Study and Application of a Wet Ball Milling Ultra-fine Method for Geological Samples

At present, there are large particles in geological samples prepared in the laboratory, which affect the representativeness of the samples and the accuracy of the analysis results^1-3. With the rapid development of modern analytical instruments, analytical methods based on inductively coupled plasma-mass spectrometry (ICP-MS) have been widely used in laboratories across the country for the simultaneous determination of multiple elements in geological samples due to their low detection limit, high analytical flux, and wide linear range^10-14. Its characteristics of high sensitivity and analytical accuracy, small sample injection volume, and high requirements for sample representativeness are incompatible with the sample preparation technology that serves as the foundation of laboratory analysis work. Therefore, there is an urgent need to research and develop sample preparation methods that reduce sample size and improve representativeness, in order to meet the needs of ultra trace element detection.　　The main methods for preparing ultrafine geological samples include air flow pulverization and ball milling. Air flow pulverization is widely used in the preparation of geological standard material samples, but the sample size prepared in one step is large, which is prone to sample grading and requires secondary mixing^16-17. Dry ball milling is a common geological sample preparation method in laboratories. Prolonging the ball milling time is beneficial for sample refinement, but it can easily cause contamination by the characteristic elements of the ball milling tank material. Wet ball milling can form a slurry between the sample and the grinding aid, which is conducive to the flow of the sample and the grinding ball during the ball milling process, increasing the friction between them, and achieving the goal of further refining the sample²¹.

To establish a method of preparing ultra-fine geological samples by wet ball grinding with water as the grinding aid.

Weigh 20g samples into the ball mill tank made of agate material, and add 20mL water and grinding balls (8 pieces of Φ10mm balls, 16 pieces of Φ5mm balls, and 48 pieces of Φ2mm balls). Cover the lid, and grind at 300r/min for 30min. XRD were used to characterize the crystal state of the minerals during ball milling. The sampling quantity of different elements was explored by ICP-OES and ICP-MS.

(1) A method for preparing ultrafine geological samples by wet ball grinding was established in laboratory. Agate tanks were used for ball milling, and there was a low risk of sample contamination. When water was used as a liquid grinding aid and the ratio of solid to liquid (m/V) was 1, the grinding effect was better. (2) Four representative samples (rock, soil, sediment, and rare earth ore) were ball milled using this method, and the test results showed that the particle size of the samples after ball milling reached 1000 mesh. After wet ball milling of 60 samples of unknown matrix types, the D₅₀ was less than 5μm, and the D₉₀ was less than 19μm. (3) The grinding aid can reduce the surface energy of the grinding material and improve the grinding efficiency. The micromorphology of the samples prepared by ball milling showed that the large particles were significantly reduced, wet ball milling makes the particle size of each sample finer and the particle distribution more uniform, and the maximum particle diameter was reduced by about 5 times. The sampling quantity test was carried out on the rock sample (GBW07104) after ball milling, and 46 elements results were statistically analyzed. The sampling quantity was reduced to 2mg except for Mo, Cd, Cr.

After ball milling, the large particles are significantly reduced and the particle distribution is more uniform, which better solves the problem of ultrafine preparation of relevant types of samples in the laboratory, but there are some problems such as a long sample preparation process. The ultra-fine preparation technology of geological samples, combined with XRF and LA-ICP-MS, establishes a green analysis method of solid sampling with small sample quantity, and assists in the development of the detection industry.