crumbles readily into powder. Black SiC furnaces yield black amorphous zones, while green SiC furnaces produce yellowish-green variants—sometimes with color gradients. Coarse silica particles or low-carbon coke may create porous structures.

(5) ‌Secondary-grade SiC‌

Comprising α-SiC crystals (90-95% purity) too fragile for abrasive use. Distinct from amorphous β-SiC (powdery, dull), secondary-grade exhibits hexagonal crystal lattices with mirror-like luster. The division between secondary and primary grades is purely functional, though the former may retain porous structures.

(6) ‌Primary-grade SiC crystals‌

The furnace's main product: massive α-SiC crystals (>96% purity, 50-450mm thick). These tightly packed blocks appear black or green, with thickness varying by furnace power and location.

(7) ‌Graphite furnace core‌

Adjacent to the crystalline cylinder, decomposed SiC forms graphite replicas of original crystal structures. The inner core consists of pre-loaded graphite with enhanced graphitization after thermal cycling. Both graphite types are recycled as core material for subsequent furnace batches.