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

cyclosizer-typical-size-distribution

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

cyclosizer size range

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.cyclosizer-sizing

  • 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 particlescyclosizer-size-correction
    – 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