LF Refining Process Technology : Main Functions, Equipment and Features of the LF Furnace

LF (Ladle Furnace) — Ladle Refining Method

The LF ladle furnace refining process was first developed in 1971 at the Omori Plant of Daido Steel in Japan.

In China, the first LF furnace was commissioned in 1981 at Shanghai No.5 Steel Plant.

The LF furnace is one of the most important units in secondary metallurgy, used for refining molten steel after primary steelmaking.

Main Functions of the LF Furnace

Ⅰ. Reducing Atmosphere

Although the LF furnace itself does not include a vacuum system, the sealed structure between the ladle and furnace cover isolates air from the molten steel.During heating, graphite electrodes react with oxides in the slag such as:FeO, MnO and Cr₂O₃.These reactions generate CO gas, which reduces oxygen concentration inside the furnace and creates a reducing atmosphere.

During the refining process, diffusion deoxidation and precipitation deoxidation occur, providing strong reducing conditions for molten steel.This helps achieve:

• deeper deoxidation
• desulfurization
• removal of non-metallic inclusions

Ⅱ. Argon Stirring

Argon gas stirring accelerates mass transfer between molten steel and slag, promoting:

• deoxidation reactions
• desulfurization reactions

Argon bubbling also helps Al₂O₃ inclusions float to the surface faster.Under sealed LF furnace conditions, after 15 minutes of argon blowing, most Al₂O₃ inclusions larger than 20 μm can be effectively removed from the steel.

Ⅲ. Submerged Arc Heating

Three graphite electrodes are inserted into the slag layer for submerged arc heating.Advantages include:

• lower radiation heat loss
• protection of refractory lining
• higher thermal efficiency

Graphite submerged in slag reacts with oxides as follows:

These reactions:

• increase slag reducibility
• improve alloy recovery
• generate CO gas, strengthening the reducing atmosphere inside the LF furnace

Ⅳ. White Slag Refining

During LF operation, slag is strongly deoxidized to form white slag.

White slag has strong adsorption and dissolution capacity for oxides, enabling effective deoxidation of molten steel without contamination.

With argon stirring and temperature compensation, increasing slag basicity improves the sulfur distribution ratio, making it possible to produce:

• low sulfur steel
• ultra-low sulfur steel

At present, the ideal control range of (FeO + MnO) in slag for low-sulfur steel production is below 0.5% worldwide.

Equipment and Characteristics of the LF Furnace

A. Furnace Body (Ladle)

The LF furnace body is essentially a refining ladle, but it differs from a conventional steel ladle.Key features include:

• a water-cooled flange at the ladle top
• sealing with rubber rings to prevent air infiltration
• gas-tight welding for vacuum operation

At the bottom of the ladle there are:

• a sliding gate nozzle for steel tapping
• porous plugs for inert gas (argon) injection

The ratio between molten pool depth (H) and pool diameter (D) is an important design parameter.

Refining ladles generally have a relatively deep molten bath.

The distance between the molten steel surface and the ladle rim is called freeboard.

Typical values:

• Non-vacuum operation: 500–600 mm
• Vacuum operation: 1000–1200 mm

The H/D ratio influences:

• stirring efficiency
• steel-slag contact area
• thermal load on slag line refractories
• ladle lining life
• heat loss

H/D Values for Different Capacity LF Furnaces in Japan
LF Furnace Capacity /t	20	30	50	60	150
Actual Loading Quantity/t	13/23	18/33	45/50	60	100/150
Steel bag size Outer Diameter/mm Inner Diameter D/mm Total Height/mm Inner Height/mm	2200 1676 2300 1995	2400 1948 2500 2195	2924 2430 3040 2770	2600 2070 3150 2740	3900 3164 4330 4000
Melt Pool Depth H/mm	1260	1402	1348	2340	2754
H/D	0.75	0.72	0.49	1.13	0.87

B. LF Furnace Cover

The furnace cover is water-cooled to:

• maintain a strong reducing atmosphere
• minimize heat loss
• improve heating efficiency

The inner surface is lined with refractory material.A splash guard plate is suspended under the cover to prevent molten steel splashing and sticking to the ladle.The entire cover is suspended at four points on a gantry structure with adjustable chains and lifting mechanisms.

This allows flexible positioning during operation.

The furnace cover also includes:

• alloy feeding ports
• slag feeding devices
• temperature measurement and sampling ports

C. Arc Heating System

The LF furnace arc heating system is similar to that used in an electric arc furnace (EAF).

Heat is generated by electric arcs between three graphite electrodes and molten steel.

However, the LF furnace does not perform melting, and it uses submerged arc heating.

Therefore, compared with an EAF, the LF furnace can operate with lower secondary voltage.

D. Feeding System

Alloy and slag hoppers are installed on the furnace cover.

Through guide valves under each hopper, alloy or slag materials can be added in controlled quantities.

For LF furnaces equipped with vacuum systems, feeding devices are installed on the vacuum cover, using vacuum-sealed joints.

E. Slag Removal System

Before LF refining begins, oxidizing slag must be removed to allow proper white slag refining.

Two main slag removal methods are used:

Ⅰ. When the LF furnace adopts multi-station operation, a tilting and slag-removing device can be installed on the ladle car that loads the ladle. When the ladle car drives to the slag-removing station, the slag removal operation can be carried out.

Ⅱ. If the LF furnace adopts a fixed position and a movable furnace cover, the ladle tilting device needs to be set on the LF furnace base. Before refining, the slag should be removed, new slag should be added, and then the furnace should be heated for refining.

F. Powder Injection System

In LF furnace refining, powder injection equipment is often used to desulfurize, purify, and microalloy the molten steel.

The powder injection equipment includes a ladle cover, a powder injection nozzle, and a powder distributor with a 4m sliding stroke. The distributor connects to four 500kg powder hoppers. During injection, the powder is automatically weighed and mixed, then fed to the distributor via a screw feeder.

For a 50t LF furnace, the nozzle is 4500mm long, with 2500mm being replaceable. The nozzle is inserted 100-150mm from the bottom of the ladle for powder injection.

High-purity argon is used as the carrier gas during injection, with a flow rate of 200-400L/min. The typical processing time is 5-10 minutes.

Key Parameters for LF Ladle Furnace Design

Ⅰ. Furnace Capacity

The LF furnace capacity should be selected based on the maximum capacity of the primary steelmaking furnace.

Additional considerations include:

• vacuum operation
• oxygen blowing capability
• sufficient freeboard space

Ⅱ. Transformer Rated Capacity

Transformer capacity mainly depends on:

• required heating rate
• equipment efficiency

In China, LF furnace transformers are typically selected at 150–200 kVA per nominal ton.Under hot metal tapping conditions, the temperature rise rate is 3–5°C per minute.Some foreign installations reach 400 kVA per ton.

Main Technical Parameters of LF Ladle Refining Furnace
ITEM	40tLF	60tLF	90tLF	100tLF	150tLF
Steel ladle capacity/t	35~45	55~65	80~100	80~120	130~170
Steel ladle diameter / mm	2900	3150	3300	3500	3900
Steel ladle height / mm	2300	3000	4300	4800	4850
Polar diameter/mm	650	650	710	750	750
Electrode Diameter/mm	350	350	350	400	450
Transformer Capacity/kVA	6000	10000	15000	16000	25000
Heating Speed /℃/min	3~4	3~4	3~4	3~4	3.5~4.5
Free space of steel ladle/mm	600~700	800~1000	800~1000	800~1000	800~1000
Processing Cycle/min	40~90	40~90	40~90	40~90	40~70