Grinding Balls and Other Grinding Media: Key consumables
Cutting consumable costs is important for all mining operations, but not at the expense of quality. Grinding media represents low hanging fruit for most miners. Most often, the grinding media in question are grinding balls. In some mid-cap mines, grinding media makes up ~10% of total mining operational expenses (OPEX) and significant savings can be achieved.
Selecting the best-quality grinding media balls is considered a key step towards reducing mining costs. This post discusses grinding balls and quality control procedures that can be employed in order to select the best grinding balls for particular applications.
source: Metso Grinding Media Brochure.
Comminution processes are energy intensive and energy inefficient. Some figures suggest that comminution is responsible for 1% of global energy consumption. As a result, it is important to maximise the throughput for a given grinding task. Comminution costs include electricity, grinding media balls, and liners. Choosing the best quality grinding media improves the efficiency of comminution.
Grinding balls form the major portion of the consumable costs. They can account for 40% – 45% of the total cost and directly affect a mill's efficiency and wear and tear. The ball charge consists of grinding balls of several sizes and of different material qualities. Poor-quality grinding media balls have a negative impact on the entire grinding system. Low-quality grinding media balls are consumed faster, grind ineffectively, and use more electricity.
GRINDING MEDIA APPLICATION
- Grinding media steel balls are used to extract precious metals in ore mineral processing. They are most commonly used in copper and gold industries.
- Ore particles must go through the communition process: grinding -> fine grinding -> ultra-fine grinding. They are ground down to liberate precious metals from gangue materials prior to concentration processes.
- Grinding or communition is carried out in mills. The mills are partially filled with steel grinding balls or rods, commonly referred to as 'grinding media'.
- Mills need to be refilled continuously with new grinding balls as old ones wear out.
- Choice and consumption of grinding media is related primarily to the volume and charecteristics of the ore (abrasiveness, particle size, and specific energy input).
GRINDING BALL USAGE: MEDIA APPLICATION SUITABILITY:
- Tower Mills: 0.5”-1.5” grinding balls are recommended. The normal top size of media used is 10-25mm, but even smaller grinding balls can be used for very fine grinding.
- Isa Mills: <0.1” grinding balls are recommended. The Isa Mill is more efficient when using small media (i.e. high-chrome steel balls).
- Rotary Mills: Small cylpebs (<1”) are effective in ultra-fine wet regrinding operations.
- SAG Mills: 4”-6” forged-steel balls are recommended. Cast balls are not agood choice. Their outer crust is hard (typical Brinell hardness > 450). Cast grinding balls cannot sustain the high impact of SAG mill crushing forces. They are recommended for cement grinding and ultra-fine wet regrinding operations.
- Ball Mills: 1”-4” grinding balls are recommended.
BALL CHARGE QUALITY
You should always test grinding media balls from a new source or supplier. This usually requires testing pilot-scale batches of grinding balls in an industrial mill using the marked-ball test. Compare their wear rates with those of your current supplier. For the final evaluation, you may want to conduct full-charge performance trials on one or more milling circuits over a few months.
Types of grinding media
Factors used to determine grinding ball-quality operational use include:
- Frictional forces between liners and balls.
- Abrasiveness of feed material.
- Impact forces in the mill.
- Susceptibility to corrosion – especially in wet processes.
Simulations show that in some instances:
- The use of forged-steel balls, compared to low-density cast-steel balls, increases throughput by 2%-4%.
- Specific energy consumption is reduced by up to 3.5% (at constant feed/product size).
- Forged-steel grinding balls achieve considerably better results iwhen tested against other gridning media, such as cylpebs, boulpebs, or high-chromium-cast balls, taking into account the total cost-calculation of the mill process.
- Studies of SAG Mills performance show that poor quality cast grinding balls with porosity result in the production of a significant excess mass of steel scats in the SAG mill. If your milling operation is using this type of grinding ball, consider switching to higher quality balls. They offer more economic viability.
- Worn out balls can be a source of iron, especially if the rate of wear is too high. This could result in the formation of iron sulphides, which may affect downstream metallurgical processes. Reduced wear rates are essential.
PROPERTIES OF STEEL GRINDING BALLS IN CHINA
Steel grinding balls are produced from a range of alloys. Grinding media suppliers in China purchase bars from large-scale steel mills such as the Laiwu Steel Group. These bars offer different characteristics based on their hardness and amount of alloying required by the end user.
|High carbon (C) and high manganese (Mn)China Specs (B2, 60Mn, No. 45 steel)||Steel with alloying elements such as molybdenum (Mo), chromium (Cr) or nickel (Ni). These balls are especially made for ball mills and are uniformly through-hardened to 60-65 Rockwell C. They represent the highest quality of all metal grinding balls. Most operators insist on using them.|
|Cast nickel alloy||These grinding balls are also very popular. They are basically a white metal ball, and cause less metallic staining than other types of grinding media. The principal objection to these grinding balls is their rough outer surface and projecting nubs, typical of most cast balls. They require a long conditioning period before being placed into general use.|
|Stainless steel||Because of its high cost, stainless steel is only used for special work that requires an acid resistant, non- magnetic ball.|
|Forged low carbon steel||Chill iron and low carbon steel grinding balls are the cheapest metal grinding balls. They are recommended for rough grinding only, where metallic contamination is not a problem.|
CHEMICAL PROPERTIES & HARDNESS: ACCEPTABLE GRINDING MEDIA QUALITY
Steel grinding balls are produced from a range of carbon/iron (Fe/C or Fe-C) alloys. The alloy systems used include AISI 1020 mild steel, high carbon low alloy forged steel, forged martensitic stainless steel, forged austenitic stainless steel, Ni-Hard, 20% chromium, 27% chromium and 30% chromium white cast irons.
By increasing the carbon content to produce cast irons, both the hardness and wear resistance are improved significantly. The high carbon content of cast steel leads to a microstructure consisting of primary carbides instead of austenite or one of its transformation products (i.e. pearlite, martensite, etc.) as a primary phase.5 Usually, the C content of the steel balls is kept between 0.70% and 0.80%. Also, the Cr content should not be less than 0.5% in order to ensure a minimum hardness gradient.
The balance of the composition is Fe with only incidental impurities, such as sulphur (S) and phosphorus (P). Mn is added to supplement the Cr content, particularly in larger diameter balls. A high C content (i.e. >0.85%) in forged steel balls reduces toughness, which usually causes spalling. This is due to an increased tendency to retain austenite in high carbon steels.
|Cast Steel||~3.50||~3.0||1 max||530|
|Forged steel||0.80max||1.0max||1 max||675|
GRINDING MEDIA STEEL BALL HARDNESS
For the best performance, steel ball hardness must be consistent from surface to core. The hardness of cast or forged grinding balls is measured on the Rockwell or Brinell Scales. Hardness tests can be performed in conformance with the manufacturer’s specification. Minimum hardness for grinding balls varies based on the material, desired toughness by the end user, and the ore being ground.
The hardness influences the abrasion resistance of grinding balls. In order to obtain the required hardness, grinding balls should be heat treated and cooled properly to achieve good impact resistance.
Factors that lead to premature breakage of balls:
- Pre-existing flaws that concentrate stress (quench cracks, gas and shrinkage cavities,centreline shrinkage in continuous cast bar before forging, and forge laps in surfaces).
- Poor forging and heat treatment processes. These lead to high retained austenite content.
- Lack of quality control.
Impact toughness of grinding media forged steel balls. Proper heat treatment can improve the impact fatigue resistance (IFR) of grinding balls. The impact fatigue resistance decreases with increasing amount of retained austenite (Áret). Ideally, the retained austenite content should be less than 10% for high impact wear conditions.
Drop tests can be performed on the balls to evaluate their resistance to impact wear. These tests involve dropping a ball from a height of 3.4 m onto a 500 mm diameter by 150 mm thick hardened steel anvil.
Abrasive wear of grinding media forged steel balls. In comminution, high grinding media wear rates represent very high production costs. The costs should be minimised by proper materials selection, especially for grinding balls. Ideally, wear rates should be less than 1kg/ton. Marked ball tests can be performed to check the abrasive wear rates of particular grinding balls.
PRODUCTION OF GRINDING MEDIA (STEEL FORGED BALLS)
Production overview by Vitkovice Cylinders- Manufacturer based in the Czech Republic.
Steel Forged Balls manufacturing process: Roll forged & Rotary rolling
Production Video: Production of Grinding Media Forged steel balls. By Air Hammer.
- A steel bar is cut into shots, then reheated and hammered at least 30 times to compact and harden it.
- In order to maximise the throughput in the milling process, the correct grinding media is selected.
- The bar undergoes a process of hot rolling followed by rapid cooling, reheating, and hammering. This changes the microstructure from pearlitic to martensitic.
- In the forging process, the structure of the steel grain aligns and stretches.
- Stronger, more compact steel is created. This is used to make the grinding balls. The bars are heated to make them malleable enough to form near-perfect spheres.
- Finally, the balls are quenched and reheated for tempering and relieving stress.
Global capacity of the grinding media industry
There are many global players in the grinding media industry. The total capacity is more than 5 million tons per year of grinding balls and other grinding media.
|Russia & CIS||500,000|
|Middle East, SE Asia||500,000|
The grinding media market's global players
China is home to many grinding media suppliers and manufacturers; from coal-fired foundries to state of the art production facilities. The quality from China varies widely. Some of the poorest grinding media is produced here, as is some of the best. Chinese-produced grinding balls have replaced big name brands from some of the largest grinding media circuits worldwide. In some mill industrial tests of Chinese steel forged SAG balls, wear rates were found to be 19% better than Scaw Metal, West African Forgings (WAFO), Corvus and about 6% better than MolyCop.
GRINDING MEDIA STEEL BALL SUPPLIERS LIST
- Kemcore - Sourced from the highest quality Chinese manufacturers. We inspect their facilities regularly and make certain that each supplier is ISO certified. Download our pricelist here.
- Scaw metal - Grinding media supplier with production facilities in South Africa, Zambia and Ghana ($40million 50,000 metric tons grinding plant in Tema, Ghana).
- Magotteaux - Primarily high-chrome cast grinding balls. Produced in several locations around the globe.
- ME Elecmetal - South America distributor (JV) of forged grinding media from China.
- Orica - Forged grinding media from China.
- Metso (Metso: A new player in the grinding media after acquiring Santa Ana de Bolueta SA (SABO [Chile]) in late 2013. SABO was a Spanish owned, forged steel media manufacturer located in Antofagasta, northern Chile (~40 ktpa capacity).
- Proacer (Chile) - Cast steel grinding media manufacturer. Produces a limited size range of grinding balls. Located near Santiago, Chile (~60 ktpa capacity).
- Mepsa (Peru) - Cast steel grinding media manufacturer, Produces a limited size range of grinding balls. Located in Lima, Peru (~50 ktpa capacity).
- Arcelor Mittal (USA) - A small forged steel media manufacturer of a limited size range of grinding balls. Located outside El Paso, Texas (~50 ktpa capacity).
- Gerdau (USA) - A forged steel media manufacturer of a limited size range of grinding balls. Located in Duluth, Minnesota (~110 ktpa capacity).
- Donhad - An Australian-owned grinding media producer.
- MolyCop - 1.2 million capacity with productions facilities in Australia, S.America & N. America.
- AIA Engineering/Vega Industries - Indian-owned producer. Primarily high chrome cast grinding balls.
- Shree Rn Metals India Private Limited - Grinding media producer in India.
- Energosteel - Grinding media manufacturers in Ukraine.
- EVRAZ NTMK in Nizhny Tagil, Russia - Biggest player in the steel grinding balls market in Russia and CIS (EVRAZ NTMK and EVRAZ ZSMK ~ 230 ktpa capacity.)
- Vitkovice Cylinders - Manufacturer based in the Czech Republic.
- Litzkuhn-Niederwippe - Biggest manufacturer of forged steel grinding balls in Germany.
- Chinese suppliers with varying quality. Capacity up to 2 million tons.
- Craster International - Grinding media manufacturer in Zimbabwe.
- Longwear Ltd - UK Grinding media manufacturer.
- GSI–Lucchini SpA* - Operates as a subsidiary of Cevital SPA.GSI Lucchini S.p.A. Manufactures forged steel grinding balls and spheres. The company was founded in 1996 and is based in Piombino, Italy.
If you are a manufacturer and would like to be added to the list above, please email here.
Grinding Media Price Drivers
Depending on the location of the supplier, prices are formulated according to the following principles. Price can be fixed and re-negotiated quarterly or every 6 months.
- Pricing is generally based on an agreed price formula relative to the CRUspi or Scrap indices
- Freight (generally paid and managed by the supplier.) Seen as a key part of the price equation (thus proximity to the customer is often important.)
General pricing formula for China:
1. Steel Price Adjustment: When the price movement of the MySpic composite steel index published on www.mysteel.net exceeded an adjustment of 2.5% in the previous six months, the price of the product shall be adjusted for the following six months period according to the following formula: (If the steel index does not adjust by more than 2.5% in the previous six months, no adjustment to the steel price is made.) A=[B/MySpic(0)]XMySpic (1) Where, A: Updated price (FOB Price with packages)
Initial Price (FOB Price with packages)
MySpic (0): Initial MySpic steel composite index published on www.mysteel.net MySpic (1): Value of weighted average of MySpic steel composite index published on www.mysteel.net of previous six months movements.
2. Adjustment as a result of Rate of Exchange Fluctuations: Should the variation of the average exchange rate for a specific six months period (compared to the previous six months) between USD and RMB published by Bank of China exceed 2%, the Rate of Exchange will be adjusted according to the following formula: C=D X (E/F)
C: Updated price(FOB Price)
D: Adjusted USD price according to the above price Adjustment Formula.
E: Initial Exchange rate(Middle Rate) on www.boc.cn
F: Exchange Rate (Middle Rate) in the date of adjustment on www.boc.cn
3.Freight cost from departure port to destination port: Should be adjusted according to market price and added as-is to the FOB price to make up to CFR Price.
How to Improve Grinding ball Quality in your Supply Chain
The impact of low quality grinding balls on milling
- Ball chips from flaked, poor quality grinding balls result in shorter operational life of pumps, hydro-cyclones, and recirculating pipes.
- Grinding balls may be too light to have the sufficient kinetic energy to break rock.
- Mill flow rates are reduced. Inferior grinding balls cause peening when they are hammered against the discharge rate. This cushions the rocks being hammered and causes ineffective breakage.
- Energy consumption increases. The lightweight balls are pinned to the mill shell and carried to a point before leaving the shell on its trajectory, adding more load to the motor.
- Steel consumption is increased. This generates iron ions detrimental to flotation, which in turn depresses sulphide minerals, and increases sodium cyanide consumption.
EFFECTS OF BALL SCAT REMOVAL ON SAG MILL PERFORMANCE
Ball scat is fragments of grinding balls that result from using porous grinding balls. 30t of scats were removed from a total charge load of 70t. Surveying and modeling the mill revealed that the breakage rates had increased dramatically at the coarser end of the size distribution. An increase of over 10% in mill throughput was achieved by removing the ball scats from a single-stage SAG mill. This work highlights the importance of maintaining a competent ball charge in a mill.
Ref: “Starting Effect of Ball Scats Removal on SAG Mill Performance”
How to Select the Best Supplier for Grinding Media
If you want to get the best grinding ball for your money, determine value by examining ball performance and price per unit, as well as the following factors:
- Quality. Grinding media steel balls should offer consistent and superior wear rate performance compared to alternative products. Don't assume that a low price is a good deal, however. Some suppliers offer reduced prices by supplying poor quality grinding media. This can increase production costs through downtime or faster wear of cyclones and piping.
- Supply Assurance. Since grinding media is a key consumable, mines need flexible and timely product delivery. Significantly longer supply chains may increase the risk of substantial delays. Many suppliers are able to offer VMI services that eliminate long delivery times. VMI also increases unit costs, however. With careful planning, end users can import grinding media directly from overseas sources.
- Technical support. Most mines require technical support –these may include ball mill inspections, grinding media consumption monitoring, etc. This service is normally provided for free, as a value added service.
Delivery of Grinding Balls and Other Grinding Media
Grinding balls and other grinding media are generally delivered in one of two packaging types:
- In steel drums with content of approx. 900kg net on wooden pallets.
- In UV resistant PP bulk bags with the content of approx. 1,000kg net.
Grinding media is not a commodity. It represents a large portion of your total grinding cost and has a significant impact on your OPEX. Select your grinding balls with improved grinding performance and efficiency in mind, but also look for the lowest sustainable cost and maximize mill availability. A full chemical analysis, hardness profile and microstructure analysis can help you choose cost-effective grinding media balls. Implement a good quality control program to ensure consistency in the supply chain, as well.
Kemcore assists mining executives in cutting costs without cutting corners. We offer mining companies a full range of grinding media from top producers in China. Contact us now to find out how we can help you increase your profitability. Click here to learn more about our range of grinding balls and other grinding media.