Sizing and Metallurgical Testing

• Size is crucial to virtually all mineral processing
• A size range in a standard series is generally identified by the bottom size in the range
• A single size parameter is not enough to define a size distribution
• Narrow size ranges are generally best for following separation processes
• Sizes are usually arranged in a ratio series
  – Ratios of 2, √2, 4√2 are the norm
• This gives systematic coverage of a wide range of particles sizes and is convenient in many ways.
• Also relates better to the effect of size on the behaviour of the particles in many separations.

Size Distribution

• Size distributions are generally tabulated if they are to be used in further calculations
• May be plotted in many ways – useful visualisation
• Cumulative log-log has many advantages
  – Disadvantage is compression of coarse sizes
  – Compensate by expanding Y-scale
• Differential plots rely on consistency in size intervals

Size Distribution Functions

• Many functions for describing size distribution mathematically
  – none universal
  – none with valid theoretical basis
• Gaudin-Schumann the most useful to fit the fine sizes
  Y = 100 x (d/D)n
  Where ‘Y’ is the % passing size, d
  ‘D’ corresponds to the 100% passing size
  – extrapolate back
  ‘n’ is a constant corresponding to the slope

Size Distribution Expressions

• Straight line part of the narrow distribution is expressed by:
  Y = 100 x (d/111)1.65
• That for the wide distribution by:
  Y = 100 x (d/104) 0.75
• Note the wider distribution has the smaller index.
• This relationship is a straight line on a log-log plot
• Over the same size range (where the relationship applies) the differential plot will also be a straight line plot
• It will be parallel to the cumulative plot
• A good differential plot depends on accurate ratios in the size intervals

Y = 100 x [d/D]n

or log(Y) = n x log[d/D] + 2

• Where y is the % passing size d, D corresponds to 100% passing and n is a constant corresponding to the slope of the log-log plot

The straight line portion of the narrow size distribution in the fine sizes is thus expressed by:

Y = 100 x [d/111]1.65

and of the wide distribution is expressed by:

Y = 100 x [d/104]0.75

• Note that the wider distribution has a smaller index than the narrow distribution.
• This is a straight line on a log-log plot and over the same range the differential plot will also be a straight line on log-log (see graph)
• Straight line in fine sizes on log-log plot very common from typical grinding circuit
• Parallel line for differential plot goes with this (straight forward maths)
• Narrow size distributions have a higher peak on a differential plot than wide size distributions reflecting the greater concentration of material at or close to the peak value
• On a cumulative plot the slope is greater for narrow size distributions

Note although 80% passing size is the same for all distributions the rest of the distributions are very different, e.g. compare wide and narrow distributions at 3µm – narrow 0.25% passing, wide 7% passing.

Measuring Size

• Many methods using:
  – Screens
  – Settling rate
  – Light scattering
  – Laser diffraction
  – Capacitance (Coulter counter), etc.
• Desirable to get sized material in hand
  – Can analyse
  – Do mineralogical study
  – Specialised tests
• Screens are best for +38µm
• Cyclosizer best for sub-screen sizing

Screening

• Cut out accurate sample from product
• Wet screen on fine screen (38 or 43µm)
• may need to add dispersant
• removes clay,
• and fines which tend to adhere to coarse particles
• settle
• dry if nothing finer to be done
• Dry oversize – not too hot (depends on mineral)
  e.g. pyrrhotite very reactive
• Screen on appropriate stack (Ro-Tap)
• Combine minus 38µm
• Decant minus 2-5µm, settle and dry
• Cyclosize

Cyclosizing

• Cyclone principle with repeated cycling of the underflow
• Variation in dimensions of overflow tube and entry size permits progressively finer cuts
• Does not adhere to strict √2 sequence
  – 44, 33, 23, 15, 11 µm cutpoints (quartz)
  – commercial reasons!
• Uses a lot of water (can dissolve metal from very fine particles
  – temperature control for consistent sizes
• Sized fine particles available for assay

Enhanced Fine Sizing Procedure

• Advantage if we could recover all solids
• Remove finest solids by beaker decantation
  – chose conditions for cutpoint of about 5 µm (quartz)
• May now all collect solids from Cyclosizer effluent in suitable centrifuge to complete a size-mass balance on the sample
  – account for all mass
  – account for all metals
• If finest solids left in cyclosizer feed the centrifuge collection would be inefficient

Sizing by Cyclosizer

For the full procedure refer to the paper on the CD by Stewart and Restarick – Improved Cyclosizing Technique, Proc AusIMM, No.251, September, 1974, pp 9-10.

• Size fractions available for analysis
• Cut sizes affected by:
  – particle shape
  – particle SG
  – water viscosity
  – water density
• Best run at set temperature (constant sizing)
• Calibrate to give continuity with screens (graphical method on log-log best)

Basis of Size Correction

• Difference from nominal calibration mainly due to differences in SG of particles
  – primary calibration is for quartz
• Secondary differences due to particle shape
• Correction best done empirically on graph
• A further reason for temperature control is ρw
• Temperature control to maintain constant water density and viscosity

Other Particle Properties

• Bulk properties
  – size
  – shape
  – SG
  – resistance to fracture
  – magnetic response
  – mineral content
  – metal content
  – cracking
• Surface properties
  – surface area
  – mineral exposure
  – appearance, fluorescence
  – electrostatic conductivity
  – surface hydrophobicity (flotation)
• Chemical solubility
  – by component

Bold blue for most important properties for separations

Other Metallurgical Testing

• Liberation testing
  – heavy liquids
  – super-panner
  – Jones tube
  – QEM*SEM
• Crushing
  – Manufacturers tests
  – SAG & AG drop test
• Grinding
  – Bond grindability
  – Work index
• Flotation
  – Denver cell
  – ‘Bottle reaching’
  – locked cycle
• Magnetic/electrostatic/gravity
  – specialised equipment available
  – generally consult supplier e.g. Gekko, Kelsey
• Leaching (CIL)
  – big range of tests
  – consider separately