Crushing & Grinding
The purpose of crushing and grinding is to reduce the size of the potential ore particles to that where there is sufficient REE mineral liberation and the size/size distribution is suitable for the chosen downstream mineral recovery process. This allows for the subsequent REE mineral recovery processes to function in an economically efficient manner. Not only is the average particle size a key parameter, the distribution of sizes is important too, with the narrower the size distribution, the better. Each possible mineral recovery process has a certain size range of best performance, and the final product of the crushing/grinding circuit must match the chosen process size requirements.
Proper grinding is critical in order to get good metallurgical test results, as the entirety of downstream processing is dependent on optimal grinding and recovery at this stage. Due to this, sometimes the recovery and grades of first stage of processing concentrates are seen to be lower than expected, and elevated REE values are found in the tailings. Occasionally, the REE mineral grains are weathered, hydrothermally altered, or otherwise very fine, and traditional methods of grinding and processing are insufficient for creating the proper size/size distribution to make a first stage mineral concentrate. Alternative processing routes must be investigated.
The first stage of the crushing/grinding process in a production facility begins with blasting of the ore body, where the blasted material is composed of a very wide range of sizes. Very fine material from the zones surrounding the blast holes, to large boulders exist, which require additional breakage. Sometimes soft ores can be directly recovered with excavating equipment.
The material to be crushed from exploration projects comes from drill cores, trenching, chips or surface samples.
Crushing is performed in stages using a combination of jaw, cone and impact crushers and is performed dry. The material is moved by mechanical means with conveyor belts. Sometimes the blasting and crushing fines are washed off the ore, and pumped as a slurry directly to the grinding circuit. The upper size limit for crushing is approximately 1 meter, and the final crusher product is about 1 cm.
Grinding is performed at production scale almost exclusively with ball mills, although ball mill/rod mill combinations have been used in the past. For lab scale work, small ball and rod mills are used. The grinding media for a ball mill is usually steel balls sized from about 1 to 10 cm in diameter, and for rod mills about 1 to 10 cm in diameter, with the length being slightly shorter than the length of the rod mill. The grinding circuit product can be made very fine, if necessary, with the majority of material finer than 20 microns (um).
Lab / Pilot Scale Rod Mill
The product of the grinding circuit is usually separated into fine and coarse fractions, with the fine fraction being sent to the downstream processing circuits and the coarse material being returned to the grinding circuit for additional size reduction. It is a good idea to remove the fines as soon as possible in the grinding process, as regrinding of the fine fraction often leads to even finer material that can be difficult to recover.
The grinding circuit product is characterized before use in the mineral processing circuit. Terms like p80 = 106 um are used, which means that 80 wt% of the material will pass a sieve containing openings of 106 um. The staff responsible for the grinding process needs to work closely with the project mineralogist to ensure that the correct amount of REE mineral liberation has been obtained in the grinding circuit product. Only after the mineralogist has given his/her approval of the grinding circuit product, should mineral processing begin. It is sometimes useful to measure mineral processing recovery as a function of grinding circuit product size. This helps the grinding engineers to make the necessary adjustments to the physical components of the grinding circuit.