Comparing Sample Preparation Methods

In my last article, I showed how adequate sample preparation is recommended to significantly improve results when using portable XRF in the mining field – especially for the XRF analysis of elements lighter than calcium. In this article, I’ll be discussing two different types of equipment and methods for preparing your samples: a direct rock sampler and an electric hammer mill. The direct rock sampler allows the collection of fine powders from a rock face or even drill core. The device can be attached to either an AC (110/220V) or DC (18-24V) powered commercial angle grinder. The special diamond cutting blades easily generate a sample by cutting slots, or even making a line cut, on the rock face or drill cores. The powder is collected in a sample tube. This material can then be cupped or pelletized for XRF analysis. This type of sample collection and preparation can result in a spatially resolved, quantitative rock-face analysis in minutes, in the field. Take a look at this short direct rock sampler video to see a demonstration of how to properly collect samples using this tool. To obtain best assays from drill cores, this method of sample preparation is strongly recommended. The electric hammer mill is a powerful device, weighing less than 5 kg, but provides rapid milling of samples and delivers a homogenized, fine powder in just seconds. This mill operates at 150W at 12VDC, so you can use it for hours and run it off a conventional automobile battery. First you need to chip pieces off the rock. Rock chips must be less than 8 mm to feed the mill, so you should use a “crusher tool” to reduce the size of larger rocks. When you make your favorite smoothie, you don’t drop in giant hunks of ice; the large ice pieces could jam the blades and your drink would be called a chunky instead of a smoothie. It’s the same way with milling your samples. Similar to the direct rock sampler method, the fine powder that results from the milling can also be cupped or pelletized. Here’s a video demonstration on how to collect samples using an electric hammer mill. Both of these methods convert the sample into a fine powder so it can then become a pellet. Just like when children fill a pail with beach sand and then flip it over to become a cylindrical castle, you can use a press — just like a pail. Just a few things to keep in mind… The press should be on a firm surface. When compacting the powder, tap gently on the sides to ensure the powder slides to the bottom of the press, then gently tap on the top until the powder doesn’t compact anymore. Give the press one solid firm hit on the top to form the pellet. That’s ONE solid hit; if you hit it several times you will destroy the pellet and have to start all over again. Remember that you are producing solid powder tablets without binder so you have to be firm but gentle. How many times did you see that beach sand castle slide into a heap because the sand wasn’t compacted enough in the pail? To see a side-by-side comparison of steps, see the diagram page of the mining sample prep tools specification sheet.  Both of these methods make it easy to set up a full laboratory in the field and help ensure you get the most accurate results as possible.