|Tungsten carbide||10-100 g||Fast, tough, but contaminates samples with W, Co, C, and traces of Ta (not Nb, oddly enough)|
|Medium Al2O3||10-60 g||Slow, brittle, no significant contamination where normal rocks are concerned|
|Small Al2O3||1-10 g||Slow, brittle, no significant contamination where normal rocks are concerned, only for very small samples|
The orange thing is a sound and safety box, with the shaker inside (both are Rocklabs®). The crushing vessels fit onto the shaker. It is on wheels so it can be moved about. The ring and puck mills crush small rock fragments to a fine powder that can later be chemically analyzed. This sort of powdering is not good for mineral separations. Samples put into the mills must have first been crushed to fingernail size or smaller, such as by using the hydraulic press. We have stainless steel 1/4″ (6 mm) sieves to help make sure all pieces are appropriately small.
Tungsten carbide vessel
This is our Rocklabs® tungsten carbide vessel. The outer parts are made of steel, which rusts, so keep it as dry as possible except when cleaning it. Tungsten carbide is a dense ceramic, similar in hardness to aluminum oxide. By itself it is difficult to manufacture into shapes, so tungsten carbide powder is usually mixed with a cobalt metal binder, and such is the case here. Crushing rocks in this vessel contaminates them with tungsten, cobalt, and also trace tantalum (but not niobium, strangely enough). It also contaminates the sample with carbon, but for most rocks the amount is trivial. If you aren’t interested in analyzing those elements, the large vessel size, toughness, and efficient crushing makes this the best vessel.
The insides are lined with a layer of tungsten carbide. The tungsten carbide ring and puck go into the bowl. The dust seal is a rubber o-ring on the lid, which is glued in place so don’t try to remove it.
The pre-crushed samples should be well-mixed, and properly split so that the fraction crushed to powder is representative of the whole. A scale helps ensure that no more than 100 grams of your sample is put in the vessel at once, though using the weight-calibrated volume of a small container is faster. Too much sample won’t hurt the vessel, but crushing rate slows down dramatically. Pieces that are too big can jam the ring and puck, and halt crushing completely. Never crush <10 grams of rock in this vessel, or put in only a few rock pieces, because that will probably damage the vessel. A paper weighing boat is shown here. If you use one, change it for every sample.
Put the ring and puck into the bowl, and pour the pre-crushed rock, distributing it about evenly in the spaces inside and outside the ring. Wipe away particles and dust from the rim and tops of the puck and ring. Otherwise the lid won’t seat properly. Put on the lid and carry it to the shaker. Don’t drop it! The vessel is heavy, fragile, and very expensive.
The shaker and orange box can become dusty. Clean things up to reduce the chance of contaminating your sample. The cardboard spacers shown here must be put in the vessel hole on top of the shaker. They prevent the threaded clamp from coming down too far. If it does that, it comes off its guide pin, and it’s a pain to get it back on.
Make sure there are at least three pieces of cardboard in the vessel hole. Clean them if they get dusty.
The vessel should fit nicely in its position under the clamp and on top of the cardboard. Turn the clamp handle to lower the clamp until it starts to press on the top of the vessel and you feel resistance. Then tighten the clamp another 1/3 turn. Close the orange lid.
Circled in white is the 208 V, 3-phase outlet for the shaker motor. Above the outlet are black and red start/stop buttons, and above those is large red ON/OFF lever switch. Be sure the lever switch is off, then plug in the shaker. Push in the black START button, and leave it there. Don’t use the push buttons because they like to fall off. Turn the shaker on using the lever switch. A crushing time of 2-3 minutes is commonly good for 50-100 g samples. Coarse rock and particularly hard or tough minerals like garnet and quartz may take longer. Loads closer to 100 g take longer than loads closer to 10 g. When the time is up, turn off the shaker, open the cabinet, and raise the clamp.
Before dumping out the powder, look at it and feel it with your clean fingers to see if it is fine enough. If it is still too coarse, put the lid back on and grind it for a longer time. If your samples are all similar, you don’t have to test the grain size every time. Just find out the proper amount of time for the first sample, and do all the rest the same. When the sample is properly crushed, open the vessel and pour, brush, or gently tap to get all the powder onto a clean sheet of paper. Don’t drop the vessel parts! Pour the powder to a clean storage container.
Cleaning the vessel parts prevents cross-contamination. When cleaning, remember that the vessel parts are heavy, so be careful. A plastic scrub brush and water can be used for cleaning. Scrub the vessel parts in hot tap water, then briefly rinse in deionized water and put the parts on a towel to drain and dry. The warm parts will largely dry by themselves in a few minutes. Use paper towels or kimwipes to get any residual drips. The entire work space should also be cleaned between samples. It will give you something to do while a sample is being crushed.
The steps above show the crushing process using the tungsten carbide vessel, but the other two vessels are used in essentially the same way. Below are some details about the aluminum oxide vessels.
Medium aluminum oxide vessel
This our Specs® aluminum oxide grinding vessel. The white material is aluminum oxide containing a small quantity of MgO and SiO2 as a binder. It is rated up to 100 g of sample, but in practical terms the useful range is 10 to 60 g. The low density of aluminum oxide makes crushing less efficient than tungsten carbide. This vessel is appropriate for samples that must remain free of tungsten, cobalt, and/or tantalum contamination. The vessel material is very hard and wears extremely slowly, so Al2O3 contamination is not a worry, unless a chip of the ceramic breaks off. Five minute grinding times, or more, are typically necessary. This vessel only has a puck, a bowl, and a lid, but no ring. The rubber o-ring dust seal is removable and should be washed after each run, like the other parts. The plastic thing in the image lower left is an adapter plate that sits on top of the vessel. It permits clamp pressure to be distributed closer to the lid rim rather than just the center.
Put the puck is in the middle of the bowl, and put the o-ring back in place. Pour your sample evenly around the puck. Brush off sample dust that falls on the rim and on top of the puck, or the seal may not be good.
Put the lid on the vessel, and the plastic adapter plate on top of the lid, with the machined side up. Except for smaller sample size, the removable o-ring, the adapter plate, and longer crushing times, using the shaker, getting the sample out, and cleaning are the same as with the tungsten carbide vessel.
Small aluminum oxide vessel
We also have this small Rocklabs® binder-free aluminum oxide grinding vessel. It can be run with 1 to 10 grams of sample, and is particularly good for fine-grained rocks or pure minerals. The vessel material is very hard, but rather low density so its crushing rate is rather slow. Five minute grinding times, or more, are typically necessary. While hard, the vessel is very brittle and somewhat friable. Tiny, ~100 µm chips occasionally flake off the puck. The aluminum oxide vessel only has a puck, a bowl, and a lid. The large white thing in the image is a plastic adapter plate that lets the small vessel fit onto the shaker. Put the puck in the middle of the bowl before pouring in pre-crushed sample. As with the other vessels, brush off any sample from the rim and top of the puck.
Here the lid is on the vessel, and the vessel is in the plastic adapter plate. There is no rubber o-ring. The dust seal is made by the lid and bowl faces themselves, so keeping those faces free of sample dust is important. Except for smaller sample size and the adapter plate, putting it on the shaker, removing powdered sample, and cleaning are the same as with the other two vessels.