How to Select the Right Ball Mill for Your Mineral Processing Operation
The SAG mill vs ball mill is basically a type of heavy industry workhorse equipment that helps in processing the mineral. When we need liberation of the value-bearing mineral from the rock matrix that contains it (the gangue), it would probably be a flotation, a leaching a magnetic separation, or any other such separation method. Choosing the right ball mill would be a factor greatly impacting the concentrator's capital budget, and incorrect choice of the machine can cost the operation millions over its operation life. We have compiled here the list of technical and commercial criteria that you might want to consider.
Where Ball Mills Fit in the Comminution Circuit
Comminution circuit of hard-rock metallic mineral operations typically consists of 4-5 stages of reduction in the size of ore:
Primary crushing—gyratory or jaw crushers which reduce sizes to approximately 150-250mm; blasting reduces in-situs (undisturbed) rock to ROM fragments, with top size around 800-1,500mm;
Secondary/tertiary crushing—cone crushers which reduce sizes to approximately 10-25mm;
Grinding (rod, ball, or SAG mill)—comminution to liberation size, typically 75-150 microns.
Re-grinding (optional)—if a harder or more disseminated ore is present, ultra-fine grinding can be up to 20-25microns.
As a rule of thumb, comminution grinding mill selection consumes between 40-60% of all the comminution energy of the circuit, so mill choice&operation really is the energy&production lever of the mine plant.
Ball Mill, Rod Mill, SAG Mill - Which One is Right for Me?
Besides ball mills themselves, the Sanlands ball mill series catalog also includes other types like rod and SAG mills.
Crumbly hard ores which can be crushed by themselves without the help of ball grinding medium can be crushed in SAG mills - feed -250 mm, product -1 to 12 mm; such mill would also do away with need for secondary crushing;
Feed -25 mm to rod mill, with product -1 to 4 mm; coarse grinding before ball milling to a fairly uniform product, so that grinding can be carried out efficiently.
Ball mill: feed-25 mm, product 75-150 microns; most of the concentrate is obtained by this ball grinding. Milling the rock to liberation is a standard for most minerals concentrate facilities;
Ball mill re-grind: feed -200 microns, product 25-45 microns; liberation of very fine disseminated minerals by means of ultrafine grinding.
Crushing+ballmilling circuits are the comminution circuits of choice in most small- & medium-sized copper, gold, lead-zinc, and iron operations; SAG mill is used for very large (over 20,000 tons per day) operations, as a rule.
Sizing: The Bond Work Index Calculation
Ball mill sizing begins with the Bond Work Index (Wi) measure of grinding difficulty based on representative laboratory testing on ore samples. Some typical work indices are as follows:
Limestone: 11-13 kWh/t
Copper porphyry: 12-15 kWh/t
Iron ore (taconite): 14-17 kWh/t
Quartz/gold ore: 13-16 kWh/t
Some super hard ores (e.g. some platinum ores): 18-22 kWh/t
For instance, a plant having throughput of 1,000 tonnes per day, grinding the product where the grinding work index equals 14 kWh/t generally needs about 530 kW of the grinding power. Add 15-25% margin to account for circuit inefficiencies and motor sizing, and the total mill power requirement goes up to about 650-700 kW.
Mill Geometry: Diameter, Length, Aspect Ratio
The dimensions of a ball mill are given by the ball mill D × L format meaning diameter × length expressed in meters. For example, Sanland has such models as the 2136 mills which are 2.1 meters in diameter and 3.6 in length and also the 1830 (Spanish-language markets).
Other common geometrical aspects to consider include:
D/L ratio—usually between 0.5 to 2.0; smaller ratios (i.e., longer mills) will yield finer particles at a lower capacity per volume whereas larger ratios favor greater capacity with coarser product.
Mill volume determines the ball-load volume, which in turn determines both the power and throughput of the mill.
Trunnions bearings vs. slide-shoe bearings—influences both the maintenance and the noise; normally big mills run on slide-shoe bearing.
Drive Configuration
There are three drive types mainly used in today's operations:
Single-pinion drive—this is the simplest and up to about 5,000 kW.
Twin-pinion drive—mid to large-size mills have most commonly this drive type of around 5,000 - 15,000 kW.
Gearless or direct drive (wraparound motor)—used for very large mills (more than 15,000 kW) at the cost of higher initial capital if a high degree of reliability is justified.
Ball Charge and Operating Parameters
mineral processing equipment are designed to normally contain about the following amounts:
Ball Load:
Ball Charge Level: 30-40% of the ball mill inner shell.
Mill rotation speed: 70%-80% of critical.
Slurry solids content: The grinding medium for the ore is between 65-75% by weight of the water.
Ball size distribution: Ball mix sizes (typically sizes of 25, 40 and 60, 80 mm) that can work out all size ranges of the particles.
For the parameters given, the commissioning and first year of operation should be used for fine-tuning, as they will differ from the theoretical values based on what is observed from the actual grinding process of the ore.
Closed-Circuit vs Open-Circuit Operation
Milling the ore with a grinding mill under closed-circuit—by returning the undersize of the screen or cyclone back to the mill for further grinding—has been the standard practice for more than half of the concentrators operating world wide these days. It not only results in better particle-size uniformity of the final product but also reduces overgrinding, therefore leading to greater mill efficiency, and the product is also more adaptable to variations in ores hardness. The downstream classifier, most commonly a hydrocyclone, is the main part of the circuit where ball mill discharges go through classification and are returned as the ball mill feed; the efficiency and cut size of the classifier have a direct influence on the sizing of the ball mill product.
Service, Wear Parts, and Lifetime Economics
You can count like a cone crusher, the biggest cost a ball mill will run into during its lifetime is not even the capital investment but wear parts and the electricity bill. Over the mill's life, the main reasons for the expense in the operation will be as follows:
Mill liners usually need to be relined by replacement every 12-24 months; the selection of the right alloy material will largely decide mill performance.
Grinding media (balls) - they're being added in a continuous and never-ending manner; in ball mills the grinding media are usually replaced after a given time; typical specific consumption is 0.4-1.0 kg/tonne.
Energy - generally the cost would be between 12-25 kW/ton according to different ores and different types of grinding size.
Trunnion and bearing maintenance - a well-maintained mill will have these parts lasting quite long while if neglected they're expensive.
Sanlands promise about fast parts availability really reflects how much mill operation cost savings can be made in terms of reliability of operation: dependable machinery equipped with service, spare parts and technical expertise.
Contact Sanland for Your Mining Project
For ball mill sizing, selection, and specification on your specific project—including drive type, geometry, and integration with the rest of your comminution circuit—contact Sanland's engineering team:
Email: sales@sanland.com
China HQ: +86-24-2583-0521 ext. 8121 (Shenyang)
North America: +1-705-988-5866 (Sanland Canada, Val Caron, ON)
