|Tungsten carbide||10-100 g||Fast, tougher than the others but contaminates samples with W, Co, C, and traces of Ta.|
|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, suitable for very small samples.|
Tungsten carbide vessel
The ring mill is designed to crush small rock fragments to a fine-grained powder that can later be analyzed for trace elements or other geochemical characteristics. The ring mill should be used after large samples have been crushed to fingernail size or smaller, such as by using the RockLabs® hydraulic crusher/breaker. We have stainless steel 1/4" (6mm) sieves available to help make sure all pieces are appropriately small.
The ring mill is surrounded by an outer high-strength steel bowl and lid.
The inside of the steel bowl and lid are lined with a layer of high-purity tungsten carbide, which is a brittle ceramic almost as hard as aluminum oxide, and much denser and tougher. The tungsten carbide ring and puck go into the bowl. The dust seal is made with a rubber o-ring. This o-ring is glued in place, so don't try to remove it.
The high density and large size of this vessel make it ideal for quickly crushing large amounts of coarse sample. However, the tungsten carbide contains a cobalt carbide binder, so samples will be contaminated with tungsten, cobalt, carbon, and also traces of tantalum. Oddly enough, our vessel seems to be effectively free of niobium based on analyses of crushed, cleaned quartz crystal.
The pre-crushed samples should be well-mixed, and properly split so that the fraction crushed is representative of the original sample. A scale helps ensure that no more that 100 grams of your sample is put in the vessel at once, though using a weight-calibrated volume of a small container is quicker. Too much sample won't damage the vessel, but results in inefficient crushing and can even jam the ring and puck. Never crush <10 grams in this vessel, or put in only a few rock pieces, as this probably will damage the vessel.
A coarsely crushed sample is weighed on scale before putting it in the Mill. A weighing boat can be made from paper to hold the fragments, but change the paper with each sample.
Pour the sample split into the vessel bowl evenly distributing it between the ring-puck and ring-bowl spaces.
This shows the positions of the ring and puck inside the vessel before the sample is added. 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.
Next, prepare the shaker. Needless to say, you should clean the top of the shaker before using it to lessen dust contamination.
These cardboard spacers must be put in the vessel hole on top of the shaker. They keep the threaded clamp from coming down too far and coming off its guide pin.
Make sure there are at least three pieces of cardboard to prevent damage to the clamp system. Clean them if they get dusty.
Put the vessel on the shaker, on top of the cardboard.
The vessel should fit perfectly in its position under the clamp. turn the 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 1/4 turn.
Once the bowl is secure, close lid of the the sound-dampening cabinet.
Circled in white is the 208 V, 3-phase outlet for the shaker motor. Above the outlet are the black and red start/stop buttons, and above those is the large red ON/OFF disconnect switch. Be sure the disconnect switch is off, then plug in the shaker. Push in the black START switch, and leave it there. Don't use the push buttons as they tend to pop off. Turn the shaker on using the disconnect lever. A crushing time of 2 minutes is commonly suitable for 50-100 g samples of many rocks. 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. You can also sieve a small portion to see if any grains are still too big. For guidance, see the sample prep section of a geochemistry text. If it is still too coarse, put the lid back on and grind it for a longer time. If the samples are 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, disassemble the vessel and collect all the powder onto a clean sheet of paper. Pour, brush, or gently tap the rock powder off of the ring mill parts. Don't drop the parts! Transfer the powder to a clean storage container.
Cleaning the vessel parts prevents contamination of one sample with the next. Note: The vessel parts are heavy, so be careful handling them during cleaning.
A good plastic scrub brush and water can be used for cleaning. Scrub the vessel parts in hot tap water, then rinse in deionized water and put on a towel to drain and dry. The warm vessel parts will largely dry by themselves in a few minutes. Use paper towels to get any residual drips. Thorough cleaning is necessary to prevent contamination between samples. The top of the shaker and inside of the orange plastic housing should also be cleaned as necessary. The entire work space, from the balance to the tabletop, should be cleaned between samples.
Large aluminum oxide vesselWe have a medium-size Specs® aluminum oxide grinding vessel. The grinding faces contain a small quantity of MgO and SiO2 to act as a binder for the Al2O3 particles. It is rated up to 100 g of sample, but in practical terms the range is 10 to 60 g. The low density of aluminum oxide makes it less efficient at crushing large amounts at a time, or for especially hard or tough minerals like garnet or quartz. This vessel is appropriate for a wide variety of rocks that must be prepared free of carbon, tungsten, cobalt and/or tantalum contamination. The vessel material is very hard and wears slowly, so contamination of the sample is not much of a worry. 5 minute grinding times, or more, are typically necessary.
The aluminum oxide vessel only has a puck, a bowl, and a lid. Unlike the tungsten carbide vessel, the rubber o-ring dust seal is removable and should be washed after each run, like the other parts. The plastic thing on the lower left is an adapter plate that sits on top of the vessel. It permits the pressure of the clamp to be distributed closer to the rim of the lid ranther than just the center.
Here the puck is in the bowl and the o-ring is back in place. It is ready for sample to be added and the lid to be put on. Remember to brush off sample dust that falls on the rim and on top of the puck, or the seal may not be good.
Here the lid is on the vessel, and the plastic adapter plate is sitting on top of the vessel with the machined side up. Except for the smaller sample size, the removable o-ring, and the adapter plate, use of this vessel is the same as with the larger tungsten carbide vessel.
Small aluminum oxide vesselWe also have a small Rocklabs® binder-free aluminum oxide grinding vessel. It can be run with 1 to 10 grams of sample, and is particularly suited for fine-grained rocks or pure minerals, and samples that must be free of carbon, tungsten, cobalt and/or tantalum contamination. The vessel material is very hard, but rather low density so its crushing efficiency is not as great as the tungsten carbide vessel. 5 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 on the left is a plastic adapter plate that lets the small vessel fit onto the shaker.
Here the lid is on the vessel, and the vessel is in the plastic adapter plate. This vessel has no rubber o-ring. The dust seal is made by the polished lid and bowl faces themselves, so keeping the matching faces free of sample dust is important. Except for the smaller sample size and the adapter plate, use of this vessel is the same as with the larger tungsten carbide vessel.
The above information was provided with the aid of the RockLabs® ring mill Instruction Manual. This web page was originally prepared by Mike Nigro.