AU4827997A

AU4827997A - Method for controlling the pulp level in a group of flotation cells

Info

Publication number: AU4827997A
Application number: AU48279/97A
Authority: AU
Inventors: Peter Bourke
Original assignee: Outokumpu Technology Oyj
Current assignee: Metso Corp
Priority date: 1997-12-10
Filing date: 1997-12-10
Publication date: 1999-07-01
Anticipated expiration: 2017-12-10
Also published as: AU745153B2

Description

1

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: Actual Inventor/s: Address of Service: Invention Title: Outokumpu Technology Oy Peter BOURKE SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "METHOD FOR CONTROLLING THE PULP LEVEL IN A GROUP OF FLOTATION CELLS" The following statement is a full description of this invention, including the best method of performing it known to us:- (File: 20260.00) METHOD FOR CONTROLLING THE PULP LEVEL IN A GROUP OF FLOTATION CELLS The invention relates to a method for controlling the pulp level in a group of flotation cells using only one level stage to produce a substantially continuous froth concentrate discharge rate.

It has been common practise when operating forced air flotation devices to manually adjust either the pulp height level or flotation air rate or both in order to achieve a continuous froth concentrate discharge rate over the froth lip. Other factors, which can affect the pulp level and the flotation air rate and thereby impact on the froth concentrate discharge rate, are: 1) pulp density of new feed slurry, 2) apparent pulp density in the flotation device, 3) hydraulic gradient between flotation devices and 4) changes in a new feed rate to the flotation device. The nett effect of these variables on small flotation devices with less than 16 cubic metres volume is small, since the ratio of froth thickness relative to the cell depth is sufficiently large to offset any effect from small changes in the pulp level and consequently there is no significant effect on the froth discharge rate from the flotation device.

With the introduction of large flotation cells up to 150 cubic metres in size in recent years, it has been necessary to have automatic control of both the pulp level and the flotation air rate. The reason for this change is due to the fact that the froth concentrate thickness relative to the cell depth is now significantly smaller than was previously the case with smaller volume flotation devices and as a consequence small changes in an air rate have a significant effect on the pulp level and hence the froth discharge rate from the flotation device. Even with both automatic air control and pulp level control large flotation cells are still susceptible to significant hydraulic gradient changes between a group of flotation cells, since the pulp level control is operating on only the last cell in each cell bank and as a 2 consequence the froth discharge rate from a group of cells can be erratic and uneven.

The above problem would not occur if it were possible to have steady state conditions where the feed rate to the head of the flotation circuit is always constant. Unfortunately, with the advent of large fully autogenous and semiautogenous grinding circuits the blending of ore is not perfect and the hardness of the ore is also varying with the nett effect that the recirculating load within the grinding circuit is in a constant state of flux or change. While these changes are not large, they are sufficient to cause small changes in pulp density and slurry flow rate to the head of the flotation circuit which in turn cause changes in the apparent pulp density and hydraulic gradient between flotation cells and as a consequence fluctuations result in the froth concentrate discharge rate.

The object of the present invention is to improve or substantially obviate some of the drawbacks of the prior art and to create an improved method for controlling the froth concentrate discharge rate from a group of flotation cells operating with only one level control stage. The essential features of the invention are listed in the enclosed claims.

In accordance with the invention the flotation device contains a group of flotation cells wherein at least two flotation cells are connected to each other so that the material to be treated in the flotation device is fed to the first flotation cell in the process flow direction and the material rejected in the flotation process is discharged from the last flotation cell of the group. According to the method of the invention the pulp level in each flotation cell of the flotation device is maintained substantially at the predetermined set point value for each flotation cell during the whole operating period. The predetermined set point value can be the same for each flotation cell or each flotation cell can have the value of its own for the pulp level. However, the desired predetermined set point value for each flotation cell is maintained substantially the whole operating period. This is achieved so that the flotation cell having an outlet for rejected material in the flotation device is provided with two control systems, one for the pulp level and one for the constant gas flow to the cell, and each flotation cell before the flotation cell having an outlet for rejected material are provided with one control system for the pulp level.

The control system for the pulp level in the flotation cell having an outlet for rejected material is carried out by at least one electronic PID controller or at least one electronic PLC system which receives inputs from at least one pulp level indicator and compares this input against the predetermined controller set point value of the pulp level and in case this level will change, opens or closes the discharge adjusting member for rejected material from the flotation device in order to maintain the pulp level at the predetermined controller set point value. The other control system connected to the flotation cell having an outlet for rejected material also contains at least one PID controller or at least one PLC system which receives inputs from a gas flowmeter and compares the input against the predetermined controller gas set point value and opens or closes the gas control member to maintain a substantially constant gas flow to the cell.

In the flotation cell or cells before the flotation cell having an outlet for rejected material the control system for the pulp level contains at least one pulp level indicator and at least one gas control member together with a PID controller or a PLC system which uses as its set point the desired predetermined pulp level set point value which is registered in the control unit. If the pulp level in the cell is different from the desired predetermined set point value, then the controller will open or close the gas control member to bring the pulp level back to the predetermined set point value. Although a PID controller or a PLC system has been mentioned as a control system for the pulp level, it is also possible to use other respective control systems.

Applying the method of the invention to a group of flotation cells connected to each other, the pulp level in each flotation cell is maintained substantially at the desired level during the substantially whole operating period by controlling the adjusting member for the discharge of rejected material in the flotation cell having an outlet for rejected material and in the other flotation cells the feed of gas for the float creation. Controlling the pulp level in a group of flotation cells according to the invention produces a substantially continuous froth concentrate discharge rate from each cell.

The method of the invention allows a group of flotation cells connected to each other in an operating manner to maintain substantially the desired pulp level in each flotation cell using only one level stage. Furthermore, the method of the invention is different from feed forward or cascade control systems which continuously monitor or change the pulp level set point value to maintain a product target flow rate or hopper level. The method of the invention aims to control the actual flotation process in each individual cell and hence offers a new level of process control. The method of the invention also makes it possible to operate a group of flotation cells with each cell maintaining substantially a constant deviation in level such that different amounts of froth concentrate can be discharged from each cell. The predetermined set point value for the pulp level in each flotation cell is thus related to the froth concentrate discharge rate in each flotation cell as well as being a function of air hold up and hydraulic gradient between flotation cells. This method can also be used to complement existing cascade or feed forward control systems where maintaining a consistent concentrate grade is vital.

The invention is described in more detail with reference to the appended drawing, which is a schematical cross-section illustration of the preferred embodiment of the invention.

According to the figure of the drawing a group of three flotation cells 1, 2 and 3 is connected to each other in operating manner so that the rejected material from the cell 1 flows to the cell 2 through an aperture 4 and the rejected material from the cell 2 flows to the cell 3 through an aperture 5. The material to be treated in flotation cells 1, 2 and 3 is fed to the cell 1 through the inlet 6 which is connected to the aperture 7 in the cell 1. The rejected material from the group of the flotation cells 1, 2 and 3 is removed from the cell 3 through the aperture 8 which is connected to the outlet 9. The product of this flotation process, froth concentrate, is removed from the flotation cells 1, 2 and 3 via froth launders 10 mounted in the upper part of the cells 1, 2 and 3. The froth concentrate is transferred through the froth bed 24 to the launders Each flotation cell 1, 2 and 3 is provided with a pulp level indicator 11, 12 and 13 and with an automatic air control valve 14, 15 and 16 respectively. These pulp level indicators 11, 12 and 13 as well as the air control valves 14, 15 and 16 are separately connected to a control unit 17 which contains four separate PID loops.

A PLC device can also be used as a control unit. The bottom valve 18 for rejected material in the outlet 9 and the air flowmeter 19 installed before the air control valve 16 of the cell 3 are also electrically connected to the control unit 17. In the drawing there are also illustrated for each cell 1, 2 and 3 a rotor with its rotating mechanism 20 and the air supply pipe When the group of the flotation cells 1, 2 and 3 is in operation, the flotation cell 3 operates at a predetermined pulp level set point 23. In the embodiment of the Figure the pulp level is maintained at the same predetermined pulp level set point value 23 for each flotation cell 1, 2 and 3. A PID loop controls the pulp level 23 using the signal from the float level indicator 13 to open or close the bottom valve 18 thus maintaining the pulp level at the predetermined value. A PID loop (11) is also connected to the flotation cell 3 and this PID loop (11) maintains a constant air flow to the flotation cell 3 via the flowmeter 19 and the air control valve 16.

The flotation cell 2 is provided with a pulp level indicator 12 and an air control valve 15. The PID loop (111) is set up to control the pulp level 22 in the flotation cell 2 to the same set value as in the flotation cell 3 using the pulp level indicator 12 and the air control valve 15. Hence the control system can vary the air supply to manipulate the air hold up in the flotation device to achieve the same remote pulp level set value as in the cell 3.

The flotation cell 1 is also provided with a pulp level indicator 11 and an air control valve 14 which are used in a PID loop (IV) to control the pulp level 21 in the flotation cell 1 to the same set value as in the flotation cell 3. Hence the control system can vary the air supply to manipulate the air hold up in the flotation device to achieve the same remote pulp level set value as in the cell 3.

The operation is described more particularly in the following example.

Example When there is a step change, for instance the solids content from 38% to 39%, in the pulp density of the material fed from the inlet 6 into the flotation cell 1, the pulp level 21 drops since there is less flow of slurry and the hydraulic gradient is smaller. The pulp level indicator 11 picks up this change away from the remote set point value and the PID loop (IV) automatically opens the air control valve 14 to increase air flow to the flotation cell 1 to maintain the same pulp level 21.

When the pulp density change reaches the flotation cell 2 then the same response will occur as in the flotation cell 1. The pulp level indicator 12 picks up a change away from the remote set point value and the PID loop (111) automatically opens the air control valve 15 to maintain the same pulp level 22.

When the pulp density change reaches the flotation cell 3 then the float level indicator 13 picks up this change and the PID loop (II) automatically closes the bottom valve 18 for the discharge of the rejected material of the flotation device to maintain the same pulp level 23 in the flotation cell 3.

Claims

1. Method for controlling pulp level to produce a continuous froth concentrate discharge rate in a flotation device containing a group of at least two flotation cells connected to each other in an operating manner so that the slurry to be treated in the flotation device is fed to the first flotation cell in the process flow direction and the material rejected in the flotation process is discharged from the last flotation cell, and each flotation cell providing with a mechanism for creating a froth bed above the pulp level and with a pulp level indicator and a gas control member, characterized in that the pulp level in the flotation device is set at the predetermined set value by feeding a substantially constant gas flow to the flotation cell having an outlet for rejected material, and that the set value of the pulp level is maintained at the predetermined set level for each flotation cell during the substantially whole operating period by controlling in the flotation cell having an outlet for rejected material the discharge of rejected material and in each flotation cell before the flotation cell having an outlet for rejected material the feed of gas for the froth bed creation.

2. Method according to the claim 1, characterized in that the predetermined set value for the pulp level is the same for each flotation cell.

3. Method according to the claim 1, characterized in that each flotation cell has the predetermined set value of its own for the pulp level.

4. Method according to any of the preceding claims, characterized in that the adjusting member for the discharge of rejected material of the flotation device and the pulp level indicator in the flotation cell having an outlet for rejected material are electrically connected to the same electronic control circuit.

Method according to any of the preceding claims, characterized in that the pulp level indicator and the gas feeding control member in each previous flotation cell before the flotation cell having an outlet for rejected material are electrically connected to the same electronic control circuit.

6. Method according to any of preceding claims, characterized in that the predetermined set value of the pulp level is adjusted by the gas flowmeter connected to the gas control member of the flotation cell having an outlet for rejected material.

7. Method according to any of preceding claims, characterized in that the gas flowmeter, the gas control members, the pulp level indicators and the adjusting member for the discharge of rejected material are electrically connected to the control unit.

8. Method according to any of preceding claims, characterized in that the predetermined set point value for the pulp level is related to the froth concentrate discharge rate for each flotation cell.

9. Method according to any of preceding claims, characterized in that the predetermined set point value for the pulp level is related to the changes in the hydraulic gradient and air hold up between flotation cells.

10. A method for controlling pulp levels substantially as herein described with reference to the accompanying drawing. DATED this 10th Day of December, 1997 OUTOKUMPU TECHNOLOGY OY Attorney: STUART M. SMITH Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS