EP1613434B1

EP1613434B1 - Measuring froth stability

Info

Publication number: EP1613434B1
Application number: EP04719877A
Authority: EP
Inventors: Brett Triffett; Johannes Jacobus Le Roux Cilliers
Original assignee: Technological Resources Pty Ltd; University of Manchester
Current assignee: Technological Resources Pty Ltd; University of Manchester
Priority date: 2003-03-13
Filing date: 2004-03-12
Publication date: 2011-05-18
Anticipated expiration: 2024-03-12
Also published as: ATE509704T1; DK1613434T3; WO2004080600A1; EP1613434A1; EP1613434A4; ZA200507463B; ES2371311T3; PL1613434T3; AU2010212522A1; AU2004218778A1; AU2010212522B2; AU2003901142A0

Abstract

A method of measuring froth stability (as described herein) of a froth in a cell of a flotation circuit for a slurry of a mined mineral containing valuable material and gangue materials is disclosed. The method includes a step of measuring one or more than one froth stability parameter using a measurement column arranged to extend downwardly through the froth in the cell to a location below an interface between the froth and the slurry in the cell. A method of controlling the operation of a flotation cell that is based on the froth stability measurement method is also disclosed.

Description

The present invention relates to recovering valuable material from mined minerals by means of froth flotation of a slurry of the mined materials.
The present invention relates particularly to a method of measuring froth stability in a cell of a flotation circuit for recovering valuable material from a slurry of mined minerals containing valuable material and gangue material.
The present invention also relates to an apparatus for measuring froth stability.
The present invention also relates to a method of controlling the operation of a flotation cell using froth stability as a control parameter for the method.
It has been long understood that the froth phase of a flotation cell has a significant influence on the overall performance of the flotation process.
A froth is a three phase structure comprising air bubbles, solids and water. The bubbles are defined by a thin water film or lamellae, which separates two bubbles, while the intersection of three lamellae results in the formation of a narrow water channel called a Plateau border. The entire froth is therefore made up of a continuous network of narrow water channels in which water and solid particles can flow. The solids contained in the froth are either valuable material attached to lamellae or a mixture of valuable material and gangue material contained freely within the Plateau borders.
A froth is a highly dynamic system in which solids and water movement is governed by the following processes:

The flow of air bubbles from the pulp froth interface to the top surface of the froth. Typically, 5-10% of the air entering the froth is carried over a cell weir into a concentrate launder and the other 90-95% leaves the top of the froth as bubbles burst. As the bubbles flow upwards they carry valuable material which is directly attached to the bubble lamellae. The upward flow of bubbles also drags a portion of the water contained in Plateau borders upwards along with its load of entrained particles (both valuable material and gangue material).
Bubble coalescence. Water contained in the thin film lamellae defining each bubble tends to flow towards the Plateau borders. As this takes place the lamellae become thin, and eventually break, resulting in the coalescence of two adjacent bubbles into a single larger bubble. The coalescence process releases attached particles into the Plateau borders. The thinning and rupture of lamellae at the top surface of the froth results in bubbles bursting. This results in loss of air from the froth and release of attached particles into the Plateau borders.

The term "froth stability" is understood herein to mean the ability of bubbles in a froth to resist coalescence and bursting.
A more stable froth will have less coalescence and bursting events, a smaller mean bubble size and may carry more water. All of these factors will ultimately determine the structure and volume of the froth (water, solids and air) carried over the cell weir into the concentrate launder and therefore the recovery of attached and unattached (carried in Plateau border) particles - in other words, the valuable material recovery and concentrate grade.
GB 1 287 274 discloses an apparatus for controlling the thickness of froth material above the liquid slurry of a flotation cell comprising probe means movable in position relative to the froth layer such that a first signal is provided upon the probe means sensing a first predetermined surface of the froth layer and a second signal is provided upon the probe means sensing a second predetermined surface of the froth layer, and means for controlling the thickness of the froth layer in response to the first and second signals.
WO 01/34304 discloses a method for detecting froth flow rate in a slurry-processing recovery apparatus such as a froth flotation cell whereby a sensor disposed along a froth flow path extending from an outlet of the apparatus is operated to determine location of froth-air interface along said froth flow path. The location of the froth-air interface or froth level is related to the rate of froth production and enables feedback control of operating parameters of the froth flotation cell to maintain cell operation in an optimal range.
US 4,552,651 discloses a method for controlling the separation of coal from a mixture of coal and refuse in a froth flotation device by measuring the differential back pressure between two gas bubbler tubes immersed to different depths into the body of pulp in the device to produce a first control signal representative of the pulp density, and adjusting the rate of addition of a froth enhancement additive to the froth flotation device responsive to changes in said first signal; a second signal, produced by measuring back pressure of a single bubbler tube and representative of the pulp level in said device, can be corrected for changes in density by combining it with said first signal and then utilized to control liquid level in the cell by adjusting the rate of withdrawal of refuse therefrom.
SU 1717237 discloses a floatation process regulation unit having a float with pulp level measuring electrode, whereby its shaft can interact with the position detector connected to the pulp level stabilization regulator, electrodes for froth thickness measuring, a regulator for thickness stabilization and froth mineralization. The unit is simplified by providing with relay slave mechanisms and transistor amplifiers. The froth thickness measuring electrodes are connected to transistor amplifiers bases, whose manifolds are connected to one of relay slave elements outlets, whose other outlets are connected to the pulp level measuring electrode. The slave elements are connected to the froth thickness stabilization and mineralization regulator.
It is evident from the above-described processes that a more stable froth will recover a greater amount of valuable material attached to bubbles and within the Plateau borders.
It is also evident from the above that a more stable froth will also recover more gangue material.
Accordingly, from a viewpoint of maximising recovery and concentrate grade, there is an optimum froth stability for any given flotation cell and any given operating conditions for that cell.
The term "operating conditions" is understood herein to mean:

(a): chemical conditions (frother, collector, pH and other modifiers or contaminants) of the cell;
(b): grade of hydrophobic particles in the cell feed (valuable material and gangue material);
(c): slimes and clay content in the cell feed;
(d): particle size of the cell feed;
(e): air rate for the cell; and
(f): pulp density of the slurry supplied to the cell.

Each of these variables can change rapidly or gradually over time and can significantly influence froth stability and the overall flotation performance.
A method by which froth stability can be measured online in a flotation circuit for the purposes of process monitoring and control would add great value to any flotation operation.
The present invention provides such a method.
According to the present invention there is provided a method of measuring froth stability (as described herein) of a froth in a cell of a flotation circuit for a slurry of a mined mineral containing valuable material and gangue material, which method includes a step of measuring one or more than one froth stability paremeter below using a measurement column arranged to extend downwardly through the froth in the cell to a location below an interface between the froth and the slurry in the cell.
The froth stability parameter is any parameter that provides information on the stability of the froth in the cell that can (a) be measured directly by means of the column or (b) be derived from measurements made using the column.
Two preferred froth stability parameters that are measured directly by means of the column are:

(a): the rate or velocity of movement of froth up the column from a pre-determined starting height to a maximum height of the froth in the column; and
(b): the maximum height attained by the froth in the column.

Preferably the method includes washing the column to collapse the froth in the column to the pre-determined starting height, for example the interface between the slurry and the froth, and thereafter repeating the above-described measurement step and measuring one or more than one froth stability parameter.
According to the present invention there is also provided an apparatus for measuring froth stability of a froth in a flotation cell in a plant and for controlling operating conditions of a flotation cell which includes:

(a): a measurement column arranged to extend downwardly through the froth in the cell to a location below an interface between the froth and the slurry in the cell; and
(b): a means for measuring the above mentioned froth stability parameters directly by means of the column or indirectly from measurements made using the column; and
(c): a means for processing data measured directly or indirectly from the column and adjusting one or more than one of the above mentioned operating conditions of the cell to optimize cell performance.

According to the present invention there is also provided a method of controlling the operation of a flotation cell which includes the steps of:

(a): measuring froth stability (as described herein) of a froth in the cell in accordance with the method described above;
(b): inputting froth stability data into a model that relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade) to assess the cell performance; and
(c): adjusting one or more than one of the operating conditions of the cell (as described herein) to optimize cell performance.

The term "froth stability data" is understood herein to mean data that is directly measured by means of the column or is derived from directly measured data.
Preferably the method includes repeating the measurement of the froth stability during the course of the operation of the cell and adjusting operating conditions of the cell based on the froth stability data.
The model may be any suitable model that relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade) to assess the cell performance.
The model may be a fundamental model derived from theoretical considerations.
Alternatively, the model may be based on comparing measured froth stability data and data on the historical operation of the cell.
One particular model is a model that is being developed by the applicant.
The development of the model has been supported by testwork carried out by the applicant to determine how to measure selected froth stability parameters in a single laboratory batch flotation cell and along the rougher bank of a flotation circuit at one of the mines operated by the applicant.
As indicated above, the model relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade).
The model is a fundamental model and is based on foam physics and interprets the effect of froth structure on flotation. The model links the flow rate of valuable material, gangue material, and water to froth structure. The mass flow rate of valuable material, gangue material, and water are related to the flow rate of bubble surface area and the total volumetric flow rate of Plateau borders overflowing the weir. These last two parameters can be estimated through analysis of video images of the overflowing froth.
The testwork program carried out at the applicant's mine involved the use of a column 30cm square by 165cm high constructed of perspex. The objective of the program was to investigate how to measure froth stability parameters.
The column was inserted into the pulp phase to a depth of 30cm and an operator manually recorded the level of the rising froth with time.
At the end of the test, when the froth had reached a stable maximum height, the data was entered into a spreadsheet and the appropriate parameters were calculated.
Figure 1 shows a typical column froth height versus time curve generated during the testwork. The graph has raw data as well as a "fitted model" for the data. The fitted model is a separate model to the previously described model.
Of significance is the close fit of the fitted model to the raw data.
The fitted model has the form shown in equation (1) where H₀ is the maximum height the froth reaches and τ is a fitted stability parameter. $H = H_{0} (1 - e^{- t / τ})$

Other modes may be appropriate also.
Figure 2 and 3 are graphs of column froth height versus time for selected operating conditions.
What is clear from Figures 2 and 3 is that alterations to operating conditions resulted in significantly different froth stability curves. These differences can be used to clearly explain differences in metallurgical performance, ie recovery and concentrate grade, of a cell.
An extension of the froth stability curves discussed above comes with consideration of the bursting fraction (1 - a) of the froth.
If all of the air entering the froth from the pulp was retained in the froth then the rise velocity within the column would be equal to the superficial gas velocity Jg (where Jg equals the gas flow rate per unit area of the column). This would be the case if bubbles at the surface of the froth did not burst and release their contained air. This is obviously not the case with only a small fraction of the airflow retained in the froth.
It may be expected that the value of α in the column and the value of α for the entire cell at a given froth depth will be different. The relationship between a in the column and the actual a achieved for the entire cell is the subject of current research.
Given the considerations in the preceding paragraph, the following equation can be presented, relating the froth rise velocity (u) to the superficial gas velocity and the fraction of air retained in the froth (α): $U = J_{g} (1 - α)$
Given that the instantaneous rise velocity can be calculated from the froth stability curve and the superficial gas velocity can be measured, then the instantaneous bursting fraction (1-α) can be calculated for a given froth height.
The result is a plot of alpha versus height shown in Figure 4.
These graphs take the form: $α = α_{i} (1 - H / H_{0})$

where α_I is the fraction of airflow retained in the froth
at a froth height of zero.
Using equations 1, 2 and 3 the following additional equations can be developed to assist with interpretation of the data: $H_{0} = τ J_{g} α_{I}$
$U = J_{g} α_{I} - H / τ$
$\sum = H_{0} / J_{g}$

where Σ is the dynamic froth stability factor.
In summary, the above-described testwork determined how to measure two particular froth stability parameters, namely the maximum height attained by the froth in the column and the rate or velocity of movement of froth up the column from a pre-determined starting height to a maximum height of the froth in the column.
Figure 5 is a conceptual diagram of an apparatus for measuring froth stability in a flotation cell in a plant and for controlling operating conditions of a flotation cell.
The main, but not only, operating conditions that can be adjusted in response to froth stability measurements include reagents (frother, collector, pH modifier or other modifier), air rate, pulp density, particle size and ore blend.
With reference to Figure 5, the apparatus includes a column (6) that is constructed from 300mm diameter perspex pipe with a wear resistant and replaceable HDPE extension piece (9) which, in use, is inserted into the pulp in a cell.
While the original column used in the testwork at the applicant's mine was square, it is anticipated that a circular column will provide better movement of the froth as there is no interference from the corners. Having said this, a square column would still suffice.
The column (6) is constructed in a number of sections so the measurement height of the column can be reduced if necessary.
The maximum height shown in Figure 5 will allow for two metres of froth, which might typically be expected in a high grade rougher cell, whereas a shorter column might be used for a scavenger cell where less froth is generated.
Figure 5 illustrates a gridmesh walkway above the cell on which to secure the column (6). This may not always be the case and an alternate securing arrangement may be required. The column (6) is secured to the gridmesh via a securing plate (7) and the depth that the column is inserted into the pulp can be adjusted slightly via the level adjustment bolts (8).
When the column (6) is used at its maximum height the adjustable length tie down bars are required to minimise any bending of the column as a result of the pulp movement at the base.
If the column is used in a shortened form the tie down bars may not be required.
In use, the froth height inside the column (6) is measured by an ultrasonic level sensor, although any other suitable means of continuously monitoring the froth level will suffice.
In this case, the froth height data is monitored by a commercially available Citect monitoring and control system, which collects the data and performs the calculation of the froth stability parameters described previously.
Any other suitable means of continuously logging and calculating the appropriate froth stability parameters for process control purposes will suffice.
Froth stability data is supplied to the above-described model that relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade) and the model assesses the cell performance and, if required, initiates adjustments to selected operating conditions of the cell to improve the cell performance.
Once the froth stability parameters are calculated to a satisfactory level of accuracy in a first measurement cycle, a water solenoid valve (2) is actuated to wash down the froth. The measurement sequence, data input to the model, and adjustment of cell operating conditions is then repeated. Typically, the sequence requires a 20-60 minute period and can be repeated on a continuous or periodic basis during the operation of the cell. The measurement sequence period may be any suitable period.

Claims

A method of measuring froth stability of a froth in a cell of a flotation circuit for a slurry of a mined mineral containing valuable material and gangue material, which method includes a step of measuring one or more than one froth stability parameter including:
a) the rate or velocity of movement of froth up the column (6) from a predetermined starting height to a maximum height of the froth in the column (6);
and/or

b) the stable maximum height attained by the froth in the column (6)
using a measurement column (6) extending downwardly through the froth in the cell to a location below an interface between the froth and the slurry in the cell.
The method defined in claim 1 wherein the froth stability parameter is (a) measured directly by means of the column (6) or (b) be derived from measurements made using the column (6).
The method defined in claim 1 includes washing the column (6) to collapse the froth in the column (6) to the pre-determined starting height at the end of a measurement step and thereafter repeating the measurement step and measuring one or more than one froth stability parameter.
An apparatus for measuring froth stability of a froth in a flotation cell in a plant and for controlling operating conditions of a flotation cell which includes:
(a) a measurement column (6) arranged to extend downwardly through the froth in the cell to a location below an interface between the froth and the slurry in the cell ; and

(b) a means for measuring the rate or velocity of movement of froth up the column (6) from a predetermined starting height to a maximum height of the froth in the column (6); and/or
the stable maximum height attained by the froth in the column (6); and

(c) a means for processing data measured directly or indirectly from the column (6) and adjusting one or more than one of the following operating conditions of the cell
- chemical conditions (frother, collector, pH and other modifiers or contaminants) of the cell ;

- grade of hydrophobic particles in the cell feed (valuable material and gangue material);

- slimes and clay content in the cell feed ;

- particle size of the cell feed;

- air rate for the cell ; and

- pulp density of the slurry supplied to the cell.
to optimize cell performance.
A method of controlling the operation of a flotation cell which includes the steps of:
(a) measuring froth stability of a froth in the cell in accordance with the method defined in any one of claims 1 to 3;

(b) inputting froth stability data into a model that relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade) to assess the cell performance ; and

(c) adjusting one or more than one of the following operating conditions of the cell
- chemical conditions (frother, collector, pH and other modifiers or contaminants) of the cell ;

- grade of hydrophobic particles in the cell feed (valuable material and gangue material);

- slimes and clay content in the cell feed ;

- particle size of the cell feed;

- air rate for the cell ; and

- pulp density of the slurry supplied to the cell.
to optimize cell performance.
The method defined in claim 5 includes repeating the measurement of the froth stability during the course of the operation of the cell and adjusting operating conditions of the cell based on the froth stability data.
The method defined in claim 5 wherein the model is a model that relates froth stability and the performance of the cell (in terms of recovery of valuable material and concentrate grade) to assess the cell performance.