CA2193360A1

CA2193360A1 - Thickener mud gauge

Info

Publication number: CA2193360A1
Application number: CA002193360A
Authority: CA
Inventors: Vince Robert Dooley
Original assignee: Individual
Current assignee: Alcoa of Australia Ltd
Priority date: 1994-06-29
Filing date: 1995-06-27
Publication date: 1996-01-11
Also published as: WO1996000885A1; AUPM654094A0; EP0767897A4; EP0767897A1; BR9508193A

Abstract

An apparatus for determining the concentration of solids or for measuring the interface between a liquid and settled solids in a settling vessel, comprising: (a) electrical conductivity or resistivity measuring means adapted to monitor electrical conductivity or resistivity within a liquid in a vessel; and (b) monitoring means which is capable of recording electrical conductivity or resistance levels within the vessel and which is capable of determining the concentration of solids from the electrical conductivity or resistivity results. Where the apparatus consists of a conductivity sensor (1) which is in electrical communication with and suspended by a cable (2) from a winch (3). The winch is controlled by the operation of a motor (4) which is controlled by a monitoring means (5). The conductivity sensor signals are transferred from the cable (2) by sliprings (6) to a conductivity meter (7). The conductivity data is then passed from the conductivity meter to the monitoring means (5) which measures positional data from a shaft encoder (8) and controls the position of the conductivity sensor by instructing the winch (3) to raise, lower or hold the sensor in any position inside the settlement vessel. The monitoring means is adapted to continuously examine conductivity data from the sensor.

Description

_ WO 96/0088S PCI/AU9~J0037~
21 93:~60 , ~THICKENER MUD GAUGE~

THIS INVENTION relates to a Illetllod and an apparatus for the dete""in~tiGn of the co"c6i)tralion of solids in a liquid. The ",etl,od and appara~us are particularly useful in the alumina processi"g industry for determining the 5 in~, race between red mud and green liquor.

There a number of dirrarent types of detectors used in processing plants to measure and record the solid content of liquid slurries. In this respect - measur~",6"l of the level of the settled solids and of the solids conc6nt~alion profile in settling tanks thickeners or like vessels is essenlial for the effective 10 control of such plants.

Measu,el"6i,ls based on turbidity are most co"""on and give excellent results inclean liquids but have a limited ~pplic~1ion range in liquids co"")~only encountered in for exc,,,,pl~ mineral p,ocessing. One explandtion for this is that they require frequent ",aintenance in processes where precipilate scales 15 form on the dete~tor.

Systems based on ull~asonic absG"~tion are also used extensively but are unreliable in many processing industries due to te",peral,Jre li",itations and an ~ uncontrollable instability often due to air e,lt,air""6nt on the sensor or the effect of currents in the liquid.

20 More rec6lltly natural' rAdioAclivity has been used in for example ",i"eral processin~ industries as an alle",dtive measure to determine settled solids levels and solids concent, ation profiles in processes where there are radionucleotides present in the solids. However the dete""ination of the solids content using such a measure is often difficult to achieve since the radioactivity 25 presen~ in minerals and ores is generally low and these devices require time to accumulate sur~ic;enl counts for measu~",enl pu"~oses. Further they are e~.e"sive to manufacture and are subject to significant amounts of electrical WO 96/0088S 2 1 9 3 3 6 0 PCI'IAU9S/00377 noise which has been found to produce false read;ngs or skew or distort the results obtained.

Another system that has been fl~l Itly adopted as a means to measure levels of settled solids and solids COIlCelltldtiGll p~rileS in setlle",ent tanks and the like 5 has been based on dete~o,s which measure sonic reflections. However such systems have not proved reliable. Reasons for failure are unknown but could be due to air entrainment on the dele~lo,s.

It has long be appreciated that the dete,-"inaLion of the solids content in liquids used in many p(ocessi"g industries is essenlial to dele"";ne the throughput of a10 particular process. The presence of excess solids in a liquid often results in considerable production loss and high costs in cleaning up conLa",inaled solution. Hitherto there has been no reliable ,"etl,od or instrument available to measure the conce,lt,dtion of solids or to detect the i"le,rdce betw~en liquids and settled solids in settling tanks thickeners or like vessels.

15 In accorda"ce with the present invention there is provided a " ,etl ,od of dete""ining the c~ncerlt~ation of a solid in a liquid in a vessel the Illetllod co,np,ising the steps of:

(a) monitoring the electrical conductivity or resistivity of the liquid in the vessel; and 20 (b) dete""i"iny the concenlration of solids in the liquid from the electricalconductivity or resistivity ll,ereof after cor,eotion for any te""~erature changes in the liquid.

In a prefei,~d form of the invention there is provided a ",etl,od of measuring the inte,rdce between a liquid and settled solids in a vessel the ",etl,od co",~risiny 25 the steps of:

W0 96/0088S 2 1 9 3 3 6 0 PCIIAU9~J00377 (a) ",onitoting the elecl-ical conductivity or resistivity of the liquid 7in the vessel at ~lirferenl levels therein;

(b) detell-,i,lillg the con~r,t,dlion of solids in the liquid at tne di~rent levels by using the elect, ical conductivity or resistivity of the liquids or the solids after colrectiGIl for any te,~"~e~ture ~,anges in the liquid in the vessel;
and (c) selecting the level where conductivity or resistivity cha"ges subst~rltially relative to the conductivity or resistivity of a clean liquid and/or a liquid contai,)ing solids.

Also according to the prese"t invention there is provided an apparatus for determining the concenl~ation of solids in a liquid in a vessel the apparal-ls comprising in co")binalion:

(a) electrical conductivity or resistivity measuring means aclapted to monitor elect, ical conductivity or resistivity within the liquid in the vessel; and J

15 (b) monitoring means ~ssoci~d with the electrical conductivity or resistivity measuring means the ,),onit~ring means being capabl~ of recording electrical conductivity or resislance levels within the vessel and being capabl~ of dete""ining the cor,centr~tiGn of solids from the elect,ical conductivity or resistivity results.

20 The term "liquid" used herein is inlencled to include but is not limited to any liquid solution hol"ogenised liquids slurries and particulate material in gaseous suspension which behave as a liquid.

It has also long been known that varying levels of natural eleot, ical conductivity or resistivity is present in all g~o'ogi~l ",ale,ia!. Su,p,isingly it has been found 25 that bulk ele~l ical conductivity or resistivity of a liquid at a given temperal.lre is a function of the electrical conductivity of the liquid the ele~t, ical conductivity of w096~ 2 1 9 3 3 6 0 PCI/AU9S/OO~

the solids and the cGnc~rd,dtiGn of the solids in the liquid. Ther~fore when conductivity measure",ents are coll~ted for te",perature cl,anges or when te"",era~.lre is cor,~ ,lt the cG"ce"~.~io" of solids can be e~li,nated from measufe",e"t of liquid conductivity relative to that of clean liquid.

5 The IlleUlod and apparatus of the invention may be employed in any process where it is i",po,ldnt to ",onilor the solids cG~Itent in a liquid in a vessel and where the solids exhibit some ele~t, ical conductive or resistive properties. For example the ",eU,od and apparatus may be applied to brine tanks settling tanks slurry tanks any tank with a solid/liquid intelrace calcium ca,60nale 10 clarifiers and the like. It is convenient tl,ereror~ to explain the present invention by way of example in the context of its applic~lioll to the alumina processing industry where it is used to determine the intel race between red mud(i.e Bayer mud) and green liquor (i.e the alumina-6ea,i"9 solution).

Alumina is typically prorluced from bauxite by the Bayer pr~cess. In this 15 "~eU~od aqueous ~ustiç soda is used to treat bauxite under high temperature and pressure. During this p,ocess aluminium hydr~icle present in the bauxite dissolves in the CP~IStiC solution forming a sodium alu",i"dte. The insoluble residues form red mud which is primarily co",posed of the oxides of iron silica and clay. In stages s~hseq~ ~ent to the Bayer ~rocess sodium ahJ",i. ,a~e solution 20 and caustic soda are separated from the red mud and other insoluble ",alerials.
Coarse waste particles (e.g sand etc.) are se~,ard~ed from solution in large settling vessels. The red mud is then separa~ed from the green liquor by a gravity sedi~"entation meU,od in large dia",eter vessels known as thickeners.
The undel nO~H from these vessels is known as ~red mud~ while the overflow is 25 known as green liquor~. Red mud removed as u"de,now is washed in large washing tanks to remove v~ hl~ ~ustic soda for reuse. Inrol",dlion cG~Icelllirlg the i,lte"~hase between red mud and the green liquor is i""~o,ldnt to improve efficiencv and to institute process controls and auto",~ion.

W096l00~8S 21 g3~0 PCI/AUg5/00377 Eleclncal conductivity or resistivity may be measured by any means known in the art. r, eferdbly that means is capable of wiU Islanding high te~llperature and a caustic envir~ll",~nl. Elect~ical conductivity may for example be measured using ele~ll odes or toroidal sel Isor~.

5 The concent,ation of solids in the liquid in a vessel has been found to be a function of the electlical conductivity or resistivity and of the telllpel~ re of the liquid in that vessel. Where the te"lperat.~re of the liquid is consistent throughout the volume of the vessel elecl~i~al conductivity or resistivity bares a direct inverse relationship to the concentraliGn of solids in the liquid. Thus - 10 te."peralure measure"lenls are not generally required where temperal.lre is conslanl. However the teil,per~tlJre of the vessel is preferably ",Gnilored to ensure te""~eralure consistency throughout the vessel whilst conductivity resistivity measure,llenls are taken.

rlocess fluids encountered in the alumina industry are generally conductive but 15 solids such as mud and sand behave as insulators. The presence of dispersed solid particles reduce the fluid conductivity by a ratio which is de~,endenl on the volumetric collcellt,dtio,l of solids. This plinc;ple applies for any combination of solids and fluid where the ratio of their conductivities exceeds a value known as the Critical Conductivity Ratio (CCR). The theorelical conductivity of a slurry or 20 mixture conta6~ing spherical pallicle-e of any size distribution is given by the relalionship:

K mi~cture = K nUid (1 F)3/2 where K is conductivity and F is solids fraction by volume. The effect of solidson fluid conductivity is depicted in Figure 1.

25 In practice the reduction in conductivity c~used by the introduction of solids such as red mud to Bayer process fluids has been found to be slightly g,ealer than that pl e~l;~ed by the model for sphe, ical particles.

W096~5 21 93360 I'CTMU9~U77 At low volumetric conce, Itl dtions of solids in thicken~,a and washers and for the purpose of mud level measu,e",e,)ta the following simple linear relationahip maybe used to determine solids c~"cenl,alion.

Solids in gpl = A(K flUid - K m~duro) IK nuid where A is a e~ e,i",entally dete""ined consla,lt.

If the te""~erature of the liquid in the vessel is known to vary the monitoring means is plefer~bly connected to a te,t,per~ture measuring device which is capable of determining te",perat,Jres of the liquid at di~renl levels within thevessel. Where a te""~erature measuring device is used that device should be carz~la of wilhatanding high te",peratures and a c~ustic enviro,-",ent.
r,e~e~bly the te",perature measuring device when ~)rese,)t is in communication with the l"ol ,iloring means which in tum directs temperature datato be recor,led at the same time that ele~ ical conductivity or resistivity readi"~as are taken. The tempe, dture measuring device may for example be a ll,ei",islor a tl,e",~oco~lrle a r~sisla"ce te",perature device (RTD) or some other thermal dete~tor. Further the tel"perature measuring device may be ,uosilioned in comb.nation with the conductivity or resistivity measuring means as a single sensor unit.

The p,esent invention will now be desuibed by way of e~a",pl_ only with rerere"ce to the acco",pdn~ing draw "9-~ of which:

Figure 2 illustrates one form of the e~le_t,ical conductivity measuring device of the presenl inve, Ition;

Figure 3 is a graph showing the output data from 5 dirrere"l liquid col,centn3tions measured over varying ele t,ical conductivity and tel"perature regimes; and ~ wogC/0088s 21 ~3360 PCI/AU9~J00377 Figure 4 illlJall dt~S an alle" ,a(i~e form of the ~lec~l ical conductivity measuring device of the preseu~ invention.

Figure 2 depicts an appar~tus for determining the conce"t,dtion of solids or formeasuring the inte,race betwcon a liquid and settled solids in a setlling vessel5 consi~tiny of a conductivity sensor 1 which is in ele~l.ical communication with and suspended by a cable 2 from a winch 3. The winch is controlled by the operation of a motor 4 which is controlled by a ",oniLoring means 5. The conductivity sensor signals are l,ansre"ed from the cable 2 by sliprings 6 to a conductivity meter 7. The conductivity data is then p~ssed from the conductivity10 meter to the monitoring means 5 which measures positional data from a shaft e"coder 8 and controls the posilio" of the conductivity sensor by instructing the winch 3 to raise lower or hold the sensor in any pocilion inside the selllel"enlvessel. The ",or,itori,-g means is adapted to continuously excl",ine conductivity data from the sensor.

15 In one mode of operatiol) the conductivity sensor 1 is lowered at a predetermined speed into the setlling vessel. The mo-lilulilly means 5 recGrds the posilion of the conductivity sensor 1 and the ele~,ocûnductivity at cJirreren~
points. If a te",peralure sensor is present the monitoring means would also measure te""~erature at each point. From such data the inte,race between 20 green liquor and red mud may be idenliried as indic~ted by a s~ sl~nlial change (e.g decrease) in conductivity. The ",oniloring means 5 then indicates the position of the i"te, race by any suitable means.

The present ~nvention may also be used to control the level of red mud in a settling vessel. For this purpose the conductivity sensor 1 may be variably 25 positioned within the setlli-,g vessel. The locatiGn of the conductivity sensor 1 will co,-espond to the maximum height that the red mud may rise to accord;"g to process control req-.;re,l)en~s. The ,-,o-)ito,ing means 5 continuously examinesele~,ical conductivity data from the conductivity sensor 1. When electrical conductivity reaches a predete~ ~,ined value which ~"esponds to the maximum w096/0088s 21 93~60 PCI/AU9S/0037'7 height to which the red mud may rise the ~Gnito~ing means would produce a signal which auto",alically i"iliales wiU,dr~ 31 of red mud from an outlet in the bottom of the s~tllirlg vessel.

The metl,od of the present invention may also be used in rake- equipped tanks 5 for example to control the level of red mud in a liquid (i.e tanks equipped with aulo,,,c,lic stirrers which are capabla of mai.)taining solids liquid). For thispurpose the "~oniloring means 5 is in elEctncal communication with the drive motor for the rake and is adapted to raise the conductivity sensor 1 when the rake a~.roaches and holds the output signal constanl until the rake p~sses.
10 The ",o"ilorin~ means may then reposilion the sensor at the last point of measure",ent prior to the rake approacl,iny and resume recording data.
Allair,ali~/ely the l"onitori"g means may r~ t,oduce the sensor in to the liquidand co"""ence continually recording data until it reaches the bottom of the vessel whereupon it retums to the surface. When ele t,ical conductivity 15 reaches a pre- dete",~ined valve which cor,esponds to the maximum co"cer,l,dtion of suspended solids in a liquid the moniloring means would signalthat the pre-dete""ined concenl,ation was reacned. In addition to or in the altemative the r"or,ilo,ing means would also direct an outlet means to open to release the liquid suspensio". The same ,nonitori"g means may also direct the 20 delivery of further liquid and or solid into the vessel. The liquid in this instance pre~e,ably consisls of one or more flocculants (ie. a setlli"g and clanricdtion aid).
While the foregoing Illethod is des~ibed for use in raké- e~uipped vessels it would be ap~,,eci ~ed that the same ~,etl,od would be applicable to vessels devoid of a rake appar~llJs.

25 Figure 3 illustrates g~phically test results showing the relationship betweente",perature eleul,ical conductivity and solids concelll~dlioll from a mud slurry in a sodium al~",li"ate solution. The points on the graphs of Figure 3 show thatat a co"stant temperature elecl, ical conductivity decreases as solid concentration increases.

_ wo 96~s 2 1 ~ 3 3 6 ~ PCI'IAU9S/00377 _ 9 _ Figure 4 illusl.at~s an alle",dti~e form of bhe invention wherein bhere is provided a doughnut style toroidal conductivity det~tor 10 suspended on a cable 12 which may be in.s!~'~'ed with high té",~erature teflon, from a winch 14. The de~ector is conne- tecl to the cable by an underwater break~ay oonnector 16.
5 The cable is conn~..t~d to the winch and is in ele t~ical communicaliG" with aconductivity ba"~",itler 18. The conductivity ~a,)smiller is mounted in the winch drum 20 to avoid the need to pass low level de~eclor signals across slip rings.
The conductivity signal wiring p~sses along the inside of the winch main shaft 22 to a rotary connector 24 mounted on the opposile end of the shaft. From 10 there the signal is conne-.tecJ back to a small modular style controller (notshown) such as PLC which is used to control the detector and record data from the deteotor.

The winch 14 is driven by a motor 26 having an integral worm reduction gea, box 28 which serves to reduce the speed and prevent runaway of the winch 15 under the weight of the detel;tor. Further speed red~ ~ction is achie\r0d through a timing belt and/or pulleys to give a winch speed of approximately 3mlmin.

The present a,cparatus has a pulley on the motor output shaft which is mounted to a slip clutch 30 so that in the event of the cletector being caught up on sol"elt,ing inside the fluid containing vessel then the slip clutch will allow the 20 cable to peel off the drum and cJisconnect U ,ere~, u,. ..

A quick releare socket (not shown) is used to wire the dete~or cable to the winch drum.
-A seconcbry shaft 32 may be used to drive a rotary limit switch 34 with upperand lower winch setlinys. Also mounted on this shaft are gears which operale a 25 pru~i",ity switch 36 for posilion measu,e",enl by the controller.

~ wo~oo~ 2 1 9 3 3 6 0 PCr/AU9510W77 P, efe, ably the conductivity l~nsmitler 18 is mounted centrally in the winch drum on a n~nged collar (after removing the terminal cover from the t~a"s",itter). The conductivity t~ans",iller is pr~er~bly a i"i~upn~cessor based tr~ns",ittar.

The dete~tw is pre~erably able to operale reliably in caustic solution at 100-5 120~C (eg 106~C). The cable should also be able to ~,ll,slt,nd the sameenvi, ~.r" "en~.

Addilionally the present invention includes a wash system (not shown) to keep the winch cable clean and ,ni"i~,ises scale build up on the dete~;tor. For example the wash system may have two dirrere,)t types of wash nozles.
10 Straight nozles should be used to wash the cable while deflection nozzles should be used to wash the detector. The no~ les are mounted to the appa, atus in close pro,~i",ily to the cable and the detector to provide a washing ll,erevf.
rre~ra~ly there is provided a plurality of deflected spray w~sher~. One op~,osing pair should be adjusted to deflect up~ards and the other pair 1 5 downwards.

When the prasenl invention is used in rake mounted vessels limit switches need to be mounted on the rake drive ",e~,anis", to provide an interlock so thatthe unit will operate in the safe zones betv/ccn rake p~cses In operalion the appa,dt.ls irliliates a scan each time the rake limits make a 20 transition from an unsafe zone to a safe zone. The winch lowers the delector into the vessel updating a rerer~"ce position as the dele~ r leaves the upper limit setting.

If there is a clear zone within the thickener fluid the controller reco,ds the fluid conductivity in that zone to use as a reference to ~lcul~te solids. Once in the 25 scan range the controller continuously c~ '~'es the solids conoent,dlion fromthe conductivity readings and the refer~nce reading. The controller calçul~tes the position of the dete.tor by counting the proximity pulses from the gear teeth.

_ wo 96~0088s 2 1 9 3 3 6 0 PCI'IAU9S/00377 It counts on both positive and negative l.allsilio.,s so that each pulse represents 10mm or 1cm.

The outputs are !q~d~1ed during the scan and are held until the next scan.
When the output rea~,es a preset point the unit ceases scanning and retums to 5 the upper limit posilion.

A wash system cleans the cable when the detector is being raised from the vessel and operales after each cycle to minimising scale growth on the dete~Lor.
Those skilled in the art will appreciate that although a specific e"lL,odi",ent of the present invention has been illusl~ated and des~ ibed above, variations in the 10 form of this embodiment can be made without depa,ling from the present inventive CCil ,cept.

Claims

THE CLAIMS defining the present invention are as follows:

1. A method of determining the concentration of a solid in a liquid in a vessel comprising the steps of:

(a) monitoring the electrical conductivity or resistivity of the liquid in the vessel; and (b) determining the concentration of a solid in the liquid from the electrical conductivity or resistivity thereof after correction for any temperature changes in the liquid.

2. A method of measuring the interface between a liquid and a settled solid in a vessel comprising the steps of:

(a) monitoring the electrical conductivity or resistivity of the liquid in the vessel at different levels therein;

(b) determining the concentration of solids in the liquid at the different levels by using the electrical conductivity or resistivity of the solids after correction for any temperature changes in the liquid in the vessel; and (c) selecting the level where conductivity or resistivity changes substantially relative to the conductivity or resistivity of a clean liquid and/or liquid containing solids.

3. A method according to any one of the preceding claims wherein the interface between the liquid and settled solids is calculated from conductivity or resistivity results according to the formula:

solids = A(K fluid - K mixture)/K fluid where A is an experimentally determined constant and K is the conductivity of the fluid in the vessel.

4. A method according to any one of the preceding claims in which the solids form a mineral processing slurry.

5. A method accordingly to claim 4 wherein the mineral processing slurry is the underflow from a settling vessel or suspended solids in a thickener vessel.

6. A method according to claim 5 wherein the slurry is a red mud slurry from an aluminium processing operation.

7. An apparatus for determining the concentration of solids in a liquid in a vessel, said apparatus comprising:

(a) electrical conductivity or resistivity measuring means adapted to monitor electrical conductivity or resistivity within a liquid in a vessel; and (b) monitoring means which is capable of recording electrical conductivity or resistance levels within the vessel and which is capable of determining the concentration of solids from the electrical conductivity or resistivity results.

8. An apparatus according to claim 7 wherein the electrical conductivity or resistivity measuring means is in communication with the monitoring means through a winching device that is capable of altering the position of the measuring means within the vessel.

9. An apparatus according to claim 8 wherein the winching device is controlled by the monitoring means

10. An apparatus according to anyone of claims 7 to 9 wherein the electrical conductivity or resistivity measuring means is capable of withstanding high temperatures and a caustic environment.

11. An apparatus according to anyone of claims 7 to 10 wherein the monitoring means is in communication with a temperature measuring device which is capable of determining temperatures of the liquid at different levels within a vessel.

12. An apparatus according to anyone of claims 7 to 11 wherein the temperature measuring device is capable of withstanding high temperatures and a caustic environment.

13. An apparatus according to claim 11 or 12 wherein the temperature measuring device is in communication with the monitoring means and is attached to the electrical conductivity or resistivity measuring means wherein the monitoring means directs temperature data to be recorded at the same time that electrical conductivity or resistivity readings are taken.

14. An apparatus according to claim 7 for use in rake-equipped vessels, wherein the monitoring means is triggered by the rake in the rake-equipped vessel to raise the conductivity sensor from inside the vessel when the rake approaches the sensor, hold the output signal constant until the rake passes thesensor and then take one or more further measurements as the sensor is introduced back into the liquid in the vessel.

15. An apparatus according to any one of claims 7 to 14 wherein the monitoring means is adapted to control the concentration of solids in a liquid suspension by instructing an outlet means to open or close in response to the concentration of suspended solids in the liquid.

16. An apparatus according to any one of claims 7 to 14 wherein the monitoring means is adapted to control an outlet means at or near the base of the vessel which is capable of releasing solids from the vessel.

17. An apparatus according to anyone of claims 7 to 16 wherein the monitoring is adapted to control the delivery of further liquids or solids into the vessel.

18. An apparatus according to claim 17 wherein the further liquid is a flocculant.

19. An apparatus according to any one of the preceding claims wherein the apparatus includes a wash system which is capable of washing the apparatus after it has been removed from the liquid.

20. A method of measuring the interface between a liquid and settled solids in a vessel substantially as hereinbefore described with reference to the examples and/or the accompanying drawings.

21. An apparatus for determining the concentration of solids in a liquid in a vessel substantially as hereinbefore described with reference to the examples and/or the drawings.