1/5/2017 Course Objective Kwame Nkrumah University of Science & Technology, Kumasi, Ghana • Determination of the constituents of ores and metallurgical products for: – prospecting METE 256 ASSAYING – ore reserve calculations – control of processes (gravity concentration) – recovery calculations – smelter schedule Dr. Anthony Andrews – bullion sales, etc Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering www.knust.edu.gh Course Outline Course Assessment • Sampling • Quizzes – 10 points – Methods of sampling • Mid Exam – 20 points – Sampling dividing techniques – Weight of samples relative to size of particles • Final Exam – 70 points • Statistical evaluation of data • Metallurgical testing – Bottle roll test, Column leach test, Acid digestion, Fire assaying, Diagnostic leaching • Characterization and instrumental methods of analyses www.knust.edu.gh www.knust.edu.gh Fire Assaying - Introduction Fire Assaying - Background • Many methods have been developed and refined over the years, but “ Fire Assay ” remains a favoured method for determining the • The particular fire assay method under discussion is total gold content of a sample. aimed only at measuring • In this method, a pulverised mineral sample is dissolved using heat Gold and Precious Metals and fluxing agents. • Precious metals are extracted from the melted material using • Variations of fire assay can be used for other metals, molten Lead (Pb). however, in most instances other analytical methods are • The precious metals are then separated from the Lead in a favoured secondary process called “ cupellation ”. • The gold content of the precious metals collected is then determined, using a variety of analytical techniques. www.knust.edu.gh www.knust.edu.gh 1
1/5/2017 Fire Assaying – Applications Traditional Fire Assay Method (After Sample Preparation) • Soil samples 1. Sub-sampling & Catch-weigh 2. Fluxing • Exploration drill samples 3. Firing 4. Cooling & Separation • Grade control 5. Cupellation 6. Parting & Dissolution • Mill solutions 7. Analysis • Tailings www.knust.edu.gh www.knust.edu.gh Sampling Significance of Sampling • A process of taking a portion from a bulk of material and • Convenience in size for transportation and testing using that portion to represent the bulk of material. Or • A sample is a small amount of material removed from a • Obtain the desired information at the smallest cost bulk, such that it contains all the components in the proportion in which they occur in the original lot. • Entire bulk may be inaccessible, too massive or too • Why Sample??? dangerous to deal with. E.g human blood www.knust.edu.gh www.knust.edu.gh Important Considerations in Categories of Sampling Sampling • Exploratory • Representative of the bulk – Samples taken during prospecting, exploration and proving of a • Results from analysis of the sample should be appropriate to mine predict the behaviour of the bulk • No sample can provide absolute information about the bulk • Controlled • Statistical technique – provide an estimate within probability limit – Samples taken to determine the content of specific constituents • All the components in the bulk should have equal chance of in a given environment reporting into the sample • Pre-sampling preparation to reduce biasness www.knust.edu.gh www.knust.edu.gh 2
1/5/2017 Principles of Sampling Methods for Sampling Material in a Lab • The distribution of values in an ore body is never Stratified or Unstratified uniform • When is this sampling • The results of the sampling shall represent as truly as technique used? possible the average metallic content of the ore/bulk • Where will you take a sample material from? • Each single sample must represent a true average of that portion of bulk from which it is taken www.knust.edu.gh www.knust.edu.gh Methods for Sampling Material in a Methods for Sampling Material in a Lab Lab Grab sample Random – chance • Simplest, quickest, and most • Where will you take a sample flexible method • from? It can be carried out on small quantities using spatulas, or on large quantities using shovels Systematic – orderly • This method uses the least Mixing a sample on a rolling mat. equipment, but also is the most • Where will you take a sample Mix by first drawing corner A so that prone to human biases and has a the sample rolls towards C, then from? higher variance between samples drawing corner B to corner D, then than other methods. drawing corner C to corner A, then corner D to corner B, then repeat. www.knust.edu.gh www.knust.edu.gh Methods for Sampling Material in a Sample Dividing Methods Lab • The sample does not pass Composite sample through the sample device and hence prone to error • Individual samples combined as single sample • Sample is taken from the surface where it may not be typical of the mass. • Shake sample before Scoop sampling sampling. www.knust.edu.gh www.knust.edu.gh 3
1/5/2017 Sample Dividing Methods Sample Dividing Methods Chute-Type Riffle Sampler Coning and quartering www.knust.edu.gh www.knust.edu.gh Sample Dividing Methods Comparison of Lab Sample Devices Rotary Riffle Splitter Standard Deviation of Sampling Method Samples (%) Cone & Quarter 6.81 Grab Sampling 5.14 Chute-Type Sample Splitter 1.01 Rotary Riffle 0.125 www.knust.edu.gh www.knust.edu.gh Sampling Problems and Sampling Problems and Requirements Requirements • Problems in sampling centers on: • Degree of representativeness is based on heterogeneity – Nature and efficiency of sampling process • Issues with variations in the distribution of components within the – Weight reduction in the lab bulk such as: – Size segregation – Correctness in the interpretation of data – Mineralogy – Reliability of results – Chemical composition – Accuracy of results – Grade – Precision of results – Moisture content – Biasness in sampling and measurement – Weight • Incorrectness of the above will result in sampling error – Shape www.knust.edu.gh www.knust.edu.gh 4
1/5/2017 Size effect on sample integrity Sampling Calculations using Gy’s Method • Mineralogy, grade and moisture content may vary with size • This method is a general-purpose calculation to determine the minimum size of sample needed to ensure that it will be • Bulk material …Gross sample…Lab…Measurement representative of the whole lot, within specified limits. – Samples for lab measurement are obtained by standard techniques – Samples for lab measurement can be size-biased Before using, approximate estimates of the following will be needed: • Coarse samples presents challenges in size volume reduction • The content of the species of interest in the lot (assay) • Smaller volume samples are more representative when particle • The general shape of the particles size is fine • The densities of the various species and phases present • The particle size distribution • The degree of liberation, and the grain size www.knust.edu.gh www.knust.edu.gh Sampling Calculations using Gy’s Sampling Calculations using Gy’s Method Method Basic Equation: Basic Equation: When W is much larger than M, the equation is simplified to: www.knust.edu.gh www.knust.edu.gh Gy’s Equation – Working out C Calculating with incomplete information 𝑁 − 1 1 𝑇 2 = 𝑔𝑚𝑛𝐸 3 Make the following conservative assumptions: 𝑀 • f = 0.5 (normal blocky particles); Where C is fglm • g = 0.75 (narrow size distribution. Use g = 1 if the sample is • f = particle shape factor (describes the shape of the particles) obviously monosized and 0.25 for broad size distribution); • g = granulometric factor (describes how much variation there is • l = 1 (grains are as large as the particles) in the size of particles) • l = liberation factor (how close to liberation the material has • The value of m will still need to be calculated, based on your best been ground) estimate of the assay of the sample and the densities of the • m = mineralogical composition factor (describes how much of components of interest. a rock is made up of the element of interest at a given grade) www.knust.edu.gh www.knust.edu.gh 5
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