Inside the Submerged Arc Furnace: Thermal & Electrical Kinematics

01Submerged Arc Furnace (SAF) Hardware Architecture

The industrial production of Metallurgical-Grade Silicon (MG-Si) is executed inside massive Submerged Arc Furnaces (SAF) rated between $10 MW$ and $50 MW$ of electrical power. Unlike open-arc steelmaking furnaces, the electric arcs in an SAF are deeply submerged beneath a permeable bed of solid raw materials known as the furnace charge (or burden).

Process schematicSchematic · source diagram

Key Hardware Components

Søderberg Self-Baking Electrodes: Three massive vertical carbon/graphite electrodes (up to $1.5 meters$ in diameter) supplied with three-phase alternating current (AC).
Refractory Hearth & Shell: A water-cooled cylindrical steel shell lined with high-density carbon blocks and silicon carbide ( $SiC$ ) refractories at the bottom tap hole.
Rotating Furnace Body: Many modern SAFs slowly rotate or oscillate ( $1 revolution per 12 - 24 hours$ ) to break up crust formation and maintain uniform burden distribution.

02Multi-Zone Thermal Topology & Internal Gas Kinematics

The interior of a silicon SAF operates under a steep vertical temperature gradient divided into two primary reactive zones:

Zone 1: Upper Low-Temperature Zone ( $110 0^{\circ} C - 150 0^{\circ} C$ )

In the cooler upper region of the furnace shaft, descending solid carbon materials react with rising silicon monoxide gas ( $SiO$ ) generated in the lower zone to form solid silicon carbide ( $SiC$ ):

SiO (gas) + 2 C (solid) \to SiC (solid) + CO (gas)

This reaction is critical for furnace efficiency; it recycles volatile silicon gas before it can escape out the top exhaust, converting solid carbon into reactive $SiC$ .

Zone 2: Lower High-Temperature Zone ( $> 190 0^{\circ} C$ )

In the immediate vicinity of the electric arc plasma cavity, quartz melts and reacts with the newly formed silicon carbide to produce liquid elemental silicon:

SiO_{2} (liquid) + SiC (solid) \to Si (liquid) + SiO (gas) + CO (gas)

SiO (gas) + SiC (solid) \to 2 Si (liquid) + CO (gas)

*FIG. 1*A highly detailed photorealistic interior view of a 30MW Submerged Arc Furnace cross-section

03Operational Parameter Matrix & Process Control

Maintaining continuous, steady-state operation requires delicate balance across electrical, material, and thermal parameters:

Operational Parameter	Nominal Control Range	Consequence of Out-of-Spec Operation
Operating Temperature	$190 0^{\circ} C - 210 0^{\circ} C$ (Arc Cavity)	$< 180 0^{\circ} C$ : Furnace freezes; $> 220 0^{\circ} C$ : Excessive $SiO$ gas loss.
Charge Porosity & Size	Quartz: $20 - 80 mm$ ; Woodchips: $50 - 100 mm$	Low porosity causes gas blowouts ("blowholes") and electrode tipping.
Fixed Carbon Ratio	$92% - 98%$ of stoichiometric requirement	Under-coking leaves unreduced $SiO_{2}$ ; Over-coking forms excess $SiC$ crust.
Electrical Power Density	$15 - 30 kW/cm^{2}$ electrode surface	Improper arc positioning leads to shallow hearth heating or refractory damage.

*FIG. 2*A detailed 2D engineering cross-section diagram of a Submerged Arc Furnace for silicon production

04Tapping and Refining Operations

Liquid silicon collects at the bottom carbon hearth at approximately $180 0^{\circ} C$ . At regular intervals ( $1 - 2 hours$ ), the tap hole is drilled open.

[ Molten Silicon Tap ] ---> [ Ladle Oxygen/Air Refining ] --->Schematic · source diagram

Oxidative Ladle Refining: Oxygen or air enriched with chlorine gas is bubbled through the molten liquid silicon in the ladle. Calcium ( $Ca$ ) and aluminum ( $Al$ ) impurities oxidize preferentially before silicon, forming a floating slag layer ( $CaO-Al_{2} O_{3} - SiO_{2}$ ) that is skimmed off.
Casting & Crushing: The refined molten silicon is poured into shallow iron molds, cooled, and mechanically crushed into uniform granules ( $10 - 50 mm$ ) ready for chemical processing into trichlorosilane.

*FIG. 3*A clean industrial process flowchart titled 'SAF Operational & Refining Kinematics'

→Prerequisites & continue reading

Prerequisites: Understanding of carbothermic reduction fundamentals and industrial thermodynamics

Also in Topic 1.1, Quartzite Extraction & Carbothermic Reduction:

Tier 1 · From Sand to Silicon: The Journey of Quartzite Reduc…Tier 3 · Thermodynamics of Carbothermic Reduction and Impurit…