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Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana
METE 256 ASSAYING
Dr. Anthony AndrewsDepartment of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
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Course Objective
• Determination of the constituents of ores and
metallurgical products for:
– prospecting
– ore reserve calculations
– control of processes (gravity concentration)
– recovery calculations
– smelter schedule
– bullion sales, etc
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Course Outline
• Sampling
– Methods of sampling
– Sampling dividing techniques
– Weight of samples relative to size of particles
• 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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Course Assessment
• Quizzes – 10 points
• Mid Exam – 20 points
• Final Exam – 70 points
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Fire Assaying - Introduction
• The particular fire assay method under discussion is
aimed only at measuring
Gold and Precious Metals
• Variations of fire assay can be used for other metals,
however, in most instances other analytical methods are
favoured
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Fire Assaying - Background• Many methods have been developed and refined over the years,
but “Fire Assay” remains a favoured method for determining the total gold content of a sample.
• In this method, a pulverised mineral sample is dissolved using heat and fluxing agents.
• Precious metals are extracted from the melted material using molten Lead (Pb).
• 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.
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Fire Assaying – Applications
• Soil samples
• Exploration drill samples
• Grade control
• Mill solutions
• Tailings
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Traditional Fire Assay Method(After Sample Preparation)
1. Sub-sampling & Catch-weigh
2. Fluxing
3. Firing
4. Cooling & Separation
5. Cupellation
6. Parting & Dissolution
7. Analysis
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Sampling
• A process of taking a portion from a bulk of material and
using that portion to represent the bulk of material.
Or
• A sample is a small amount of material removed from a
bulk, such that it contains all the components in the
proportion in which they occur in the original lot.
• Why Sample???
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Significance of Sampling
• Convenience in size for transportation and testing
• Obtain the desired information at the smallest cost
• Entire bulk may be inaccessible, too massive or too
dangerous to deal with. E.g human blood
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Important Considerations in
Sampling
• Representative of the bulk
• Results from analysis of the sample should be appropriate to
predict the behaviour of the bulk
• No sample can provide absolute information about the bulk
• Statistical technique – provide an estimate within probability limit
• All the components in the bulk should have equal chance of
reporting into the sample
• Pre-sampling preparation to reduce biasness
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Categories of Sampling
• Exploratory
– Samples taken during prospecting, exploration and proving of a
mine
• Controlled
– Samples taken to determine the content of specific constituents
in a given environment
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Principles of Sampling
• The distribution of values in an ore body is never
uniform
• The results of the sampling shall represent as truly as
possible the average metallic content of the ore/bulk
material
• 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
Lab
Stratified or Unstratified
• When is this sampling
technique used?
• Where will you take a sample
from?
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Methods for Sampling Material in a
Lab
Random – chance
• Where will you take a sample
from?
Systematic – orderly
• Where will you take a sample
from?
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Methods for Sampling Material in a
Lab
Grab sample
• Simplest, quickest, and most flexible method
• It can be carried out on small quantities using spatulas, or on large quantities using shovels
• This method uses the least equipment, but also is the most prone to human biases and has a higher variance between samples than other methods.
Mixing a sample on a rolling mat.
Mix by first drawing corner A so that
the sample rolls towards C, then
drawing corner B to corner D, then
drawing corner C to corner A, then
corner D to corner B, then repeat.
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Methods for Sampling Material in a
Lab
Composite sample
• Individual samples combined
as single sample
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Sample Dividing Methods
Scoop sampling
• The sample does not pass
through the sample device
and hence prone to error
• Sample is taken from the
surface where it may not be
typical of the mass.
• Shake sample before
sampling.
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Sample Dividing Methods
Coning and
quartering
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Sample Dividing Methods
Chute-Type Riffle Sampler
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Sample Dividing Methods
Rotary Riffle Splitter
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Comparison of Lab Sample Devices
Sampling MethodStandard Deviation of
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
Requirements• Degree of representativeness is based on heterogeneity
• Issues with variations in the distribution of components within the
bulk such as:
– Size segregation
– Mineralogy
– Chemical composition
– Grade
– Moisture content
– Weight
– Shape
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Sampling Problems and
Requirements
• Problems in sampling centers on:
– Nature and efficiency of sampling process
– Weight reduction in the lab
– Correctness in the interpretation of data
– Reliability of results
– Accuracy of results
– Precision of results
– Biasness in sampling and measurement
• Incorrectness of the above will result in sampling error
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Size effect on sample integrity
• Mineralogy, grade and moisture content may vary with size
• Bulk material …Gross sample…Lab…Measurement
– Samples for lab measurement are obtained by standard techniques
– Samples for lab measurement can be size-biased
• Coarse samples presents challenges in size volume reduction
• Smaller volume samples are more representative when particle
size is fine
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Sampling Calculations using Gy’s
Method• This method is a general-purpose calculation to determine the
minimum size of sample needed to ensure that it will be
representative of the whole lot, within specified limits.
Before using, approximate estimates of the following will be
needed:
• The content of the species of interest in the lot (assay)
• The general shape of the particles
• 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
Method
Basic Equation:
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Sampling Calculations using Gy’s
Method
Basic Equation:
When W is much larger than M, the equation is simplified to:
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Gy’s Equation – Working out C
𝑆2 = 𝑓𝑔𝑙𝑚𝐷31
𝑀−1
𝐿
Where C is fglm
• f= particle shape factor (describes the shape of the particles)
• g= granulometric factor (describes how much variation there is
in the size of particles)
• l = liberation factor (how close to liberation the material has
been ground)
• m = mineralogical composition factor (describes how much of
a rock is made up of the element of interest at a given grade)
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Calculating with incomplete
information
Make the following conservative assumptions:
• f = 0.5 (normal blocky particles);
• g = 0.75 (narrow size distribution. Use g = 1 if the sample is
obviously monosized and 0.25 for broad size distribution);
• l = 1 (grains are as large as the particles)
• The value of m will still need to be calculated, based on your best
estimate of the assay of the sample and the densities of the
components of interest.
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Calculating with incomplete
information
• The liberation factor, l, is a measure of the degree of dispersion
of the valuable material through the bulk, and of the homogeneity
of the material.
• It is calculated from the expression:
𝑙 =𝐿
𝑑
Where:
L = the size where the values are essentially completely liberated
(grain size), cm
d = sieve size
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Calculating with incomplete
information
• The composition factor (m), is calculated from the formula:
𝑚 =1− 𝑎
𝑎1 − 𝑎 𝑟 + 𝑎𝑡
Where: r = specific gravity of the valuable component
t = specific gravity of the remainder of the material
a = fractional average assay of the valuable substance
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Gy’s Equation
• Simplified version of Gy’s equation:
𝑊 ≥ 125000𝑑3
W = weight, g
d = diameter of the largest particle (cm)
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Work Example
• A sample of 200 g is to be taken and used for fire
assaying from a bulk sample of weight 5 kg with
average particle size 10 mm. How fine should the
material be crushed before a representative sample can
be taken?
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Home Work
Bulk Materials Parameters:
• Materials of Interest: CuFeS2 in a silica matrix, 1.5% Cu
(4.3318% CuFeS2); Top Size = 1.5 cm; CuFeS2 grain size = 0.01
cm.
• Desired sampling accuracy: ±0.02% Cu, certainty of 0.99 (2.576
standard deviations)
• CuFeS2 specific gravity = 4.2; Overall specific gravity = 2.8;
Broad size distribution.
Determine the minimum sample weight (in grams) needed for
testing.
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Important Terminologies
• Replicates: - samples of the same size that are carried through an
analysis in exactly the same way.
• Precision: - the closeness of data to other data that have been
obtained in exactly the same way.
• Accuracy: - the correctness of measurement or closeness of a result
to its true or accepted value.
• Outlier: - an occasional result in replicate measurements that
obviously differs significantly from the rest of the results.
• Bias: - a measures of the systematic error associated with an
analysis.
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Important Terminologies
• Error:- a measure of deviation of the observed or calculated value
from the true value
• Absolute error:- the difference between the measured value and
the true value.
𝐸𝑎 = 𝑥𝑖 − 𝑥𝑡
• Relative error:- absolute error divided by the true value
𝐸𝑟 =𝑥𝑖 − 𝑥𝑡𝑥𝑡
× 100
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Random Error
• Caused by unknown and unpredictable changes in the experiment
• Examples of causes of random errors are:
– electronic noise in the circuit of an electrical instrument,
– irregular changes in the heat loss rate from a solar collector due
to changes in the wind.
• Random errors often have a Gaussian normal distribution
The Gaussian normal distribution.
m = mean of measurements.
s = standard deviation of measurements.
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Systematic Error
• Occur with measuring instruments when the calibration of the
instrument is not known correctly.
– Instrument has linear response
• Two types of systematic errors
– Offset or zero setting error
– Multiplier or scale factor error
Systematic errors in a linear
instrument (full line).
Broken line shows response
of an ideal instrument
without error.
The accuracy of
measurements is often
reduced by systematic
errors
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Gross Error
• Gross errors occur occasionally, are often large, and may
cause a result to be either high or low.
• Gross errors lead to outliers
• Gross errors can be avoided by using two suitable
measures
1. Proper care should be taken in reading, recording and
calculating data.
2. By increasing the number of experimenters
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Sample Extraction Methods
• Samples can be taken by hand or by using machines
– Hand sampling is slow and prone to bias
• Automatic sampling is done by mechanically driven
sampling cutters designed to cut the falling ore or pulp at
predetermined intervals
• Bulk sampling can be done in continuous streams or
stationary systems
• General rule in sampling:
Whenever possible, a sample should be taken when the
material is in motion
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Stationary and Continuous Streams
Stationary
• Heaps
• Drums
• Trucks
• Bars
• Bags
• Bucket conveyor
• Slurry in container
Continuous
• Ore on conveyor belt
• Slurries in pipes
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Sampling from a moving streams
• The ratio of the cutter width to the diameter of the largest particle
should be made as large as possible with a minimum value of
20:1
• The collecting device should cover the whole stream
• The device/cutter should be presented at right angles to the
stream.
• The speed of the cutter should be constant
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Sampling from a static slurry/liquid
Eg, Slurry in a buckets
• First agitate to suspend particles
• Scoop from various sections or pour whole content
depending on number of containers and stage of
sampling
• Filter and dry slurry
Eg, Reagent or water in container
• First agitate to homogenize system
• Scoop from various sections or pour whole content
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Sampling from heaps and dumps
Eg, heap leach pads
• May demand the use of pipe and
auger sampler
• Pipe should be long enough to
reach the bottom of the heap to
be sampled
• Samples can be taken at various
depths
• After sampling, pipe is
withdrawn, and sample
discharged www.knust.edu.gh
Sampling from drums and bags
• First sample may be taken
randomly
• Additional samples
systematically
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Sampling from metals and alloys
• Chipping the corners of the bar
• Drilling holes in the bar and using the material that
comes out of it is as sample.
• Sawing through the bar and using the dust as sample.
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Grab Sampling
• Taken with a scoop, shovel, hand, bottle , pipe, etc
• Can follow fixed pattern
• Rapid and cheaper but unscientific
• For bulk sampling, composite may be better
• For lab samples, standard dividing techniques may
provide a more representative sample
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Automatic Vs Manual
• When?
• Where?
• Advantages?
• Disadvantages?
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Sampling on metallurgical plants
• To acquire information on ore entering the plant for
treatment.
• To inspect the condition of the ore at selected points
during its progress through the plant.
• To check the performance of the plant against set targets
• To correct malfunctions, reduce losses, and improve upon
recovery
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Typical Sampling Points in a Plant
• Head feed
• Comminution
• Crushing
– product
• Grinding circuit samples
– Ball mill discharge
– Classifier return
– Classifier overflow
• Concentration
• Flotation products
• Gravity concentration
– Concentrate
– Tailings
• Leaching
• Solution
• Solids
• Loaded carbon
• Desorption
• Eluate
• Barren carbon
• Pretreatment
• Roasting
• Biooxidation
• Smelting
• Bullion www.knust.edu.gh
Sampling from Metallurgical PlantHead sample
• Location:
– Crusher product and mill
feed
• Data taken/Test performed:
– Moisture content to correct
for dry tonnage
– Size analysis to check
crusher performance and
correct size for mills
– Tonnage by a weightometer
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Sampling from Metallurgical Plant
Grinding circuit samples
• Location:
– Ball mill discharge, classifier overflow and underflow
• Data taken/Test performed:
– Pulp density/solid-liquid ratio to control coating of balls.
– Density and particle size from classifiers to check classifier
performance and effect on mill throughput, leaching feed, etc.
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Sampling from Metallurgical Plant
Flotation products
• Location:
– Concentrate
– Tailings
• Data taken/Test performed:
– Grade by fire assaying
– Partial chemical analysis
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Sampling from Metallurgical Plant
Biooxidation
• Location:
– Primary reactors
– Secondary reactors
• Data taken/test performed:
– Acidity,
– Temperature,
– Fe2+/Fe3+ ratio,
– Sulfide, carbon, arsenic, gold grade,
– Bacterial activity, redox potential, www.knust.edu.gh
Sampling from Metallurgical Plant
Leaching/cyanidation/adsorption circuit
• Location:
– Head and tail tanks
– All tanks
– Carbon recovery screen
• Data taken/Test performed:
– pulp density,
– pH
– cyanide level,
– dissolved oxygen,
– carbon content and grade
Loading of carbon
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Sampling from Metallurgical Plant
Elution/desorption/stripping circuit
• Location:
– Soaking/preheating tanks
– Stripping tank
• Data taken/Test performed:
– Gold in solution
– Loaded and barren carbon grade
– Caustic-cyanide strength
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Sampling from Metallurgical Plant
Electrowinning
• Location:
– Cell solution
– Cathode
• Data taken/Test performed:
– Gold in solution
– Loaded cathode
– pH
– Temperature
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Sampling from Metallurgical Plant
Smelting
• Location:
– Smelter feed
– Furnace
– Smelter products
• Data taken/Test performed:
– Purity of gold bullion
– Gold value in slag
– Gold value in calcined cathode
– Temperature www.knust.edu.gh
Basic Statistical Analysis
• A subset of the population is used to estimate the population
• The sample will therefore be a representative of the population
PopulationSample
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Measure of Central Tendencies
• Three common ways to measure central tendency:
– Mean
– Median
– Mode
• Mean is based on quantitative data whereas median is
based on position and mode is based on frequency
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Measure of Central Tendencies
• Find the mean, median and mode.
• The sample mean, y, is given by:
• where n is the sample size and yi are the measurements
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Measure of Central Tendencies
Difference between precision and accuracy
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Symmetrical Distribution
• Mean, Median and Mode are all the same, mound shape,
no skewness
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Right Skewness
• Mean to the right of the Median
• Long tail on right
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Left Skewness
• Mean to the left of the Median
• Long tail on left
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Measures of Variability
WHAT IS VARIABILITY?
• Variability refers to how "spread out" a group of scores is.
Bar chats of two quizzes
Quiz 1 Quiz 2
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Range
• The range is the highest score minus the lowest score
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• The interquartile range (IQR) is the range of the middle 50% of the
scores in a distribution.
• It is computed as follows:
QR = 75th percentile - 25th percentile
Quiz 1 Quiz 2
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Variance
• The variance is defined as the average squared difference
of the scores from the mean
where σ2 is the variance, μ is the mean, and N is the population
where s2 is the estimate of the variance and M is the sample mean
Population variance
Sample variance
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Variance
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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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Standard Deviation
Normal distributions with standard deviations of 5 and 10.
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GOLD
• Classification of gold ores
• Host materials
• Characteristics of gold ores
– Equipment used
– Parameters to observe
• Analyses
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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 goldwww.knust.edu.gh
Classification of Gold Ores
Classification of gold ores and typical recovery with traditional methods
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Most common causes for refractoriness and double refractory ore
appearance www.knust.edu.gh
Classification of Gold Ores
• Non-refractory;
– placer,
– free-milling,
– oxidized
• Refractory
– Ultrafine gold particles in the matrix of sulphide minerals
– Carbonaceous materials
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Types of Gold Deposits
• Placer ores
• Oxidized ores
• Primary ores
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Gold Ore Types
• Main ore types• placers
• oxidized
• free milling
• silver rich
• iron sulphide bearing
• arsenic sulphide bearing
• carbonaceous
• copper bearing
• antimony bearing
• gold telluride bearing
easy processing
refractory
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Gold in its ore (host material)
• Tiny particles (< 75 μm)
• Minute concentration (<0.001%)
• Highly disseminated in the gangue (unwanted) materials
(99.999%)
• Recovery depends on particle size of gold and degree of
association with unwanted materials
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Traditional Gold Recovery
• Recovery rate of refractory gold ores can be improved
through roasting.
• Laboratory roasting is done in an electric furnace.
• Use of lead and other reagents in laboratory smelting.
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Characteristics of Gold Ores
• Gold grain size distribution
• Type of gangue minerals
• Mineral associations and alterations
• Mineralogical mode of occurrence
• Variations of the above items within the same ore body
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Characterization Methods• X-ray fluorescence (XRF)
• X-ray diffractometer (XRD)
• Optical microscope (OM)
• Scanning electron microscope (SEM)
• Infra-red spectrophotometer (IRS)
• Raman spectrometer (RS)
• X-ray photoelectron spectroscopy (XPS)
• Atomic emission spectrophotometer (AES)
• Volumetric titrator
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Parameters to look out for
• 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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XRD Analysis
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Analysis• Determination of:
– Moisture content and pulp density
– Dissolved oxygen, pH and lime addition
– Cyanide consumption
• Aqua regia
• Acid digestion
• Cyanidation
• Bottle roll test
• Column leaching
• Diagnostic leaching
• Fire assaying www.knust.edu.gh
Determination of Moisture
Content
𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 % =𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (𝑔)
𝑊𝑒𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔)× 100%
𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑔 = 𝑊𝑒𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑔 − 𝐷𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡 𝑔
• If 100 tonnes of ore is treated and the grade is 0.002 g/t,
find the quantity of metal recovered assuming the
moisture content is 5%.
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Determination of Pulp Density
Determining solids fraction
• To determine the percent solids of a slurry from the
density of the slurry, solids and liquid
∅𝑠𝑙 =𝜌𝑠(𝜌𝑠𝑙 − 𝜌𝑙)
𝜌𝑠𝑙(𝜌𝑠 − 𝜌𝑙)
• ∅𝑠𝑙 = 𝑠𝑜𝑙𝑖𝑑𝑠 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑖𝑛 𝑠𝑙𝑢𝑟𝑟𝑦 𝑚𝑎𝑠𝑠
• 𝜌𝑠 = 𝑠𝑜𝑙𝑖𝑑𝑠 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
• 𝜌𝑙 = 𝑙𝑖𝑞𝑢𝑖𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
• 𝜌𝑠𝑙 = 𝑠𝑙𝑢𝑟𝑟𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
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Determination of Pulp Density
• Liquid mass from mass fraction of solids
∅𝑠𝑙 =𝑀𝑠
𝑀𝑠𝑙× 100
𝑀𝑠𝑙 =𝑀𝑠
∅𝑠𝑙= 𝑀𝑠 +𝑀𝑙
𝑀𝑙 =𝑀𝑠
∅𝑠𝑙−𝑀𝑠
∅𝑠𝑙 = 𝑠𝑜𝑙𝑖𝑑 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑖𝑛 𝑠𝑙𝑢𝑟𝑟𝑦
𝑀𝑠 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑠
𝑀𝑙 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑
𝑀𝑠𝑙 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑙𝑢𝑟𝑟𝑦
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Determination of pH and Lime
𝑝𝐻 = −log[𝐻+]
• Use to determine the pH of cyanide solutions.
– Ex: pH > 10.5
• Precipitation of salts
– pH < 8.5
– Decreases cyanide efficiency
• pH meter and a probe are used to measure pH
– Calibration required (Buffer solution)
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Determination of Cyanide
Consumption
• Leaching of gold with cyanide
– Rolling bottle with perforated lid
– Columns (cylinders with perforated base)
– Miniature tank with a stirrer
– Beaker placed on a shaker
– Bottle/beaker/container with magnetic stirrer
• pH of solution is raised to about 11, before cyanide
addition
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Bottle Roll Test
• Weigh 1 kg of sample
• Prepare 50% pulp density
• Adjust pH
• Add cyanide
• Agitate by rolling bottle for 72 hrs
• Take solution samples at time intervals (1,2,4,12, 24 hrs)
• Take 100 g samples to determine the tailings grade
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Column or Percolation Leach
Test• Design parameters used for heap leaching
• Crush ores
• Mount in columns
• Irrigate with cyanide
• Several columns mounted to determine the appropriate
particle size, strength of cyanide etc.
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Acid Digestion
• Dissolution of metals/elements using acid
• High temperature and/pressure
• Fume chamber with extractor is required
• Small quantities of material can be used
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Acid Digestion• Perchloric acid (HClO4)
– Used for wet ashing when sample contains carbonaceous
material
• Hydrofluoric acid (HF)
– Used to digest silica to release occluded minerals
• Aqua-regia
– Used to determine gold in samples
– Mixture of HNO3 and HCl
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Diagnostic Leaching 1. Water leaching (if tailings material)
2. Cyanide leaching of liberated gold
3. Digestion with dilute hydrochloric acid to break down weak
components like carbonates, followed by cyanide leaching
4. Digestion with nitric acid to break down/oxidize components like
sulfur, followed by cyanide leaching
5. Roast at 750oC to decompose carbonaceous matter, followed by
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 gradewww.knust.edu.gh
Conversion of grade from g/l of
solvent to g/t of ore
Question
Suppose a 50g sample was digested with acid and then
filtered into a 100 ml volumetric flask and topped to the
mark with distilled water. If the AAS reading is 3.5 mg/l,
estimate the grade of ore in g/t.
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Cynanidation
hrs
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Cynanidation
Leaching time (hrs)
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Gold RecoveryQuestion
A Bottle roll test was conducted on 2 kg of a soil sample at 50%
solids. 50 g of the tailings was digested with aqua-regia and topped to
10 ml. The data obtained after AAS is presented in Table 1. Find the
head grade and percent recovery for each period and plot a suitable
graph. Are there preg-robbers in the sample?
Time, h Gold in solution, mg/l
2 1.75
4 1.52
8 3.10
16 4.52
24 4.35
Tailings 0.92www.knust.edu.gh
What is Fire Assaying?
• Quantitative method for the
determination of Au, Ag, Sn, Cu, Hg,
Pb and the platinum group of metals
• Consist of crucible fusion of
weighed amount of sample with
suitable reagents
• Two major stages are fusion and
cupellation
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Objectives of Fire Assaying
• The valuation of a mining property
• The basis for buying and selling various materials
• In plant quality control
• Accounting and inventory requirements
• Environmental considerations.
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Steps in Fire Assaying
1. Pulverizing/Sampling
2. Mixing of sample and fluxes
3. Crucible fusion (assay furnace)
4. Cupellation (assay/cupellation furnace)
5. Parting and Annealing OR
6. Acid digestion and AAS finish
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Fire Reagents
• Borax (sodium tetraborate)
• Silica
• Soda ash (sodium carbonate)
• Litharge (lead oxide)
• Carbon (in the form of flour or charcoal)
• Nitre (potassium nitrate)
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Borax (Na2B4O7)
• Anhydrous borax melts at 741oC to form a viscous glass,
which becomes more fluid at elevated temperatures
• It is a strongly acidic and readily dissolves almost all
basic metal oxides
• The borax melts to form a colourless transparent glass
𝑁𝑎2𝐵4𝑂7 → 𝑁𝑎2𝐵2𝑂4 + 𝐵2𝑂3𝑍𝑛𝑂 + 𝐵2𝑂3 → 𝑍𝑛𝐵2𝑂4
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Soda Ash (Na2CO3)
• Na2CO3 is a strong basic flux and fuses most readily with
silica to form fusible slag
• It is used as an oxidising and desulphurising reagent
• Na2CO3 melts at 850oC, and at 950oC, dissociates
partially evolving CO2 and liberating some free alkali
𝑁𝑎2𝐶𝑂3 → 𝑁𝑎2𝑂 + 𝐶𝑂2𝑁𝑎2𝑂 + 𝑆𝑖𝑂2 → 𝑁𝑎2𝑆𝑖𝑂3
𝑁𝑎2𝐶𝑂3 + 𝑁𝑎2𝑆𝑖𝑂3 → 𝑁𝑎𝑆𝑖𝑂4 + 𝐶𝑂2
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Litharge (PbO)
• Litharge is a readily fusible basic flux
• It acts as an oxidising and desulphurising agent
• It melts at 883oC and reacts with the reducing agent to
liberate metallic lead
• This metallic lead provides the lead rain, which collects
the noble metals in the sample to form a lead button upon
solidifying
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Red Lead (Pb3O4)
• Used as alternative to litharge
• Additonal oxidising effect during fusion
• More expensive than litharge and hence not commonly
used
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Nitre (KNO3)
• KNO3 is a powerful oxidising reagent that melts at 339oC
• It decomposes at about 400oC, liberating oxygen
• Nitre oxidises sulphides to sulphates, and arsenides to arsenates.
• 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
• Serves two purposes:
– They reduce sufficient litharge to produce the Pb button, which
collects the values
– They reduce any ferric oxide present in the sample to the
ferrous state so that it can be slagged
• Sources of reducing agents:
– Those added to the charge
– Those already present in the charge
• Most effective reducing agent is carbonwww.knust.edu.gh
Theoretical Reducing Power
• Denotes the amount of grams of metallic Pb that is
produced from 1 g of reducing agent added to PbO
2𝑃𝑏𝑂 + 𝐶 → 2𝑃𝑏 + 𝐶𝑂2[C = 12; Pb = 207]
𝑅𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 =414
12= 34.5
Flour or starch as reducing agent
12𝑃𝑏𝑂 + 𝐶6𝐻5𝑂10 → 12𝑃𝑏 + 5𝐻2𝑂 + 6𝐶𝑂2
𝑅𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 =2486
162= 15.3
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Oxidising Agent
• The presence of sulphides, arsenic or antimony makes it
necessary for oxidising agents to be added
• Examples of oxidising agents:
– Red lead (Pb3O4),
– Manganese dioxide (MnO) and
– Nitre (KNO3)
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Theoretical Oxidising Power
• Denotes the amount of grams of Pb prevented from being
reduced by 1 g of oxidising agent
2𝐾𝑁𝑂3 → 𝐾2𝑂 + 2𝑂2
2𝑂2 + 4𝑃𝑏 → 4𝑃𝑏𝑂
𝑂𝑥𝑖𝑑𝑖𝑠𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 =828
202= 4.1
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Fluxes
Reagent Reasons
Litharge
Used to provide lead to collect the precious metals. It
is also a strong basic flux and reacts with metallic
oxides and silica to form a slag. By far the most
expensive component of a fire assay flux.
Soda Ash
A powerful basic flux that is usually the principal
component of fire assay flux. It reacts with silicates to
form a slag
Borax
An acidic flux that lowers the fusing point of all slags.
It forms fusible complexes with limestone and
magnesite
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Fluxes
Reagent Reasons
Silica
An acidic flux that forms the principal component of
many samples. Small amounts are present in the flux to
prevent attack on the fire assay crucibles when assaying
samples deficient in silica.
NitreA powerful oxidising agent added to the flux when
assaying samples containing sulfides
Flour A source of carbon used to reduce the litharge to lead.
SilverA small amount is added to the flux to provide a
collection medium for the precious metals
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Crucible Fusion• Heating (fusion) of the charge in a
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
Crucible Fusion – Possible
Layers
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Characteristics of Metallic
Phase
1. A minimum amount of impurities
2. A bright, soft, malleable button
3. A button close to the desired weight
4. A complete recovery of the noble metals
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Slag Properties
1. It should have a comparatively low formation temperature.
2. It should be pasty at its formation temperature.
3. It should be thin and fluid when heated to somewhat above its
melting point.
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
6. It should not attack the material of the crucible to any great
extent.
7. Its specific gravity should be low.
8. When cold, it should separate readily from the lead, and be
homogeneous.
9. It should contain practically all the impurities of the sample.
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Preparation of Pb Button for
Cupellation
• After fusion, the melt is poured into a mould and allowed
to cool until the Pb solidifies.
• The lead is detached from the slag by striking at the
junction of the Pb and slag with a hammer
• The cone-shaped button is hammered into a rough cube
on an anvil.
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Cupellation• Cupellation is the oxidation of the lead and subsequent absorption
into a small shallow porous cup called a cupel
• 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
• Cupel must be dry before being placed into the muffle furnace and
then raised to cupellation temperature of 950oC before button is
added
• The furnace should have an ample supply of air for the oxidation
of Pb
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Steps in Cupellation
1. Preheating of the cupels to drive off any water, organic
matter, and carbon dioxide.
2. Increase temperature to 950oC and place lead buttons.
3. Lead button melts covering the cupel with a dark scum
composed mostly of litharge.
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
• Pb/Cu ratio should be around 16 at least, to ensure the absence of
free copper in the system
Cu2O-PbO phase diagram
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Parting of the Gold Beads
• This is the process of separating the Au and Ag obtained after
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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Parting of the Gold Beads
• The concentration of Ag should be high. If low, more silver should
be added (Ag:Au at least 3)
• Addition of Ag to the fusion is termed inquartation
– Ag-Au ratio should be known
• Recupeling the doré bead with three times its weight in silver
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Annealing
• After parting, the bead is annealed and weighed
• Annealing is a heat treatment done to:
– avoid weighing extraneous materials
– provide opportunity to examine the gold bead for
impurities
– destroy porosity and so prevent the absorption of
moisture and gasses
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Weighing
• Sensitive balance used (1/500 of a miligram)
• Maximum load capacity is 1 or 2 g
• Conducive environment
• Express weights in proportion in the material sampled
• 1 Assay Ton = 29.166 g of ore
• 0.001 g of gold in a sample weighing 29.166 gwww.knust.edu.gh
Fire AssayInstrumental analysis - AAS
• Some laboratories complete fire assaying by recourse to
digestion of the prill and AAS analysis instead of parting,
annealing and weighing
• This method reduces problems associated with parting
such as:
– handling of tiny gold beads,
– incomplete silver dissolution, and
– inefficiencies associated with weighing on a 4-6 d.p electronic
balance.
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Fire AssayInstrumental analysis - AAS
• In this technique, silver-gold prills are digested with
boiling nitric acid and hydrochloric acid
• Gold content in the resulting solution may be determined
by AAS analysis
• Detection level of gold by fire assay/AAS method is
0.01ppm
• The grade per tonne of ore can then be calculated
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Fire AssayInstrumental analysis – ICP-OES
• ICP-OES has the advantage of being
able to analyze gold and other
elements such as PGEs in one reading
• Results compared to known and
verified standards
• Detection level Gold by Fire Assay /
ICP-OES method = 0.01ppm
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Difficult SamplesElement Comment
Copper May be reduced to the metal during fusion and reports in the
lead button. This can then inhibit cupellation, making it
impossible to recover the precious metals. Alternatively, it may
react with pyrite to form a matte that will preferentially absorb
gold.
Nickel Reacts similarly to copper with regards to cupellation, but will
create problems at far lower concentrations (> 0.5% in the lead
button). A combination of nickel and copper will create far
greater problems than either of the elements individually.
Antimony Completely miscible with molten lead. More than 2% antimony
in the lead button may cause cracking of the cupel, resulting in
low gold recovery. It will also form antimonides with copper,
nickel or iron (speiss) which will preferentially absorb gold
Arsenic Will form arsenides with copper, nickel or iron (speiss),
resulting in low gold recovery. www.knust.edu.gh
Difficult Samples
Element Comment
Tellurium Extremely detrimental to the recovery of precious metals during
cupellation when present in amounts of > 0.5% in the lead
button. It lowers the surface tension of the precious metal prill,
allowing some of the doré to be absorbed by the cupel.
Selenium Behaves similarly to tellurium.
Sulphur
(Sulfides)
Can cause problems by forming mattes with copper, nickel or
iron compounds, resulting in low gold recoveries. Will also
produce large lead buttons if using a high litharge flux. This can
be controlled by adding a calculated amount of oxidant.
Carbon
(organic
matter)
Can cause major problems during fire assaying due to the
formation of lead shot within the slag, leading to low lead button
weights. This will result in low gold recoveries
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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