Copper mining needs smart choices. The grinding circuit is the heart of the plant. It costs the most money. It uses the most power. Many miners ask a big question. Should I use a SAG mill? Or should I use a Ball mill. This article helps you decide. We look at facts. We look at data. We use SBM experience. We keep it simple. You will learn to pick the right one. This saves you money.
Let us start with the basics. A ball mill is a classic grinder. It is a long cylinder. It rotates on its axis. Inside there are steel balls. These balls are the grinding media. The mill spins. The balls drop. They hit the ore. This is called impact. The balls also roll. They rub the ore. This is attrition.
The ball mill needs fine feed. The feed size is usually 10mm to 20mm. It grinds very fine. The product is often 74 microns. Or even smaller. The length is usually bigger than the diameter. The ratio of L/D is 1.5 to 2.5. This ensures the ore stays inside longer. It gets ground well. SBM ball mills use rolling bearings. This saves energy. It reduces friction.
Why do we care about the motor? The motor turns the shell. The speed is key. It must be critical speed. Usually 70% to 80% of critical speed. If too fast the balls fly. If too slow the balls slide. We need the cascade effect. This breaks rocks best.
SAG stands for Semi-Autogenous Grinding. It is different. It uses the ore itself to grind. Plus it uses some steel balls. About 8% to 15% volume is balls. The rest is big rocks. The feed size is huge. It can take 200mm rocks. Sometimes 300mm. This is a big advantage.
The SAG mill is short and fat. The diameter is big. The length is short. The ratio D/L is usually 2 or 3. This design lifts rocks high. They fall hard. This is high impact. It breaks big rocks fast. It replaces two crushers. You skip secondary crushing. You skip tertiary crushing.
But it has a weakness. It needs the right ore. The ore must be hard. It must not be too soft. If ore is soft it breaks too fast. Then there is no grinding media. The mill stops grinding. This is called “critical size” buildup. We must watch out for this.
We need to see the numbers. Data speaks truth. Here is a table. It compares the two mills. This data comes from SBM labs. And industry journals like Minerals Engineering.
| Parameter | SAG Mill | Ball Mill |
|---|---|---|
| Feed Size | 150mm – 300mm | 10mm – 20mm |
| Grinding Media | Ore + 10% Steel Balls | 30-40% Steel Balls |
| Length/Diameter Ratio | Small (0.5 – 1.0) | Large (1.5 – 2.5) |
| Energy Efficiency | Lower (High fluctuations) | Higher (Stable load) |
| Maintenance Cost | High (Liner wear) | Moderate |
| Suitability | Hard, blocky ores | All ore types |
Why look at this? It shows the limit. SAG eats big rocks. Ball mill eats small rocks. SAG uses more power per ton usually. But it removes crushers. The total system power might be less. You must calculate the Bond Work Index. This measures hardness. If index is high SAG is good.
Many clients worry. They see the motor power. A big SAG mill has a huge motor. Maybe 5000 kW. A ball mill might be 2000 kW. Is SAG too expensive? Not always.
Think about the crushers. A ball mill needs fine feed. You need a jaw crusher. Then a cone crusher. Maybe two cone crushers. And many screens. And many conveyors. These all use power. They all need parts. They all need workers.
The SAG mill removes them. It takes ore from the primary crusher. It does the work of three machines. So the total cost drops. The CAPEX is lower. You buy fewer machines. The plant is smaller. This is why big mines love SAG. SBM engineers calculate this for you. We check the “Total Comminution Energy”.
This is a big pain point. You run the mill. The liners break. You stop production. You lose money. Why does this happen. It is often the trajectory. The balls hit the liner. They should hit the ore. If the speed is wrong they hit the shell.
In SAG mills it is worse. The rocks are big. The impact is huge. The liners must be strong. We use Cr-Mo steel. Or rubber composites. SBM uses special wave liners. They lift the charge better. They protect the shell.
Solution involves monitoring. Check the filling rate. For ball mill keep it 40%. For SAG keep it 25%. If the mill is empty liners die. You must feed it constant. Use a VFD drive. Adjust the speed. If ore changes slow down. This saves the steel.
Let us look at a real job. The location is Peru. The altitude is 4000 meters. The air is thin. The client had a problem. Their old plant was slow. The ore was hard copper porphyry. The bond index was 16 kWh/t. They used three stage crushing. It was dusty. It broke often.
The Equipment List:
Design Logic: SBM team tested the rock. It was competent. It was good for SAG. We removed the cone crushers. We installed the SAG mill. We set the CSS of the jaw crusher to 150mm. The SAG reduces it to 2mm. Then the ball mill finishes it. It goes to flotation.
The Result: The dust went down. The maintenance time went down 30%. The power bill dropped 15%. The client manager said “It runs smooth.” The SBM rubber liners lasted 8 months. This was a win. The recovery rate improved. Why? The grind was uniform.
Buying the machine is step one. Running it is the hard part. Installation matters. The foundation must be solid. A ball mill vibrates. A SAG mill jumps. Concrete must be thick.
SBM provides the “Turn-key” service. We pour the cement. We align the gears. The pinion and girth gear must match. The gap is small. Only 0.5mm tolerance. If wrong the teeth break.
For maintenance focus on lubrication. The trunnion bearings need oil. High pressure oil. It lifts the mill before it starts. This is hydrostatic lubrication. It prevents scratching. Check the oil filter daily. Change the oil every 6 months. This is cheap insurance. Do not ignore it.
Do not guess. Use a decision tree. Follow these simple steps. This is how pros do it.
SBM experts help you here. We run the tests. We simulate the flow. We give you the answer.
No you cannot. The structure is different. The SAG mill has a big diameter. The bearings are different. The motor is bigger. You cannot just change liners. It will not lift the big rocks. You must buy a new machine.
It depends on the wear rate. Usually you add balls every day. You do not change all at once. You add a little bit. Maybe 500kg a shift. This keeps the load steady. SBM steel balls are forged. They last longer. They wear round. They do not break.
For ball mills rubber is good. It is quiet. It is light. It lasts long for fine grinding. For SAG mills you need steel. Or composite. The impact is too high for just rubber. It will tear. Use Cr-Mo alloy steel. It is tough.
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