Grinding Media Balls

The use of ball mills in mineral comminution is a critical step in many extraction and production processes where size reduction to a fine powder is required. While pan milling and grinding plates is used in some small scale operations no other process can currently compete with ball milling where high product throughput is required. There are three types of ball milling processes namely autogenous milling where the products are used to grind themselves without any additional media, semi-autogenous milling where some media is introduced to assist with milling and finally ball milling where almost all the comminution is done using artificial media. Of these processes ball milling is the predominant process used worldwide.

Grinding media may consist of many different materials depending on the product milled, mill diameter etc but has to conform to certain parameters to be suitable. Major considerations are wear resistance and toughness. While some industries might use relatively exotic materials such as ceramics almost all the mineral comminution processes make use of metal media. These can be in the form of rods (rod mills), short cylinders (i.e., cylpebs) or more commonly elliptical or spherical balls produced using an iron based alloy. These have to be alloyed to produce a material with sufficient hardness to withstand the abrasive wear encountered during mill operation and can be manufactured by forging or more commonly casting. Forging is used to produce media in steel and semi-steel (very high carbon steels) where material toughness i.e., resistance to breaking due to impact is a determining factor. Forging processes can be die or roll forging, usually followed by hardening and tempering.

The more common manufacturing process is casting in sand moulds such as greensand or chemically bonded sand, with some manufacturers also using lost foam and vacuum moulding techniques. These products typically are heat treated before use. There are also processes using permanent moulding techniques to produce a chill cast product and are typically used as is without further heat treatment.

By far the most common alloying element used to produce grinding media of all types is chromium. The use of high carbon ferro-chrome has the benefit of introducing carbon as well as to produce hard chromium carbides which has very good wear resistance. These carbides are however extremely brittle so attention has to be paid to their size and morphology to prevent breakage. An important consideration is the silicon level in the alloy as this has an influence on the carbide structure with high silicon promoting massive carbides and decreased fracture toughness. For this reason, high carbon ferrochrome with low silicon levels such as plasma chrome is preferred to charge chrome.

To balance the carbon level some medium or low carbon ferro-chrome may have to be used if a cheaper source of stainless steel scrap is unavailable.

Chromium levels in grinding media varies depending on application, mill parameters, down stream processes but most importantly cost considerations. This can range from as low a 2% for chill cast material to as high as 25% in some specialized cases but generally falls in the 5 to 18% Cr range which can be heat treated to high hardness levels with acceptable toughness.

If you require any high, medium or low carbon ferro-chrome for you grinding media production requirements please contact Mineral-Loy.