A wet pan mill is a type of grinding machine used mainly in mineral processing, especially for gold ore, copper ore, and other hard minerals. It is sometimes called a gold grinding mill, Chilean mill, or roller mill. It uses heavy grinding wheels that rotate and crush ore in the presence of water, producing a fine slurry.

Wet pan mills are widely used in small and medium-scale mining operations because they are relatively simple, robust, and effective for recovering fine gold particles.

1. Basic Structure of a Wet Pan Mill

A typical wet pan mill consists of:

1. Grinding pan (circular basin)
   • A steel or cast iron bowl where ore and water are added.
2. Heavy grinding rollers (muller wheels)
   • Large wheels weighing hundreds to thousands of kilograms.
   • They rotate and roll over the ore, applying pressure.
3. Central shaft and drive system
   • Motor and gearbox rotate the rollers around the pan.
4. Scraper system
   • Helps move the slurry and ensure uniform grinding.
5. Discharge screen
   • Controls the particle size leaving the mill.

How Does a Wet Pan Mill Work?

The grinding process occurs through crushing, grinding, and mixing.

Step 1: Feeding

Raw ore is fed into the pan together with water.

Typical feed:

• Gold ore
• Quartz ore
• Rock fragments
• Other mineral-bearing materials

The water turns the material into a slurry.

Step 2: Grinding by Heavy Rollers

The motor rotates the rollers around the pan.

The rollers apply two main forces:

a) Crushing force

The weight of the rollers presses the ore against the pan surface.

Large rocks are broken into smaller pieces.

b) Grinding and shearing force

As the rollers rotate and slide, they grind the particles into fine powder.

Step 3: Mineral Liberation

The purpose of grinding is not only to make smaller particles but to liberate valuable minerals.

For example, in gold ore:

Before grinding:

Quartz rock
 ├── Gold particle trapped inside
 └── Gangue minerals

After grinding:

Fine slurry
 ├── Free gold particles
 ├── Fine quartz
 └── Other minerals

The released gold can then be recovered by methods such as:

• Gravity concentration
• Shaking tables
• Centrifugal concentrators
• Flotation
• Mercury amalgamation (where legally permitted)

Step 4: Slurry Discharge

After grinding, the fine slurry flows out through the discharge opening.

The slurry typically contains:

• 60–70% solids (depending on operation)
• Fine mineral particles
• Water

Why Use Water?

Water provides several benefits:

1. Better grinding efficiency

Wet particles are easier to grind than dry particles.

2. Prevents dust

Dry grinding creates large amounts of mineral dust.

3. Improves mineral separation

Fine particles remain suspended, allowing better recovery.

4. Reduces heat generation

Water cools the grinding surfaces.

Wet Pan Mill vs Ball Mill

Advantages of Wet Pan Mill

✅ Low investment cost
✅ Simple operation
✅ Good for gold recovery plants
✅ Can handle sticky ores
✅ High grinding efficiency for coarse gold ores
✅ Easy maintenance

Disadvantages

❌ Lower capacity than large ball mills
❌ Not ideal for very fine grinding (<10 microns)
❌ Heavy moving parts require strong foundations
❌ Wear of grinding plates and rollers can be significant

Typical Applications

Wet pan mills are commonly used for:

• Gold ore grinding
• Copper ore processing
• Lead-zinc ore processing
• Iron ore preparation
• Mineral concentration plants

A common small gold processing flowsheet is:

Jaw Crusher
      ↓
Wet Pan Mill
      ↓
Gravity Concentrator
      ↓
Gold Recovery