Analytical methods manual for the Mineral Resource Surveys Program, U.S. Geological Survey

PrincipleMost samples of naturally occurring material require some kind of physical preparation prior to chemical analysis.Samples require preparation to effect one or more of the following: (1) reduce the sample to a size that is more conveniently transported; (2) increase the sample surface area to enhance the efficiency of subsequent chemical attack;(3) homogenize the sample to ensure that a subsample is representative of the entire sample; and (4) separate the sample into components based on mineralogy, grain size, or physical and morphological criteria.Sample preparation is an important step in the analytical process.Without careful preparation, and attention to inter-sample contamination, the worth of the subsequent analyses is significantly diminished.Rock samples are reduced to !4-cm fragments in a jaw crusher.The crushed sample is split, if necessary, and fed into an operating and properly adjusted Braun vertical pulverizer equipped with ceramic plates.The sample is pulverized to approximately minus 100-mesh (<150 um) and mixed to insure homogeneity for subsequent analysis.Mineral samples with distinctive cleavage planes (i.e.mica flakes) can present a problem in pulverizing due to the crystal structure of the sample.In some methods where the quality of pulverization is critical in obtaining accurate results, shatterboxing the sample is required.The sample is placed in either a ceramic or agate shatterbox and ground until 100 percent passes a 100mesh screen. ScopeApproximately 50 samples can be processed per day. ApparatusSample cartons, 3-oz (l7/s" x 2] /£") Large sample funnel, plastic Compressed-air source, dry air, 40 psi Core splitter, if available Rock hammer Flexible hand pad, 6" x 4" x 1" (15 cm x 10 cm x 2.5 cm) Steel plate, approximately 8" x 8" x 1" (20 cm x 20 cm x 2.5 cm) "Chipmunk" jaw crusher Knife Brush, automotive parts cleaning, stiff bristle Brush, wire Vertical pulverizer with ceramic plates, catch pan, and cover plate Silicon carbide, approximately 60-to 80-mesh (250 um to 180 pm) Jones splitter, with V£" riffles (1.25 cm) Tube-type revolving mixer, with tube diameter to accommodate sample cartons Rectangular aluminum pans to fit under Jones splitter, to serve as catch pans for the crusher, and to hold the samples during various stages of processing Grease gun for lubricating equipment Kimwipes or paper towels Assortment of tools for equipment maintenance Safety precautionsEye and ear protection and a dust mask must be worn and it is recommended that a lab coat be worn.Caution must be exercised in operating the equipment, particularly the jaw crusher and the Braun pulverizer, which have the potential of inflicting serious injury if not properly and carefully used.Keep your hands, hair, and clothing away from any moving machinery parts.Remove all jewelry before you begin work.Belts on equipment must be guarded to prevent catching clothing, hands, hair, etc.Power should be turned off prior to dislodging any jammed material from the equipment with a push stick.Power should likewise be turned off prior to making adjustments to the equipment, except when adjusting the grinding plates of the pulverizer.Rock particles and fragments ejected from the crusher and grinder can cause injury, operate grinder behind safety shield.Compressed air, used to clean the equipment and work area, presents a safety hazard, especially to the eyes.Particles of debris propelled by the high velocity air stream present an additional hazard.The compressed-air stream should never be directed toward the face.A fan or hood exhaust should be used to vent dust.See the CHP for further information. Preliminary procedureCheck the Request for Analysis form (RFA) for notes on mineralogy of samples, requests for preparation that vary from standard procedure, and disposition of excess sample.Verify that the number of samples received and the field numbers on the sample collection/transport bags correspond to the number of samples and field numbers listed on the RFA.If they do not correspond, contact Sample Control.Properly label the correct number of sample cartons with the laboratory number assigned to each sample.Label both the container tops and sides using permanent ink markers, or premade labels.Affix premade labels to the tops and side of the cartons with clean transparent tape.Place the labeled sample containers in a cardboard tray labeled with the required information: (1) assigned job number, (2) submitter's last name, and (3) number of samples in the job. ProcedureCheck to see sample-size of chips produced by jaw crusher is adequate.If not, adjustments to the jaw crusher are made by varying the number of metal shims inserted behind the stationary jaw plate.Increasing the number of shims reduces the crushed rock fragment size.The spacing between the sides of the movable jaw and the cheek plates can also be varied with metal shims inserted between the cheek plates and the body of the crusher.Open the sample bag and place the sample into a loaf pan in preparation for crushing and splitting.Using a core splitter or a rock hammer and steel plate, break all large pieces down to approximately 2W x 2", a size that readily fits into the crusher.Clean the core splitter, hammer, and plate with a wire brush and compressed air prior to use and between samples.Place a second loaf pan under the jaws of the crusher to catch the crushed sample and begin feeding the sample into the jaws.Do not overload the jaws.Overloading may cause the crusher to bind.Hold the 6" x 4" x W hand pad over the mouth of the crusher to prevent rock chips from flying out of the jaws.Crush the entire sample, using more than one loaf pan if necessary.Turn the power to the crusher off and thoroughly clean the crusher mouth, jaws, and cheek plates by alternately scrubbing the components with the parts brush and blowing away dust and fragments with compressed air.Lodged rock fragments and buildup of powdered rock material can be removed with a long, thin steel knife blade.When extreme cleanliness is required to avoid low-level contamination or when ore-grade samples are being prepared, a small quantity of quartzite gravel should be crushed before crushing each sample to clean the apparatus.If such a cleaning gravel is not available, a small amount of the next sample to be prepared should be crushed and discarded with the Jaw crusher scrubbed out prior to preparing the whole sample.When necessary, split the entire crushed sample by distributing it evenly into a Jones splitter to obtain a representative split of sufficient size to fill the sample carton.Save or discard the remainder of the sample, whichever is noted on the RFA.It is suggested that the bulk material be saved until after all the samples have been pulverized.In the event of sample loss during pulverizing, additional sample is then available.Clean the splitter and splitter pans with compressed air prior to splitting the next sample.Use the knife to dislodge fragments caught in the riffles of the splitter.Turn on the Braun pulverizer and check the adjustment of the ceramic grinding plates.Plate adjustment is checked visually with the aluminum catch pan removed and by sound.The rotating lower plate should be evenly contacting the stationary upper plate and there should be a slight "skipping" sound.Adjusting the plates closer than this without any sample material present can cause the plates to chatter and bind.If the chattering becomes severe enough, the plates can crack or shatter, rendering them useless.Adjust the plates by holding the threaded adjustment shaft, at the top of the pulverizer, stationary with a 12" crescent wrench and loosening the two lower adjustment nuts.The upper adjustment nut is then slowly turned to raise the shaft.This brings the lower revolving plate closer to the stationary upper plate; the closer the spacing between plates, the finer the size of the ground sample.When the desired spacing is achieved, the lower nuts are screwed tight against the lower side of the topmost, fork-shaped, pulverizer frame piece, locking the adjustment shaft in place.Proper adjustment of the plates is verified by pulverizing several ounces of quartz sand.Sieve the ground material through a series of mesh sizes, bracketing the desired particle size.With experience, the operator can quickly determine the suitability of a grind by visually examining, and by feel of, the ground sample. Equipment maintenanceAll mechanical equipment should be lubricated at least once each week or every 250 samples.Use a grease gun containing metal-free grease (i.e.free of elements of interest in analysis) and make certain the lubricant is injected into all of the grease fittings.Do not over-lubricate and wipe excess grease from the fittings with a Kimwipe or paper towel.Check and make sure all nuts and bolts are securely tightened, prior to turning on any equipment.Check moving parts, including pulverizer belt and grinding plates, for wear.Replace worn parts.