Flotation Circuits

Contents Grinding & flotation Stage operation & Recycle Co-current & counter-current Circuit layout Plant sampling & analysis Optimisation

Grinding & Flotation Flotation circuits cannot be considered alone Comminution prepares feed for later separation With flotation separation the objective is to make the feed size distribution well suited to the flotation process This means grinding so that minerals are well liberated top size is not too large to be captured by bubbles But grind is not be too fine so fine values are lost An overall balance between grinding and flotation is needed.

Grinding & Flotation Chemical effects can also be due to comminution Grinding with steel balls can affect the surface chemistry of the minerals Also attempts to regrind flotation streams may affect pulp chemistry Such effects are complex & require careful study Optimisation of the process must rely on careful test work guided by calculations aimed to match the feed size distribution to the size-recovery behaviour of the flotation process.

Flotation Stages The aim in flotation – or any separation – is to achieve both grade and recovery at the same time Single stage operation may impose unacceptable limits Multiple stage operation is generally needed  

Single Stage Operation Consider the grade-recovery relationship for a bank of 10 cells treating a feed of 10% values (k = 1 min-1) and 90% gangue (k = 0.1 min-1) Hence the maximum grade is achieved at minimum (zero) recovery If a higher product grade than this is needed (eg 60% values in this example) it is impossible to achieve this with a single stage operation

Two Stage Operation with Scavenger Consider recycling the concentrate from the later cells to the head of the rougher

Two Stage Operation with Scavenger Consider a constant overall recovery 50%   τ / τ 0.5 1 5 10 Large sc R Concentrate grade % 44.6 49.2 52.5 65.6 72.6 Near 100

Two Stage Operation with Scavenger

Two Stage Operation with Scavenger The maximum grade is now not necessarily at the minimum recovery In general the maximum grade is achieved at some intermediate and finite recovery The improved grade is achieved at the cost of increased total residence time (ie increased plant capacity) A larger plant is needed to treat the feed plus recycle

Two Stage Operation with Cleaner Consider a similar idealized arrangement, this time recycling cleaner tails to the rougher

Two Stage Operation with Cleaner Consider again constant total recovery of 50% τ / τ Near 0 0.1 0.5 1 10 100 Large Cl R 44.6 Concentrate grade % 44.7 60.7 84.7 87.7 92.5 Near 100

Two Stage Operation with Cleaner

Two Stage Operation with Cleaner Once again the whole region can be reached   Maximum grade is again at zero recovery, but since this grade is now 100% no limitation Once again increased grade is achieved at the expense of increased total residence times, plant capacity and recycles

Multiple Stages Although it seems theoretically possible to achieve any result with a two stage circuit, it is generally much more efficient, in terms of utilising plant capacity, to have a multi-stage circuit arranged in counter flow

Multiple Stages It becomes much more difficult to represent results simply since there are too many degrees of freedom to permit a simple plot Results are calculated for a particular circuit The number of scavenger stages and cleaning stages can vary Up to about 5 or more cleaning stages are used for difficult separations

Circuit Calculations The calculation of flotation circuit performance is extremely difficult due to the complexity of the process Attempts have been made to develop circuit simulation packages to assist the metallurgist in circuit design and optimisation These packages are based on the kinetic model of flotation The first stage is to survey and analyse the circuit

Circuit Sampling & Analysis A thorough investigation and analysis of circuit performance must be based on taking steady state samples of the process streams then careful analysis of the samples The aim is to get an acurate picture of the way that the circuit is performing

Circuit Sampling & Analysis Circuit should be set in a steady operating mode Then samples accumulated over an extened period, perhaps a few hours The samples are immediately weighed wet, filtered and dried, then weighed again ( so getting pulp density estimates) The dried samples then sized & size fractions assayed (& given mineralogical analysis) Some reference tonnage is needed, generally the feed tonnage, and any other pulp flow measurements that can be taken are recorded In modern plants the process control system will record many plant operating parameters, so a printout of these data sheets is essential

Circuit Sampling & Analysis

Circuit Sampling & Analysis Here 8 process streams in total giving the following mass balances for total and component flows   Overall balance 1 = 4 + 8 (1) Rougher balance 2 = 3 + 5 (2) Scavenger balance 3 = 4 + 6 (3) Cleaner balance 5 = 7 + 8 (4) Recycle balance 1 + 7 + 6 = 2 (5)

Circuit Sampling & Analysis Not generally possible to sample all streams Major feed and product streams are most important (1,4 & 8) Some internal streams may be difficult or impossible to sample, eg 2 if 1, 6 & 7 are fed to the feed box of the rougher As many samples as possible should be taken so long as they are reliable samples.  

Circuit Sampling & Analysis Programs exist that carryout mass balance estimates on whole circuit simultaneously Rather than making individual balances these programs use experimental results to make best estimates of all the flows in the circuit Estimates must be made of the reliability of the respective analyses so that due weights can be given to the analyses in the overall calculation The result is a balanced estimate of component flows that serves as basis for the circuit analysis   A sampling survey is a major exercise, but is the only way to find out what is happening in circuit

Overall Circuit Performance Total circuit performance should be presented in terms of the major components Attempt this on a mineral basis, if necessary by estimation of minerals from assays The size by size behaviour is required to interpret the circuit performance

Overall Circuit Performance

Overall Circuit Performance Look at the shape of the mineral recovery curve At what sizes is mineral lost? Look at the shape of the feed to the plant Does it match the recovery curve?   Repeat this for all the major components including the gangue components The recovery of the other materials determines the concentrate grade It is just as important to define the gangue recovery since the aim is to minimise this

Overall Circuit Performance A plot of selectivity is a useful way of indicating the effect of size on separation It can highlight the “cost” of additional mineral recovery

Stage Performance Look at individual stages – rougher, scavenger, cleaner – in the same way Which stages are limiting the overall recovery? Are the feeds to the stages appropriate? Are the recycles appropriate?

Flotation Performance Why is the recovery limited? Why is the grade limited? Is there adequate flotation capacity? Is the grind size too large? Do the reagent conditions need adjustment?

Flotation Performance & Mineralogy A full mineralogical investigation will help solve many of these problems. It will tell you:   How the losses are divided between Liberated mineral Locked mineral What is diluting the concentrate Liberated gangue components Locked components

Flotation Performance - Mineralogy If the liberation is appropriate then losses from locked particles will be not be excessive Also if there is an excess of fine liberated mineral being lost then the grind might be too fine.   If there are significant losses of liberated material then there are two possibilities: The chemical conditions may not be appropriate – this is particularly true at the coarse sizes (check in the lab) The physical flotation conditions may not be optimised ie inadequate residence time, poor cell design, etc (check in the plant)

Mineralogical performance of galena recovery to lead concentrate

Losses of galena to the zinc concentrate

Losses of galena to the final tailings

Notes on lead performance Galena recovery is about 85% Most of this (~70%) is liberated mineral Remainder is locked mainly with sphalerite Losses of galena to the zinc concentrate (a double problem since it is wasted and also dilutes the zinc concentrate) are a little over 4% About half of this is liberated fine galena Most of the remainder is locked with sphalerite Losses of galena to the final tail are about 9% Mainly these losses are fine liberated galena The other major losses are galena locked with silicates and other minerals

Comments No indication of loss of coarse liberated galena If reagent conditions were poorly optimised, recovery would tend to drop at the coarser sizes Note the grind is already quite fine The loss of fine liberated material might perhaps be overcome with additional flotation capacity (these are the slow floating particles) Such a change must be evaluated economically (would the additional metal recovered pay for the extra equipment?) and Technically (additional entrainment of other fine minerals might lower concentrate grades – there is already entrainment of fine galena to the zinc concentrate).

Comments Loss of locked lead might perhaps be overcome by additional grinding Once again there is a cost involved and such a change would produce more fine galena - already the major component of the losses In fact since the losses are roughly split between locked and liberated galena it seems that the grind is probably about right A similar analysis should be carried out with the other components in feed, particularly sphalerite