1/5/2017
Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Course Objective • 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
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Course Outline Course Assessment • Sampling – Methods of sampling • Quizzes – 10 points – Sampling dividing techniques • Mid Exam – 20 points – 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
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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 favoured • The precious metals are then separated from the Lead in a 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 • Exploration drill samples 2. Fluxing 3. Firing • Grade control 4. Cooling & Separation 5. Cupellation • Mill solutions 6. Parting & Dissolution 7. Analysis • Tailings
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Sampling Significance of Sampling • A process of taking a portion from a bulk of material and using that portion to represent the bulk of material. • Convenience in size for transportation and testing Or • A sample is a small amount of material removed from a bulk, such that it contains all the components in the • Obtain the desired information at the smallest cost proportion in which they occur in the original lot.
• Why Sample??? • Entire bulk may be inaccessible, too massive or too dangerous to deal with. E.g human blood
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Important Considerations in Categories of Sampling Sampling • Representative of the bulk • Exploratory – 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
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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
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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 from? the sample rolls towards C, then 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
Composite sample • The sample does not pass 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.
Scoop sampling • Shake sample before sampling.
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3 1/5/2017
Sample Dividing Methods Sample Dividing Methods
Chute-Type Riffle Sampler
Coning and quartering
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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
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Sampling Problems and Sampling Problems and Requirements Requirements • Degree of representativeness is based on heterogeneity • Problems in sampling centers on: • Issues with variations in the distribution of components within the – Nature and efficiency of sampling process bulk such as: – Weight reduction in the lab – 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 – Shape • Incorrectness of the above will result in sampling error
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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
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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:
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Gy’s Equation – Working out C Calculating with incomplete information 1 1 푆2 = 푓푔푙푚퐷3 − Make the following conservative assumptions: 푀 퐿 • 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)
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5 1/5/2017
Calculating with incomplete Calculating with incomplete information information • The liberation factor, l, is a measure of the degree of dispersion • The composition factor (m), is calculated from the formula: of the valuable material through the bulk, and of the homogeneity of the material. 1 − 푎 푚 = 1 − 푎 푟 + 푎푡 • It is calculated from the expression: 푎 퐿 푙 = 푑 Where: r = specific gravity of the valuable component t = specific gravity of the remainder of the material Where: a = fractional average assay of the valuable substance L = the size where the values are essentially completely liberated (grain size), cm d = sieve size
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Gy’s Equation Work Example
• A sample of 200 g is to be taken and used for fire • Simplified version of Gy’s equation: assaying from a bulk sample of weight 5 kg with 푊 ≥ 125000푑3 average particle size 10 mm. How fine should the material be crushed before a representative sample can W = weight, g be taken? d = diameter of the largest particle (cm)
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Home Work Important Terminologies Bulk Materials Parameters:
• Materials of Interest: CuFeS2 in a silica matrix, 1.5% Cu • Replicates: - samples of the same size that are carried through an (4.3318% CuFeS2); Top Size = 1.5 cm; CuFeS2 grain size = 0.01 analysis in exactly the same way. cm. • Precision: - the closeness of data to other data that have been obtained in exactly the same way. • Desired sampling accuracy: ±0.02% Cu, certainty of 0.99 (2.576 • Accuracy: - the correctness of measurement or closeness of a result standard deviations) to its true or accepted value. • Outlier: - an occasional result in replicate measurements that obviously differs significantly from the rest of the results. • CuFeS2 specific gravity = 4.2; Overall specific gravity = 2.8; Broad size distribution. • Bias: - a measures of the systematic error associated with an Determine the minimum sample weight (in grams) needed for analysis. testing.
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6 1/5/2017
Important Terminologies Random Error • Caused by unknown and unpredictable changes in the experiment • Error:- a measure of deviation of the observed or calculated value • Examples of causes of random errors are: from the true value – electronic noise in the circuit of an electrical instrument, – irregular changes in the heat loss rate from a solar collector due • Absolute error:- the difference between the measured value and to changes in the wind. the true value. • Random errors often have a Gaussian normal distribution 퐸푎 = 푥푖 − 푥푡
• Relative error:- absolute error divided by the true value The Gaussian normal distribution. m = mean of measurements.
푥푖 − 푥푡 s = standard deviation of measurements. 퐸푟 = × 100 푥푡
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Systematic Error Gross Error • Occur with measuring instruments when the calibration of the • Gross errors occur occasionally, are often large, and may instrument is not known correctly. – Instrument has linear response cause a result to be either high or low. The accuracy of • Two types of systematic errors measurements is often • Gross errors lead to outliers – Offset or zero setting error reduced by systematic – Multiplier or scale factor error errors • Gross errors can be avoided by using two suitable Systematic errors in a linear measures instrument (full line). Broken line shows response 1. Proper care should be taken in reading, recording and of an ideal instrument calculating data. without error. 2. By increasing the number of experimenters
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Sample Extraction Methods Stationary and Continuous Streams • Samples can be taken by hand or by using machines Stationary Continuous – Hand sampling is slow and prone to bias • Heaps • Ore on conveyor belt • Automatic sampling is done by mechanically driven • Drums • Slurries in pipes sampling cutters designed to cut the falling ore or pulp at • Trucks predetermined intervals • Bars • Bulk sampling can be done in continuous streams or • Bags stationary systems • Bucket conveyor • General rule in sampling: • Slurry in container Whenever possible, a sample should be taken when the material is in motion
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7 1/5/2017
Sampling from a moving streams Sampling from a static slurry/liquid
• The ratio of the cutter width to the diameter of the largest particle Eg, Slurry in a buckets should be made as large as possible with a minimum value of • First agitate to suspend particles 20:1 • Scoop from various sections or pour whole content depending on number of containers and stage of • The collecting device should cover the whole stream sampling
• The device/cutter should be presented at right angles to the • Filter and dry slurry stream. Eg, Reagent or water in container • The speed of the cutter should be constant • First agitate to homogenize system • Scoop from various sections or pour whole content
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Sampling from heaps and dumps Sampling from drums and bags Eg, heap leach pads • May demand the use of pipe and auger sampler • First sample may be taken randomly • Pipe should be long enough to reach the bottom of the heap to be sampled • Additional samples • Samples can be taken at various systematically depths • After sampling, pipe is withdrawn, and sample discharged www.knust.edu.gh www.knust.edu.gh
Sampling from metals and alloys Grab Sampling • Taken with a scoop, shovel, hand, bottle , pipe, etc
• Can follow fixed pattern
• Rapid and cheaper but unscientific
• Chipping the corners of the bar • For bulk sampling, composite may be better • Drilling holes in the bar and using the material that comes out of it is as sample. • For lab samples, standard dividing techniques may • Sawing through the bar and using the dust as sample. provide a more representative sample
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8 1/5/2017
Automatic Vs Manual Sampling on metallurgical plants • To acquire information on ore entering the plant for • When? treatment.
• Where? • To inspect the condition of the ore at selected points during its progress through the plant. • Advantages? • To check the performance of the plant against set targets • Disadvantages? • To correct malfunctions, reduce losses, and improve upon recovery
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Typical Sampling Points in a Plant Sampling from Metallurgical Plant • Head feed • Leaching Head sample • Comminution • Solution • Location: • Crushing • Solids – Crusher product and mill – product • Loaded carbon feed • Grinding circuit samples • Desorption – Ball mill discharge • Eluate • Data taken/Test performed: – Classifier return • Barren carbon – Moisture content to correct – Classifier overflow • Pretreatment for dry tonnage • Concentration • Roasting – Size analysis to check • Flotation products • Biooxidation crusher performance and • Gravity concentration • Smelting correct size for mills – Concentrate • Bullion – Tonnage by a weightometer www.knust.edu.gh www.knust.edu.gh – Tailings
Sampling from Metallurgical Plant Sampling from Metallurgical Plant
Grinding circuit samples Flotation products • Location: • – Ball mill discharge, classifier overflow and underflow Location: – Concentrate – Tailings • Data taken/Test performed: – Pulp density/solid-liquid ratio to control coating of balls. – Density and particle size from classifiers to check classifier • Data taken/Test performed: performance and effect on mill throughput, leaching feed, etc. – Grade by fire assaying – Partial chemical analysis
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9 1/5/2017
Sampling from Metallurgical Plant Sampling from Metallurgical Plant Biooxidation Leaching/cyanidation/adsorption circuit • Location: • Location: – Primary reactors – Head and tail tanks – Secondary reactors – All tanks – Carbon recovery screen • Data taken/test performed: – Acidity, • Data taken/Test performed: – Temperature, – pulp density, Loading of carbon – Fe2+/Fe3+ ratio, – pH – Sulfide, carbon, arsenic, gold grade, – cyanide level, – Bacterial activity, redox potential, – dissolved oxygen, www.knust.edu.gh – carbon content and grade www.knust.edu.gh
Sampling from Metallurgical Plant Sampling from Metallurgical Plant
Elution/desorption/stripping circuit Electrowinning • Location: • Location: – Soaking/preheating tanks – Cell solution – Stripping tank – Cathode
• Data taken/Test performed: • Data taken/Test performed: – Gold in solution – Gold in solution – Loaded and barren carbon grade – Loaded cathode – Caustic-cyanide strength – pH – Temperature
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Sampling from Metallurgical Plant Basic Statistical Analysis Smelting • Location: – Smelter feed – Furnace Sample Population – Smelter products
• Data taken/Test performed: – Purity of gold bullion • A subset of the population is used to estimate the population – Gold value in slag – Gold value in calcined cathode • The sample will therefore be a representative of the population – Temperature www.knust.edu.gh www.knust.edu.gh
10 1/5/2017
Measure of Central Tendencies Measure of Central Tendencies
• Three common ways to measure central tendency: – Mean – Median • Find the mean, median and mode. – Mode • The sample mean, y, is given by:
• Mean is based on quantitative data whereas median is based on position and mode is based on frequency
• where n is the sample size and yi are the measurements
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Measure of Central Tendencies Symmetrical Distribution Difference between precision and accuracy • Mean, Median and Mode are all the same, mound shape, no skewness
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Right Skewness Left Skewness • Mean to the right of the Median • Mean to the left of the Median • Long tail on right • Long tail on left
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11 1/5/2017
Measures of Variability Range
WHAT IS VARIABILITY? • The range is the highest score minus the lowest score • Variability refers to how "spread out" a group of scores is.
Quiz 1 Quiz 2 Examples 1. What is the range of the following group of numbers: 10, 2, 5, 6, 7, 3, 4?
2. Here’s a data set with 10 numbers: 99, 45, 23, 67, 45, 91, 82, 78, 62, 51. What is the range?
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Interquartile Range Variance • The interquartile range (IQR) is the range of the middle 50% of the scores in a distribution. • The variance is defined as the average squared difference • It is computed as follows: of the scores from the mean QR = 75th percentile - 25th percentile Population variance Quiz 1 Quiz 2 where σ2 is the variance, μ is the mean, and N is the population
Sample variance
where s2 is the estimate of the variance and M is the sample mean
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Variance Standard Deviation • The standard deviation is the square root of the variance
• The symbol for the population standard deviation is “σ”; the symbol for an estimate computed in a sample is “s”
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12 1/5/2017
Standard Deviation GOLD
• Classification of gold ores
• Host materials
• Characteristics of gold ores – Equipment used Normal distributions with standard deviations of 5 and 10. – Parameters to observe
• Analyses
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GOLD ORES Classification of Gold Ores • Most noble metal, native occurrence • Also associated with silver, tellurium, bismuth and PGM’s • Typical ore grades: 0.5 to 20 g/t • Primary gold source – ores • Secondary gold sources – gravity concentrates – flotation concentrates – plant tailings – refinery tailings – recycled gold www.knust.edu.gh www.knust.edu.gh Classification of gold ores and typical recovery with traditional methods
Classification of Gold Ores • Non-refractory; – placer, – free-milling, – oxidized
• Refractory – Ultrafine gold particles in the matrix of sulphide minerals – Carbonaceous materials
Most common causes for refractoriness and double refractory ore appearance www.knust.edu.gh www.knust.edu.gh
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Types of Gold Deposits Gold Ore Types • Placer ores • Main ore types • placers easy • Oxidized ores processing • oxidized • Primary ores • free milling • silver rich • iron sulphide bearing • arsenic sulphide bearing • carbonaceous • copper bearing
• antimony bearing refractory • gold telluride bearing
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Gold in its ore (host material) Traditional Gold Recovery • Tiny particles (< 75 μm) • Recovery rate of refractory gold ores can be improved through roasting. • Minute concentration (<0.001%) • Laboratory roasting is done in an electric furnace. • Highly disseminated in the gangue (unwanted) materials (99.999%) • Use of lead and other reagents in laboratory smelting.
• Recovery depends on particle size of gold and degree of association with unwanted materials
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Characteristics of Gold Ores Characterization Methods • X-ray fluorescence (XRF) • Gold grain size distribution • X-ray diffractometer (XRD) • Optical microscope (OM) • Type of gangue minerals • Scanning electron microscope (SEM) • Infra-red spectrophotometer (IRS) • Mineral associations and alterations • Raman spectrometer (RS) • X-ray photoelectron spectroscopy (XPS) • Mineralogical mode of occurrence • Atomic emission spectrophotometer (AES) • Volumetric titrator • Variations of the above items within the same ore body
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14 1/5/2017
Parameters to look out for XRD Analysis • Qualitative and quantitative identification of: – Elements – Minerals – Compounds, etc. • Mineral associations • Many other things like: – Shape/size – Texture – Crack propagation – Presence of microcracks
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Analysis Determination of Moisture • Determination of: Content – Moisture content and pulp density 푀표𝑖푠푡푢푟푒 푐표푛푡푒푛푡 (푔) 푀표𝑖푠푡푢푟푒 푐표푛푡푒푛푡 % = × 100% – Dissolved oxygen, pH and lime addition 푊푒푡 푤푒𝑖푔ℎ푡 (푔) – Cyanide consumption • Aqua regia 푀표𝑖푠푡푢푟푒 푐표푛푡푒푛푡 푔 = 푊푒푡 푤푒𝑖푔ℎ푡 푔 − 퐷푟푦 푤푒𝑖푔ℎ푡 푔 • Acid digestion • Cyanidation • Bottle roll test • If 100 tonnes of ore is treated and the grade is 0.002 g/t, find the quantity of metal recovered assuming the • Column leaching moisture content is 5%. • Diagnostic leaching
• Fire assaying www.knust.edu.gh www.knust.edu.gh
Determination of Pulp Density Determination of Pulp Density Determining solids fraction • Liquid mass from mass fraction of solids
• To determine the percent solids of a slurry from the 푀푠 ∅푠푙 = × 100 density of the slurry, solids and liquid 푀푠푙 휌푠(휌푠푙 − 휌푙) 푀푠 ∅푠푙 = 푀푠푙 = = 푀푠 + 푀푙 휌푠푙(휌푠 − 휌푙) ∅푠푙 푀 • ∅푠푙 = 푠표푙𝑖푑푠 푓푟푎푐푡𝑖표푛 𝑖푛 푠푙푢푟푟푦 푚푎푠푠 푀 = 푠 − 푀 푙 ∅ 푠 • 휌푠 = 푠표푙𝑖푑푠 푑푒푛푠𝑖푡푦 푠푙 • 휌푙 = 푙𝑖푞푢𝑖푑 푑푒푛푠𝑖푡푦 ∅푠푙 = 푠표푙𝑖푑 푓푟푎푐푡𝑖표푛 𝑖푛 푠푙푢푟푟푦 • 휌푠푙 = 푠푙푢푟푟푦 푑푒푛푠𝑖푡푦 푀푠 = 푚푎푠푠 표푓 푠표푙𝑖푑푠 푀푙 = 푚푎푠푠 표푓 푙𝑖푞푢𝑖푑 www.knust.edu.gh 푀푠푙 = 푚푎푠푠 표푓 푠푙푢푟푟푦 www.knust.edu.gh
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Determination of pH and Lime Determination of Cyanide 푝퐻 = −log[퐻+] Consumption • Use to determine the pH of cyanide solutions. • Leaching of gold with cyanide – Ex: pH > 10.5 – Rolling bottle with perforated lid – Columns (cylinders with perforated base) • Precipitation of salts – Miniature tank with a stirrer – pH < 8.5 – Beaker placed on a shaker – Decreases cyanide efficiency – Bottle/beaker/container with magnetic stirrer
• pH meter and a probe are used to measure pH • pH of solution is raised to about 11, before cyanide – Calibration required (Buffer solution) addition
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Bottle Roll Test Column or Percolation Leach • Weigh 1 kg of sample Test • Design parameters used for heap leaching • Prepare 50% pulp density • Crush ores • Adjust pH • Mount in columns • Add cyanide • Irrigate with cyanide • Agitate by rolling bottle for 72 hrs • Several columns mounted to determine the appropriate • Take solution samples at time intervals (1,2,4,12, 24 hrs) particle size, strength of cyanide etc. • Take 100 g samples to determine the tailings grade
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Acid Digestion Acid Digestion
• Perchloric acid (HClO4) • Dissolution of metals/elements using acid – Used for wet ashing when sample contains carbonaceous material • High temperature and/pressure • Hydrofluoric acid (HF) • Fume chamber with extractor is required – Used to digest silica to release occluded minerals
• Small quantities of material can be used • Aqua-regia – Used to determine gold in samples
– Mixture of HNO3 and HCl
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16 1/5/2017
Diagnostic Leaching Conversion of grade from g/l of 1. Water leaching (if tailings material) solvent to g/t of ore 2. Cyanide leaching of liberated gold 3. Digestion with dilute hydrochloric acid to break down weak Question components like carbonates, followed by cyanide leaching Suppose a 50g sample was digested with acid and then 4. Digestion with nitric acid to break down/oxidize components like filtered into a 100 ml volumetric flask and topped to the sulfur, followed by cyanide leaching mark with distilled water. If the AAS reading is 3.5 mg/l, 5. Roast at 750oC to decompose carbonaceous matter, followed by estimate the grade of ore in g/t. cyanide leaching 6. Fire assaying of final tailings to determine gold in quartz, or leaching with hydroflouric acid (teflon beaker) 7. Add all the gold to get the calculated head grade www.knust.edu.gh www.knust.edu.gh
Cynanidation Cynanidation
hrs
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Gold Recovery What is Fire Assaying? Question A Bottle roll test was conducted on 2 kg of a soil sample at 50% • Quantitative method for the solids. 50 g of the tailings was digested with aqua-regia and topped to determination of Au, Ag, Sn, Cu, Hg, 10 ml. The data obtained after AAS is presented in Table 1. Find the Pb and the platinum group of metals head grade and percent recovery for each period and plot a suitable graph. Are there preg-robbers in the sample? • Consist of crucible fusion of Time, h Gold in solution, mg/l weighed amount of sample with suitable reagents 2 1.75 4 1.52 8 3.10 • Two major stages are fusion and 16 4.52 cupellation 24 4.35 www.knust.edu.gh www.knust.edu.gh Tailings 0.92
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Objectives of Fire Assaying Steps in Fire Assaying
• The valuation of a mining property 1. Pulverizing/Sampling
• The basis for buying and selling various materials 2. Mixing of sample and fluxes
• In plant quality control 3. Crucible fusion (assay furnace)
• Accounting and inventory requirements 4. Cupellation (assay/cupellation furnace)
• Environmental considerations. 5. Parting and Annealing OR 6. Acid digestion and AAS finish
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Fire Reagents Borax (Na2B4O7)
o • Borax (sodium tetraborate) • Anhydrous borax melts at 741 C to form a viscous glass, which becomes more fluid at elevated temperatures • Silica
• Soda ash (sodium carbonate) • It is a strongly acidic and readily dissolves almost all basic metal oxides • Litharge (lead oxide)
• Carbon (in the form of flour or charcoal) • The borax melts to form a colourless transparent glass
• Nitre (potassium nitrate) 푁푎2퐵4푂7 → 푁푎2퐵2푂4 + 퐵2푂3 푍푛푂 + 퐵2푂3 → 푍푛퐵2푂4
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Soda Ash (Na2CO3) Litharge (PbO)
• Na2CO3 is a strong basic flux and fuses most readily with • Litharge is a readily fusible basic flux silica to form fusible slag • It acts as an oxidising and desulphurising agent • It is used as an oxidising and desulphurising reagent
o o o • Na2CO3 melts at 850 C, and at 950 C, dissociates • It melts at 883 C and reacts with the reducing agent to partially evolving CO2 and liberating some free alkali liberate metallic lead
푁푎2퐶푂3 → 푁푎2푂 + 퐶푂2 • This metallic lead provides the lead rain, which collects 푁푎2푂 + 푆𝑖푂2 → 푁푎2푆𝑖푂3 the noble metals in the sample to form a lead button upon 푁푎2퐶푂3 + 푁푎2푆𝑖푂3 → 푁푎푆𝑖푂4 + 퐶푂2 solidifying
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18 1/5/2017
Red Lead (Pb3O4) Nitre (KNO3) • KNO is a powerful oxidising reagent that melts at 339oC • Used as alternative to litharge 3 • It decomposes at about 400oC, liberating oxygen • Additonal oxidising effect during fusion • Nitre oxidises sulphides to sulphates, and arsenides to arsenates. • More expensive than litharge and hence not commonly used • It is used most commonly for converting metallic sulphides to oxides
• The disadvantages of nitre are the possibility of oxidising silver and the tendency to cause boiling of the charge
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Reducing Agent Theoretical Reducing Power • Serves two purposes: • Denotes the amount of grams of metallic Pb that is – They reduce sufficient litharge to produce the Pb button, which produced from 1 g of reducing agent added to PbO collects the values – They reduce any ferric oxide present in the sample to the ferrous state so that it can be slagged 2푃푏푂 + 퐶 → 2푃푏 + 퐶푂2 [C = 12; Pb = 207] 414 • Sources of reducing agents: 푅푒푑푢푐𝑖푛푔 푝표푤푒푟 = = 34.5 12 – Those added to the charge Flour or starch as reducing agent – Those already present in the charge 12푃푏푂 + 퐶6퐻5푂10 → 12푃푏 + 5퐻2푂 + 6퐶푂2 2486 푅푒푑푢푐𝑖푛푔 푝표푤푒푟 = = 15.3 • Most effective reducing agent is carbon 162 www.knust.edu.gh www.knust.edu.gh
Oxidising Agent Theoretical Oxidising Power
• The presence of sulphides, arsenic or antimony makes it • Denotes the amount of grams of Pb prevented from being necessary for oxidising agents to be added reduced by 1 g of oxidising agent
• Examples of oxidising agents: 2퐾푁푂3 → 퐾2푂 + 2푂2
– Red lead (Pb3O4), – Manganese dioxide (MnO) and 2푂2 + 4푃푏 → 4푃푏푂
– Nitre (KNO3) 828 푂푥𝑖푑𝑖푠𝑖푛푔 푝표푤푒푟 = = 4.1 202
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Fluxes Fluxes
Reagent Reasons Reagent Reasons Used to provide lead to collect the precious metals. It An acidic flux that forms the principal component of is also a strong basic flux and reacts with metallic many samples. Small amounts are present in the flux to Litharge Silica oxides and silica to form a slag. By far the most prevent attack on the fire assay crucibles when assaying expensive component of a fire assay flux. samples deficient in silica. A powerful basic flux that is usually the principal A powerful oxidising agent added to the flux when Nitre Soda Ash component of fire assay flux. It reacts with silicates to assaying samples containing sulfides form a slag Flour A source of carbon used to reduce the litharge to lead. An acidic flux that lowers the fusing point of all slags. A small amount is added to the flux to provide a Silver Borax It forms fusible complexes with limestone and collection medium for the precious metals magnesite
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Crucible Fusion Crucible Fusion – Possible • Heating (fusion) of the charge in a Layers crucible to attain two products; Pb button and slag
• Litharge is added as a flux (except Pb ore)
• PbO undergoes reduction to produce lead which contain the noble metals
• Formation of matte(s) or speise must be avoided since either of these would attempt to collect some of the values www.knust.edu.gh www.knust.edu.gh
Characteristics of Metallic Slag Properties Phase 1. It should have a comparatively low formation temperature. 1. A minimum amount of impurities 2. It should be pasty at its formation temperature. 2. A bright, soft, malleable button 3. A button close to the desired weight 3. It should be thin and fluid when heated to somewhat above its melting point. 4. A complete recovery of the noble metals 4. It should have a low capacity for noble metals.
5. It should allow a complete decomposition of the sample by the fluxes.
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Slag Properties Preparation of Pb Button for Cupellation 6. It should not attack the material of the crucible to any great extent. • After fusion, the melt is poured into a mould and allowed to cool until the Pb solidifies. 7. Its specific gravity should be low. • The lead is detached from the slag by striking at the 8. When cold, it should separate readily from the lead, and be junction of the Pb and slag with a hammer homogeneous. • The cone-shaped button is hammered into a rough cube 9. It should contain practically all the impurities of the sample. on an anvil.
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Cupellation Steps in Cupellation • Cupellation is the oxidation of the lead and subsequent absorption 1. Preheating of the cupels to drive off any water, organic into a small shallow porous cup called a cupel matter, and carbon dioxide.
• The cupel should have a smooth surface and readily absorb its own weight or a little more than its own weight of Pb without cracking 2. Increase temperature to 950oC and place lead buttons.
• Cupel must be dry before being placed into the muffle furnace and then raised to cupellation temperature of 950oC before button is 3. Lead button melts covering the cupel with a dark scum added composed mostly of litharge.
• The furnace should have an ample supply of air for the oxidation of Pb 4. Molten litharge slide off the surface of the lead and absorbed by the cupel
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Cupellation
• Most of the Pb is absorbed by the cupel in the form of PbO but part is volatilised and carried away as Pb fumes
• In the course of cupellation, the furnace door may be opened slightly to allow air into the furnace to aid the process
• To prevent cracking due to colder air, the front row of cupels are not fed with Pb buttons
• The furnace temperature has to be regulated as all the metal will volatilize if temperature is too high
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Loss of Silver in Cupel Parting of the Gold Beads • Pb/Cu ratio should be around 16 at least, to ensure the absence of • This is the process of separating the Au and Ag obtained after free copper in the system cupellation
• In the determination of gold and silver, the doré beads derived from cupellation are weighed
• Separation achieved by dissolving Ag-Au alloy in acid
– HNO3 or concentrated H2SO4
• Au residue is washed, dried and annealed in muffle furnace until is bright red
• Weigh Au and determine weight of Ag
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Cu2O-PbO phase diagram
Parting of the Gold Beads Annealing • After parting, the bead is annealed and weighed • The concentration of Ag should be high. If low, more silver should be added (Ag:Au at least 3) • Annealing is a heat treatment done to: • Addition of Ag to the fusion is termed inquartation – avoid weighing extraneous materials – Ag-Au ratio should be known – provide opportunity to examine the gold bead for impurities • Recupeling the doré bead with three times its weight in silver – destroy porosity and so prevent the absorption of moisture and gasses
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Weighing Fire Assay Instrumental analysis - AAS • Sensitive balance used (1/500 of a miligram) • Some laboratories complete fire assaying by recourse to • Maximum load capacity is 1 or 2 g digestion of the prill and AAS analysis instead of parting, annealing and weighing
• Conducive environment • This method reduces problems associated with parting such as: • Express weights in proportion in the material sampled – handling of tiny gold beads, – incomplete silver dissolution, and • 1 Assay Ton = 29.166 g of ore – inefficiencies associated with weighing on a 4-6 d.p electronic • 0.001 g of gold in a sample weighing 29.166 g balance. www.knust.edu.gh www.knust.edu.gh
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Fire Assay Fire Assay Instrumental analysis - AAS Instrumental analysis – ICP-OES
• In this technique, silver-gold prills are digested with • ICP-OES has the advantage of being boiling nitric acid and hydrochloric acid able to analyze gold and other • Gold content in the resulting solution may be determined elements such as PGEs in one reading by AAS analysis • Detection level of gold by fire assay/AAS method is • Results compared to known and verified standards 0.01ppm
• The grade per tonne of ore can then be calculated • Detection level Gold by Fire Assay / ICP-OES method = 0.01ppm
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Difficult Samples Difficult Samples Element Comment Element Comment Copper May be reduced to the metal during fusion and reports in the Tellurium Extremely detrimental to the recovery of precious metals during lead button. This can then inhibit cupellation, making it cupellation when present in amounts of > 0.5% in the lead impossible to recover the precious metals. Alternatively, it may button. It lowers the surface tension of the precious metal prill, react with pyrite to form a matte that will preferentially absorb allowing some of the doré to be absorbed by the cupel. gold. Selenium Behaves similarly to tellurium. Nickel Reacts similarly to copper with regards to cupellation, but will create problems at far lower concentrations (> 0.5% in the lead Sulphur Can cause problems by forming mattes with copper, nickel or button). A combination of nickel and copper will create far (Sulfides) iron compounds, resulting in low gold recoveries. Will also greater problems than either of the elements individually. produce large lead buttons if using a high litharge flux. This can be controlled by adding a calculated amount of oxidant. Antimony Completely miscible with molten lead. More than 2% antimony in the lead button may cause cracking of the cupel, resulting in Carbon Can cause major problems during fire assaying due to the low gold recovery. It will also form antimonides with copper, (organic formation of lead shot within the slag, leading to low lead button nickel or iron (speiss) which will preferentially absorb gold matter) weights. This will result in low gold recoveries Arsenic Will form arsenides with copper, nickel or iron (speiss), resulting in low gold recovery. www.knust.edu.gh www.knust.edu.gh
Application of Fire Assay Techniques • In the precious metal industry, fire assay techniques are usually applied to ores, metal alloys and solutions Ores • A general principle is that a siliceous ore requires a basic flux, and a basic ore needs an acid flux
Bullion • The determination of precious metals in metallic alloys is referred to as bullion assaying • Received in the form of shot, borings, granules
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