CA1119985A - Method for controlling flocculant addition to tar sand tailings - Google Patents

Abstract

"METHOD FOR CONTROLLING FLOCCULANT ADDITION TO TAR SAND TAILINGS" The hot water extraction process for recovering bitumen from tar sand produces a large volume of solids-laden aqueous tailings as a waste product. The solids in the tailings stream may be flocculated by the addition of lime and the components of the stream then separated into a water-free solids phase and a clarified water phase. During flocculation, the zeta potential of the stream is monitored. It rises from an initial negative zeta potential, as the lime is added. Flocculation is terminated when the zeta potential is about zero. At this point, the tailings are in optimum condition for separation into the water-free solids phase and the clarified water phase. Separation is preferably effected by vacuum filtration.

Description

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BACKGROUND OF THE I.NVENTION
This invention relates to an improvement in the treatment of whole tailings produced by the hot water extraction process for recoveri.ng bitumen from tar sands The urgent need for alternative sources of hydrocarbons, especi.ally for use as fuels, is i:ncreasi'ngly~apparent as reserves of son-vential crude oi:l becomes us.ed up. An extensi:ve source of hydrocarb.ons. i.s in the bitumi:nous sands found in vari'ous; parts of the world. Particularly useful deposits of bitumi.nous sands are in Western Canada, where they are commonly known as tar s.ands.
~ uch. tar sands are often found near ground surface, thus they can be mi.ned and transported to an extraction plant for recovery of the heavy oi.l (bitumen).
The only commercially us.ed process for recovery of bitumen from mi.ned tar sand is the hot water extracti:on process. In accordance wi'th this process, the tar sand is -fed i:nto a rotati.ng conditi:oning vessel,known as a tumbler, and mixed wi:tK.hot water, steam, and small amounts of process ai.d. The most common process: ai.d is sodi:um hydroxi:de; its purpose i.s to assi.st i.n causi.ng the b.i.tumen to b.e released from the other consti.tuents of the tar s.and mas.s. The proces is run so that tar sand takes less than 10 mi.nutes to pas.s. th.rough the tumbler. A s.lurry emerges from the tumbler.Thi.s~ s.lurry i.s screened to remove overs;i:ze matter, such as rocks or un-diges.ted lumps of tar sand, and then i:s di'luted with add;t;onal hot water.
The screened, di.luted slurry i.s then advanced to a quiescent ~one known as.
the primar~ separati.on ves.sel (PS.V~. Because the components o-f the slurry are in only loose association (as a result of the conditioning in the tumbler), they are able to separate in the PSV under the influence of gravity. Hence the s.and~ which has a gravi:ty of a~out 2.65, si.nks to the bottom of the PSV, and may be pumped out i;n the form of an aqueous mi.xture.
The bitumen has. a dens.i.ty of around 1.00, that i.s, close to that of water.

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Left to itself, it tends therefore neither to sink nor to float. But a considerable number of air bubbles have b.een introduced into the slurry in the tumbler and these attach themselves to the bitumen globules~
In this way the bitumen, i:n an aerated state, rises to the top of the PSV and may be collected in the form OT a froth. ~ome bitumen fails to so ri.se because the s.ize of the glohules is too small or because of -failure to get aerated. This unaffected bi:tumen remains i`n the central region of the PS~ and helps make up a porti:on known as the "mi:ddlings". To i.ncrease the effi.ciency of the proces:s~ a middli`ngs stream is continuously 10 withdrawn from the PSV and advanced to induced ai:r flotation cells where, by vigorous agitati.on and the addition of external air, a second yield of bitumen i.s. obtai.ned in the form of a secondary froth. The froths are then combined and the bitumen ;;s separated from the contami:nating water and mi.neral s.olids. Th.is i.s. don~ by diluti`ng the combined froths with a 15 h~drocarbon diluent and separati.ng the diluted bi:tumen using separatory means s.uch as centri.fuges. The diluent may then be di:stilled out of the hydrocarbon product phase to leave pure i:solated bi`tumen that may then be upgraded by processes known i:n the heavy oil art.
The hot water process. is ef~i:ci:ent and has the advantage of 20 operati.ng under mi.ld condi:tions:. A dis:advantage is the producti.on of alarge volume of soli:ds-laden, aqueous. tai:li:ngs. Provision must be made ror storing these tai:li.ngs. and, at least as an i.ntermediate step, they mus.t, accordi.ng to present practice, be i:mpounded within dykes that must be constructed near the mi:ne area. S.uch` tai`li`ngs ponds bring undesirable 25 environmental effects, and cover tar s:and that is thereby rendered un-avai.lable to mi`ni.ng.
One extraction operation, producing 120,000 bbl. syntheti:c crude per day will, over the 25 year life.of the project~ create a tailings pond of around 10 s.quare mi:les i.n area. I`f, as other extraction facilities 30 are b.ui.lt, the same area of tar s.and i~s; covered7 the whole depos.it is signi:fi.cantly reduced i.n size.

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In the tailings pond, the solids are supposed to settlP to leave a layer of clear water which can be re-cycled to the extraction pro-cess. Once enough fresh water has: been taken on board, an extraction plant which is self-suffi:cient in clarified water may be obtained by this recycle proces;s.
In practice, the coarse S;Ql ids. (i:.e. the sand grains) do settle rapi:dly, but the fine s.olids (i.e. -44 mi:cron fraction) settle only slowly over a period of several years. It would be a great advantage to the tar s.and i.ndus.tr~ f a feasib.le, rapi'd water clarificati:on process could be devi:sed.
Workers.i:n the field h.ave turned for gui:dance to the water clarifi.cation art, where large volumes: of water are puri:fied for consumption as~ domesti:c ~later. As a result9 it has been proposed that coarse soli.ds be fi.rs.t settled out and the fi:ne matter s;ubsequently flocculated by conventional flocculati:ng agents. The flocculated solids could then be removed from the water b~ centri:fugi.ng or filter;:ng. Typical examples of this approach are given i:n U.S. Patents 3,487,003 and 3,502,575, i.ss.ued to Bailli:e et al and Hepp et al , respecti:vely.
A new.proces.s is the subject of patent appli:cation Canadi`an serial numher 311,6.9.6, ~n~O_S.~ o~u~e~ C~ , fi:led by J. K. Liu et al whos.e assi.gnees. are the same as. for the i:nstant inventi:on. The Li.u process ;:s. bas.ed on the di:scovery that, when ~locculant(s) are added to whole tai:li.ngs (i:.e. with the s;oli`ds unremoved), the coarse parti:cles form nuclei. to whi:ch the fi:ne parti.cles can adh.ere. What is produced i:s a preci.pitate of aggregates oF coarse and fi.ne soli'ds. In thi.s state, the mi.neral matter can be fi.ltered out without the need for a filter ai:d.
There results a fi:lter cake, small in vo'lume and wi:th little water, and a Fi.ltrate suffi:ci:ently clear for immediate. recycle to extraction. The cake is easy to di:spos.e of because i:t can be compacted to form a base for reclai:med land. Thus the cake can he di`s;posed of in the mined-out area.

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The preferred flocculant i;n th.e inventi.on of Liu et al is lime. This i.s. readily available i:n tar s.and regions and does not leave di.ssolved resi:duès that could be harmful when recycled to extraction.
The products of decomposed li:me are an i`nsoluble carbonate~ and one molecu'le of water for every reacted molecule of li`me. The present i.nventi.on, that i:s now s.ummari.zed, teaches how to optimi.ze the level of flocculant needed.

~UMMARY OF THE INVENTI:ON
To bring whole taili:ngs to a state o~ preferred readines.s for filtrati.on, flocculant i.s added to the ~hole tailings. This leads to co-flocculati:on, with the large-si:zed grai:ns: evi:dently serving as nuclei.
for the fine soli:d parti.cles to attach to. At optimum flocculant dosage the effecti.venes.s. of filtrati:on i:s maximized, leadi'ng to rapid filtration rate, lo~ cake moi.s.ture, and low solids; levels i.n the filtrate. Too little or too much flocculant prevents the f;:ltration effectivenes:s bei:ng at the maxi.mum. Further, too much flocculant i`s. wasteful of chem;:cals, and at the huge ~olumes of tai.lings involved i:n an extraction plant, this could represent seri.ous. economi.c cost.
Accordi.ng to the i:nventi.on, flocculant dosage is controlled i.n response to the tai:lings. zeta potential. Before Plocculating, the taili.ngs. have a negative zeta patenti:al. As flocculant is added, the zeta potenti.al ri.ses. At the isoelectric point where zeta potential i:s s.ubstantiall~ zero, it has been found that the taili:ngs are in the most desi.rable state for ~iltration.
Therefore, in accordance with the i:nvention, the zeta patential of the tai.lings is. moni:tored as the flocculant i:s added to and mixed with the whi.le taili:ngs. When the zeta potenti'al is about zero, the addi.ti.on of flacculant i:s: termi.nated and the flocculated taili.ngs are treated, as b~ vacuum filtration, to produce a substantially water-free soli:ds. phase and s.ubstanti.ally clarified aqueous; phase.

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Zeta potential may be monitored by meas.uring the rate at which a fine particle in suspension travels between two electrodes. This rate depends on the charge of the particle and it is this charge that is known as zeta potentia'l. ~e have further found that, although the tailings are treated i:n the whole state with flocculant, the zeta potenti:al of fi:ne particles alone i:s i:ndi:cative of the closeness of the stream to maximum fi:ltration effectiveness. When whole tailings are treated wi.th li.me flocculant, almost i:mmediatel~ the soli'ds co-flocculate and visibly begi.n to settle. A s.uhstantially clear supernatant li:quor i.s left although thi.s liquor does contai.n some finely sus.pended fihes. The flocculant may thereaFter be added in;stages.; aFter each additi`on a sample of the supernatant may he withdrawn and subjected to zeta potential analysis. The zeta potential i:ncreases from a larye negative value to zero and then increas.es: on the.posi:ti:ve s.ide. At the point of zero potential of the liquor containi.ng the Fi:nes, the whole tai:li:ngs stream i:s i.n the preferred conditi.on for filtrati.on. I:t is not necessary therefore to devise a test to attempt to determine ze.ta potenti;al of large flocs of co-preci.pitated solids, since the zeta potenti`al of fi.nes alone indi.cates the zeta potent;al state of the co-precipitated matter.
Broadly stated, the i.nvention i.s. an improvement in the hot water extracti.on proces.s wherein a soli.ds-laden aqueous whole tailings stream is generated as. a waste product compri:si:ng: treati:ng said whole tallings. stream wi.th f'locculati.ng agent and moni.tori.ng its zeta potenti.al;
controlli.ng the amount oF flocculati.ng agent added so as to raise the zeta potenti.al of the whole tailings~ from an i:niti:al negative value to ahoutzero; and then separati:ng the cons.ti.tuents; of such stream to produce a substanti.ally water-free soli.ds. phase and a suhstantially clarified a~ueous phase.
DESCRIPTION OF T!-IE DRA~IINGS
Fi.gure 1 is a plot of four vari.ables aga.inst lime added. The variables we.re: zeta potential, filtrati:on rate, cake moisture, and 501 ids i:n fi:ltrate.

Figure 2 is a plot of zeta potential versus lime added for low fines and average fines tar sands.
Fi'gure 3 is a schematic representation comparing the prior art with the present invention.
DESCRIPTION OF T~E PREFERRED'EM~ODIMENT
The feed for this i`nvention is whole tailings from a tar sand hot water eYtraction process;.
The preferred flocculant is lime. Lime is readily available in tar sand reg;ons in the form of limestone~ It i`s a well known pro-cess to convert this to lime by calcining. I:n use, the lime is mixed witn water to form a slurry and it is thïs slurry t6at i`s added as the flocculati~g agent. We have used a slurry containing a60ut lQ wt. % lime.
Since the filtrate is to be recycled to extraction~ the flocculant must be chosen from substances that will not allow build-up oF species damaging to extraction. Lime has the advantage that the ultimate reaction products of any excess of lime with carbon dioxide in the air are calcium carbonate, which has a low solubility product, and one molecule of water for every molecule of lime. Thus lime does not create compounds which de'leteriously affect extraction processing.
The data plotted in Figure 1 was developed using a Zeta-Meter*
on samples of a clarified aqueous phase derived from settled whole tailings from a tar sand hot water extraction process. The sample was introduced into an electrophoresis cell and a suitab'le direct current voltage applied, usually between 100 and 400 volts. The time required for a colloid particle to kraverse a fixed distance ;n the direction of the anode or cathode, as viewed under a microscope, was measured. The observed time was then converted to electrophoretic mobility.
The particular procedure used was as follows:

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1. Approximately 50 mL of sample was required to fill the cell. If samples appeared di.rty, as was the case with fine clay slurries, such samples were centrifuged at 15,000 rpm for 20 mi.nutes and the clear liquid decanted for analysi:s. Care was taken to ensure that the sample ~as at room temperature and the sample temperature was recorded.
2. The cell holder was posi:ti:oned on the mechanical stage of the mi.croscope. The cell, fi:lled with the sample, was placed on the holdèr. The stage was. then adj~sted to posi.tion the center of th.e cell tube di`rectly beneath the opti:cal axi:s of the microscope.

3. A thin heam of light ~as focused downward through the plate glas.s cell holder to îts mi:rror back. This beam was reflected upward and pas.sed thorugh the cell, causing reflecting colloids to ~e seèn as tiny rays of li.ght.

4. The microscope was then focused so that the positioning line of the cell was disti:nct.

5. A dlrect current voltage of lQ0 to 40.0 V was appli:ed to the cell. If the particles had no charge, they ~ould remain stati:onar~; negati.ve colloi:ds would mi:grate to-wards the anode and posi:tively charge colloids woulcl migrate to the cathode.

6. Di.s.crete particles on or near the counti.ng line were timed i.n thei.r travers;e of one or more ocular micrometer divi.sions using the manual timer. Fi:ve or ten discrete particles were timed i;n thei.r normal direction of travel.

7. The voltage appli.ed across. the cell was recorded, as were the objective magni.fi:cati.on and the fi:nal temperature of the sample.
ohjecti.ve magnificati.`on x voltage The following precautions. should be noted in carrying out the test. If the voltage is too hi:gh, the samp'le temperature may rise, causing particles to describe a s-pi:ral rather than horizontal path.
If this occurs, reduce the applied voltage. If the voltage is too low, the particles will settle out and be lost From view. For precise work one should select an opti`mum voltage and track several particles consecu-tively and then take an average.
Fi:ltration effi.ci.ency was tested by leaf filter tests.
According to this test, ~hole tai:l;ngs w.ere mi:xed with the quantity of li.me heing i.nvesti.gated and the mixture poured i.nto a funnel li.ned with filter cloth of the type that would ~e used i`n a cont;'nuous operation. We use 100 mesh U.~. standard si:eve. The cloth i:n the funnel was. supported on coarse mesh ~i:th openi:ngs of about ll4 i:nch. The funnel was drai:ned i.nto a vacuum flas.k with. a si.de arm at wh.ich poi:nt vacuum of known force could be applied. The start of l;`qu;'d bein~ pulled through the.cloth was t;.med, and a further time readi.ng was taken w.hen the surface oF the cake was. f;rst seen to be dry. (Th;s occurred qui:te suddenly and is readily repeatable.) The cake ~as s.ucked dry for a further 2 m;nutes, thi.s b.ei.ng found adequate in all cases. to remove free water. The res.i:dual cake was then tested for remai.ning water ('b.y dryi`ng a known weight); the fi.ltrate was. tes.ted for 'level of sQli.ds. (b.y further fi:ltration through f;ne mes.h, and dryi:ng and we;.ghi.ng resultant soli:ds); and the fi'ltration rate was known From the ti'me meas:urements:.
Turning now to fi:gure 1, ;'t shows that as the zeta potent;al approaches zero from the negat;.ve s;;:de, cond;.t;.on;ng of the ta;lings For filtrat;.on reaches max;:mum ef~ect;:veness:. B:eyond th;'s, lime is added unnecessar;'ly.
In the hot water extra.ct;~on proces.s, as the level of fines increases in the feed, the dQsage of s:odi:um hydroxi:de (added as; a ~rocess ai.d) mus.t li:kewi.se be i.ncreas~ed to mai:ntai.n the oil recovery at a maximum.

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It might be expected that this i.ncrease of sodi.um hydroxide would affect the quantity of flocculant needed to treat the tail;ngs. It was our surprising discover~ that, alth.ough.tai:lings hi.gh i:n sodium hydroxide began at very negati:ve values of zeta potential, li.me addition rapidly brought that potenti.al to zero. On the other hand, tailings from the s.ame type of tar sand, but low i`n sodium hydrcxi:de, while starting at a les.s negati.ve zeta potenti:al, h.ad a slawer res:ponse to lime. That i:s, fortar s.and of the s.ame fi:nes level, ~u~ treated wi:th different amounts of s.odium hydroxi.de, the slopes of the curves:linking sodium hydroxi:de to li.me vary such that the effect of s.adium hydroxi.`de is largely eli:mïnated.
Thi;s effect i.s shown i:n Fi:gure 2 whi`ch plots: zeta potential versus li.me added for a la~. fi:nes. and an average fi`nes tar s.and. For th.e low fi.n¢s. runs, the lime neede.d to bring th.e zeta potenti:al to zero was.
bet~een 300 and 350 mglkg. For the ave.rage. fines runs the dosage was 800 mg/kg. Different leve1:s. of sodium hydroxi:de added to the extracti.on process caused the ini;ti:al zeta potenti:al to vary from -23 to -42 mV
but at the i:soelectri:c poi.nt the curves: tended to collect together.
Probably sodi.um hydroxïde i:n extracti:on and li:me in . flocculation operate by reverse mechani`sms... Extraction process aïds.
deflocculate the clay matter i:n tar s:and and hence encourage releas.e of the entra.pped bi.tumen. I.t i:s possible that this is an i:ndi`rect mechanlsm whereby s.urface acti:ve agents~ are fi:rs:t produced between the sodi:um hydroxi~de and naturally occurri.ng organic aci:ds i`n the tar sand ~i:tumen and these operate on the clay. The aim of the lime by contrast is to re-flocculate the clay matter, in our case by coflocculation with the coarse so1ids. It might be expected therefore that,for the same tar sand, different levels of sodium hydroxide ~ould require varying amounts of lime.
Our finding that lime requirements are es;sentially independent of sodium hydroxide content and depend solely on fines content, is an unexpected and welcome simplification of tar sand tailin~s. m~nagem~n.t.

No test has been devised to measure the zeta potential of flocs of co-precipitated tailings solids. It is a fortunate discovery therefore that the zeta potential of the fines, left in suspension after the majority of the solids are preci:pi:tated~ may be used to indicate the zeta potential of the whole tai'li'ngs. Hence, when the zeta potential of the fines alone is brought to zero, the preci:pi`tated solids are in the ri.ght condi.tion for vacuum filtrati.on. A convenient point to measure the potential o~ the fines i:s the fi:ltrate. The small amount of fines reporting to the filtrate i.s suffi:ci:ent to allow the required zeta potenti.al measurements to he made. Flocculant slurry may then be added to the w:hole tailings to give a filtrate potenti:al of zero. In a conti.nuous pracess batch portions o~ the fi:ltrate can be withdrawn for testing or a small si.de stream may be di`verted.
Fi.gure 3 i:s a fanciful represèntati.on of tailings management by the prior art and as taught i.n the pres:ent inventi'on. In the prior art, whole tailings. are simply allowed to s.ettle under the i.nfluence of gravity. Although the coarse s.olids settle qu;'ckly the sludge of f;ne sol;ds and clays settles only ovèr a peri:od of several years due to the mutual repuls~on oF the parti.cles stabi`lizi:ng the s.ludge.
This slow settling calls for large tai:li`ngs; ponds:.
Accordi:ng to the present i:nventi:on, the whole tailings are flocculated and may be subjected to s:ome rapi:d separatory means such as vacuum filtrati.on. Although i:t i:s the condi:ti'on of the co-flocculated soli.ds that determines optimum fi:ltration, the zeta potential need be meas.ured only on the fi~nes. i:n the clear aqueous layer. Places where zeta potenti.al may he canveniently measured are marked "z.p.".
I.t i.s to be understood that the separatory means may i`nclude mere gravity settl;ng of the flocculated whole tai:li:ngs. If the soli.ds are to be formed i:nto a heach or layed down by such techniques as cone ridg;:ng, already taught i'n the mi:ning tai:li:ngs: or dyke building arts, the inYenti.on may he practi.sed i.n connecti:on with the tai:lings.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the hot water extraction process wherein a solids-laden aqueous whole tailings stream is generated as a waste product, the improvement comprising:
treating said whole tailings. stream with flocculating agent and monitoring its zeta potential;
controlling the amount of flocculating agent added so as to raise the zeta potential of the whole tailings from an initial negative value to about zero; and then separating the constituents of such stream to produce a substantially water-free solids phase and a substantially clarified aqueous phase.

2. The improvement as set forth in claim 1 wherein the constituents of the flocculated tailings are separated by vacuum filtration.

3. The improvement as. set forth in claim 1 wherein the flocculating agent is lime.

4. The improvement as set forth in claim 2 wherein the flocculating agent is lime.

5. The improvement as set forth in claim 1 wherein:
the zeta potential of the whole tailings is determined by measuring the zeta potential of fine particles in the substantially clari-fied aqueous phase.

6. The improvement as, set forth in claim 2 wherein:
the zeta potential of the whole tailings is determined by measuring the zeta potential of fine particles in the substantially clari-fied aqueous phase.

7. The improvement as set forth in claim 3 wherein:
the zeta potential of the whole tailings is determined by measuring the zeta potential of fine particles in the substantially clari-fied aqueous phase.

8. The improvement as set forth in claim 4 wherein the zeta potential of the whole tailings is determined by measuring the zeta potential of fine particles in the substantially clarified aqueous phase.