GB2241960A

GB2241960A - Electrolytic treatment of cyanide-containing liquid waste and treatment of by-product gas

Info

Publication number: GB2241960A
Application number: GB9101483A
Authority: GB
Inventors: Xinyu Yang
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-03-15
Filing date: 1991-01-23
Publication date: 1991-09-18
Anticipated expiration: 2011-01-23
Also published as: AU650252B2; ITMI910032A1; CN2063121U; ITMI910032A0; JPH04222690A; JP2603760B2; CA2031631A1; DE4040766A1; GB2241960B; IT1244362B; FR2659642A1; FR2659642B1; GB9101483D0; AU7126491A; CN1057245A

Abstract

Decyanation apparatus comprises an electrolyser (93) for receiving the liquid waste from a waste source and HCl, NaOH and NaCl from storage tanks (18, 19, 20) and a purifying tower (94) for receiving and purifying the gases exhausted from the electrolyser (93). In the electrolyser there are a pH sensor (42). an oxidation-reduction potentiometer (43), spray nozzles (31, 32) for stirring the liquid waste and a set of electrode plates (56, 57). The pH value inside the electrolyser (93) is sensed by the pH sensor (42) and controlled at about 10.5 by selectively supplying HCl and/or NaOH. The exhausted gases from the electrolyser (93) are purified in the purifying tower (94) by sprinkling liquid alkali then passing through a Paul ring filler (69 Fig. 8 not shown) layer in the tower. <IMAGE>

Description

:2:2 -11 -1- -:: CZ2, C-) DECYANATION APPARATUS AND PROCESS FOR PURIFYING

WATER FROM CYANIDE USING THE SAME The present invention relates to the purification of water, particularly to an apparatus for removing cvanide from liquid waste - a decyanation apparatus and a process using the same for purifying polluted water.

It is well known that hypertoxic cyanides exist in liquid waste from existing production processes of ore dressina, metallurgy, coking and electrcplating etc. and methcds universaliv ad o m t e d for removina cyanide such as ionic exchange, ozonization or direcc electrolytic oxidation are not efficient. For example, elect-,rolvtic oxidation i t.he curren-c not stable, therefore harmful gases are generated in the process and the processing cost is high. During the operation of the niobium anode cyanide processor, explosive gases such as hydrogen and chloramine as well as toxic cases such a nitrogen trichloride, cyanhydrin and chloride acid escaped may cause secondary pollution. In alkaline chlorine process# a chloric oxidant (chlorine, liquid chlorine sodium subchlorate, or bleaching powder etc) 'is added to the cyanide containing water so as to oxidize and decompose cyanides under 1 - alkaline condition. Since available chilor--ne mav deazade during the storage of chloric oxidants, it reacts chemically with cyanic ions in the process, generat-ing ha=,,.,'u-l aases such as c,,,anic acid and c,.anocen In addition, chloric oxidants are easv to leak out during the transportation, therefore, secondary pollution miaht be -caused. The chlorine/cyanogen equivalent weight is not easy to control in the process, which may produce excessive chlorine or cause the cyanogen content to exceed discharging standard. the cost is high.

Ton exchanae is usualiv used 'n the desal-'nazon o..E drinking water, and the processing of heavy meral ions and radioactive elements. It is effective in processiing liguid waste containing less than SOPPM of C,.-a.-cc,e-n an not suitable to -orocess licuid waste c--ntainina more than 20OPPM of cyanogen.

Meanwhile The object of the present invention is to provide a decyanation apparatus which can removehypertoxic cyanide from the liquid industrial waste and ameliorate the problems existed in the pricr art apparatus.

Another object of the present invention is to provide a novel process for purifying liquid industrIal waste using said apparatus in combination with the principles of electrochemical reastion so as to remove cyanide through electrolytic purification. The gases 2 - 1 exhausted from the process are repurified, Chus the problem of secondary pollution is ameliorated.

Embodiments of the present invention may employ three storage tanks containing HCl., NaOH and Nacl respectively to SUDD1v sAid materials via electromagnetic valves' and pipes to an electrolyser cell with their amount controlled by a flowmeter; the liquid waste is pumped into the electrolyser cell by a water collecting pump; the pE value in the electrolvser cell is controlled and displayed on a controlling board v i a a pH sensor and an ox idation- reduction potentiometer (ORP) provided within the electrolyser; a fan outside the electrolyser cell blows air into the electrolyser cell for stirring which I's further promoted by a three-dimensional eddy current generated by spray nozzles installed vertically and horizontally in the electrolyser cell; a set of electrode plates provided within of the cyanide is completely destroyed electrolysis and the gases exhausted from trolvser are sent to a purifying tower by a stored within the tower liauid alkali s-jr-d,-.kled by nozzles bf a sprinkling unit; pass through a spr nkled with liquid alkali. (Paul ring) layer in the middle section of and are transformed into CO 2 and N 2 which exhausted. There is no secondary pollution the electrolyser cell reverse in the the elecfan; there which is the gases PN f iller the tower are then caused by gases that have been processed thus.

Some embodiments will now be described with reference to the accompanying drawings in which:

1 is an embodiment of the Clecyanation appara-cus according to the present invention; Fig. 2 is the front v ew of an embodiment of the electrolyser; Fig.3 is the front view of an embodiment of the eleCtrode plate set; and Fig. 4 is a side view thereof; Fig.5 is the con-Lour of an electrode plate of the electrode plate sets in Fig.3; Fiz.6 is the sectional view of the wiring flange for the electrode plate set; F-;-a.7 is the sec-Licnal view of the supporting c.' the electrode plate set; -7-- 'z.8 is the front view of the purifying tower; is the scher,at: d-'acram of the ulDiDer and lower L supporting plates; 1-0 is the --7--onu view of an embodiment of the gas separating member of the purifying tower; Fig.11 is the top view of the water-gas separating member of the purifying tower.

Fig.1 shows an embodiment of the aDDaratus according to the present invention.

A pH acidimeter (14) and -an oxidation-reduction potentiometer (ORP) (15) are provided on a control decvanati--n board (13). A set of 6 keys on the right side of the board functions as follows: key (1) connected to the inlet pump of the electrolyser (93) controls the opera tion of the pump; kev (2) is connected to an electromagnetic valve (16) and a flowmeter of a hydrochloric acid tank (18); key (3) is connected to an electromagnetic valve (24) and a flowmeter of a alkali tank (19); kev (4) is connected to a electromagnetic valve (21) and a flowmeter of salt tank (20); the three keys (2, 3, 4), according to the display of pH acidimeter (14), control the addition of materials in the acide, alkali. and salt tanks (18, 19, 20) into the cell of the electrolyser (93) so as to match the pH value with a given valve; key (5) is connected to a electromacnetic valve (79) and a flowmeter (95) for ccntrciling the flow of licuid alkali into pur.' fying tower; key (8) connec.L-ed to a fan of the electrolyser (93) controls the fan to blow air into the electrolyser Another set o--7 6 ke,,s c_n the lower nar-of the control board is provided, among them, key (10) connected to the discharge electromagnetic valve (83) on a flange below the electrolyser (93), controls the discharge of water; silicon rectifying switch (9) controls the thyristor (84) having a phase-inverter, the output terminal of which is connected to the positive and negative electrode wiring board (33, 35) of the electrolyser (93) to reverse the polarity of the electrodes at regular intervals so as to speed up - 5 k electrolysis; key (6) is connected to a fan (77) in the passage between electrolyser (93) and the purifying tower, for controlling the fan to blow waste gases exhaus-Led from the electrolyser (93) to the purifying to-.,;er (94) and maintain a continued purifying action; key (11) is connected to a fan (82) for agent tank (80) to control the fan to give a stir in the agent tank (80); key (7) is connected to a alkali... pumip (78) of the purifying tower (9,4) and controls the pump to pump licruid alkali - from the purifying tower (94) into a sprinkling pipe (72) on the upper section of the:,-u--i--:vina tower, the 1-4auid alkali. - is then sprinkled a number of nozzles, generating a purify-ing action on the waste gases filtered by a filtering layer; ke,,( -12) is connected to a agent pumn (81) for tank (80) to control t-he pump to pumip off ac-4d, alkali and salt 4n tank (80).

In Fig. 2 the electrolyser of the apparatus =czor--; --:-.a ±o -he 'oresent -4n-jent-4cn cc-nzr-4ses an electrolyser cover (27), a water-gas separating member (28), a cylindrical body (27) and a set of electrode plates etc. Electrolyser cover (27) is a pot-shaped covering member with its central top stretched upward.Lorming a flange which is connected with another flange (26) by bolts (49) or other means. Flange (26) is connected with an exhaust pipe (25). A connection socket for a pH sensor and a connection socket for a ORP sensor are mounted in opposition to each other on cover (27).

6 - On the cylindrical body (37), a blower (30) J.S Drovided between the body and an air inlet pipe (29) to the body, and connected with a plurality o f hcrizontal and vertical blowing nozzles (31, the body. A positive wiring means (33) and a neaat4A--.e wiring means (35) arefixed on a flange (34) by means of screws or the like. Flange (34) is connected fixedly with a flange projected outward from the body and having an opening through which a set defining an anode and a cathode lower part of the body defining of electrode plates is inserted into the a electrolyser cell.

The electrode clates are connected fixedly to eac.,i c-'.'-er by means of bolts (44), nuts (45) and washers (52) all painted with a layer of anticorrosive PTFE. T 1, -L e electrode plate set is supported by a frame connected integrally with the body and projected in ward in t-he body. The body has a contour at its bottom similar to cover (271). i.e. a pct-shaped ccntour with its central --ar-: downwardly a flance --cnnectee-- with a flange (40) by bolts (39). A liquid waste inlet pipe (38) and a discharge pipe (41) are connected to extend through flange (40) in opposition to each other.

A pH acidimeter (42) and an ORP sensor (43) are disposed w i 4C. h in the body above the blowing -;-,-at4ng-ball fluviograph is installed on nozzles. A the body to ccntrol the liquid level in the body.

Fig.3 is the front view of the electrode plates set. The positive plates (56) (anode) and the negative plates (57) (cathode) are eaual in number and arranaed positive alternating with neaative with spacings between each pair of plates being preferably Smm and filled L vith insulation blocks (53). Each e'ectrcde plaze 11-Aas L.7o holes provided at i two ends respectively; within each hole a insutating ring (54) is placed. Each insulation block (53) also has a central hole so that the connecting bolts (44) can be inserted through which and the hole at the end of the plate to fix plates (56, 57) onto a supporting frame (55) and wiring flanges (34). In order to avoid corrosion, bolts (44), nuts (45) and washers (52) are preferably made of stain-Less steel or other sort of steel painted with PTFE.

Fig. 4 is the 'Lateral view of the e-".e--zrode plate set. Two rectangular copper plates (one ds positive -;znd the other as negative) are snaced aDart -11 c.r,posit--:on to each ozher. On each of which openin=s ecual in number to the positive or necative nlates 'are manner that positive electrode plates (56) are inserred fixedly into the openings on positive copper plate and negative electrode plates into negative copper plate. The electrode plates are cut to the shape shown in Fig. 5 so that the possibility of coming into contact with two L. comper clates by a electrode plate is eliminated. With the action of a phase-inverter, the pol&rities of the electrode plates reverse in regualar intervals, thus the inactivation of electrodes is effectively i 1 i 1 avoided while the bath voltage is stabized.

Fig. 6 is the sectional view of the wiring. flange (34). Flange (34) is shaped into a rectangular sleeve so that the electrode Dlate set can 11-e placed Cherei.n. A pair round holes perpendicular to the rectangular opening of the sleeve 'and in opposition to each other on the sleeve are also provided on the f lance to f ix the electrode plates (56, 57) with insulation blocks therebetween onto the flange.

Fig. 7 is the sectional view of the supporting frame (55). Supporting frame (55) is also in the form of a rectangular sleeve into which the elect-rode plate set is placed. At its upper and lower ends, holes are provided perpendicular to the rectangular opening of the sleeve so as to su--:,-c.-tedly fix the electrode clates;-i:--L.h insulaticn blocks there between into the Now referzina to Fia-11L. A water-aas seDarat-.na member (28) is provided between the cover (27) and the ,.j h = 'i n. m a,' forated plastic plates sandwiched therebetween a thin layer of fiber material for water-gas separation. A plurality of ventilating holes are provided on the side wall of the member, thus the density of waste gases may be diluted during air exhaust and the possible explosion ignized by high density hydrogen and back--.,l'ow caused by the negative pressure owing to the insufficient amont of air from blower are prevented.

Fig. 8 is the front view of the waste gas purifying tower (94). Purifying tower (94) is a cvlindrical badv constucted of three sections. The upper section ILs connected at is top with a taper cover forming a wateraas se-cara-,.--.,.a member (73) of the purif-,,-4na tc...jer; a flanae formed thereon is connected with a flance (74) by bolts (59); through the central portion of flange (74) an exhaust pipe (58) for exhausting purified rontoxic gases CO 2 and N 2 is hermetically inserted into the body. A liguid alkaline sprinkling pipe (72) with 4-5 sprinkling nozzles (71) is provided within the upper section of the body by extending the same he=.er.-Jcally therethroua.h., so that a sprinklina cha-mber (70) is formed. The middle section of the cvlindr--' cal bcdv is filled with polyhedra! PN particles (Paul Rinc) (068) forming a aas--1- 4auid reaction chember (69) for c.-.ana---.a -iauid zhase of the waste gases and h,,dr----.,.,z-ng A -_ --x-d-4z4na the waste gases. A I'lanae----2ke =or::us lower supporting plate (75) is provided beb,7een the supporting plate (76) is installed between the middle section. The lower section of the cylindrical body is divided into two parts; the upper part has a flange projected outwardly thereform which is connected with a waste gas inlet pine (64) havina a flanae at its end; and the lower part forms a liquid alkali s"-o--=-ae tank (63) which also has a flange projected outwardly therefrom and connected a pipe (66) to a liguid alkali pump, the bottom of the liquid alkali- storage tank p 1 (63) is taper shaped to increase the stability and capacity of the tower.

Fig. 9 is the schematic diagram of the upper and lower supporting plates. The upper and lower supvcrz--nc Plates (76, 75) are in the form of disc with a of apertures for filtration.

The purifying process of the present invention is now described.

Firstly, the liadid waste in a precipitating pool is pumped into the cell of electrolyser (93); Hcl (30%), NaOH (16%) and Nacl are piped from three storage tanks -he electrc,,iser call via elect--omacnetic valves into 4. in which the pH value of the liquid waste is adjuszed t, 10.5 according to the display of acidimeter (1112 the cell. The adontion --v addina acid or alkal-L into 4.

of nH acidimeter fcr controlling and adjusting waste in the electrolvser -:rz:ess, increases c=rent operation cost. Next, Nacl electrolyser cell under the automatically mcl-.itcr-.;nw, -Ihe PH valve of t.--- licuid cell shortens the reac.:-4--n -=nd of 1809/1 is added into the control of a electromagnetic valve, a flowmeter and a timer in a manner that the Nacl/liquid waste is controlled to 0. 15g/1 -3g/1 according to the concentration of the liquid waste.

Then air is blown into the electrolyser cell by blower(30) to give a stir to the liquid therein. The introduction of air-stir in the electrolyser cell prevents the precipitation of metal. cyanide and the - 1 1 - 1 formation of flacculus substance which mav adsorb cyanic icns, hence reduces concentration difference so that che undissolved chlorine accelerates the decomr)osit--on cf W. Liquid NaCH i-s nimed from the alkali tank via electromagnetic valve (24) into the storage tank wit h i n the purifuing tower aild then sprinkled onto the waste gases to be purified. The waste gas then passes through a layer of PN fillers. The main reactions in the purifying tower are.

NaOH+HW-yNaCN+H 2 0 W - +Cl 2 +201f-WO_+2Cl+H 2 0 2CHO +3CLO +H 2 0-1 CO, 2 tN 2 i-ON +3C1 WCL+2Na0H -NaCNO+NaCT+H 2 0 2NaCNO+3NaCLO+H 2 0 -2C0 2 t+N 2 f+2NaOH+3NaCl.

The liau-4d wasze containina cyanide is waste cas produced in the r)ur4.P',.-4na comDleteiv, and elec-zr---!vtic-=llv oxidized. e-lectz---lvsis is decomDosed tower, where the W bond The reaczions on anode The and purifried is destroved cu-- = in the electrolyser cell are CN-+20H--2e-.PCNO-+H 2 0 2 f-"H(:+ 4 OH- - 6 e - 2 CO 2 t.,-N 2 f+2H 2 0 40H -4e-P2H 2 0+0 2t The reaction on cathcde is 2H++2e---o H 2 f with hea.,.,-y metals are reduced and separated out; the secondary reaction is CNO-+2H 2 0-NH 4 +CO 3 2In order to accelerate the oxidization and decompo- 1 -) 1 . 1, 1 t sition of cyanide in the electrolyser cell, NaCI is added. The reaction on the anode is C17-e---P(Cl) seccndary reactj4.cns are 2 Cf- 2 e..0 Cl 2 20H +CL 2 -OC1 +Cl +H 2 0 CN +OCL +2H 2 0---PCNCL+20H CNC1+20H--.CNO +CiC+H 2 0 2CNO-+30Cl+H 2 0--.,'P2C0 2 I+N 2 t+3CL_+H 2 0 and H0ClHCl+(0) To overcome the problems existed in existing -crccess of decvanide by electzolysis, i.e. -4nstab-J-1itk--, of warrent efficiency, generation of harmful gases and high processing cost, experiments have been made by t h e inventor on the condition of elect--olyt4.--al d z a - ion, current efficiencv and the correlat-en L the related quant-;'_ties in the prccess. The Prere-uisite for a solution to the existing prQblems -cc.-nd 3 te the materal Hence, -asis of the research on titanium electrode by Dt-Nora (Italy), Damond (USA), ICI (UK), a new electrode DSA5 is developed, with which high current density electrolysis can be realized and the separating-cut of nascent oxygen and nascent chlorine is promoted.

the current efficiencv is increased.

Fur-her.more, The apparatus according to the present invention is provided with insoluble electrodes of same material in a manner that the positive and negative electrodes are equal in number and arranged in an asse.mbly with small spacinas therebetween. The electrodes used- in the apparatus according to the present invention are c--zzable of resisting high-density curren-c and cn- =n=--.-,pclarities automatically. Due to the salt adhered on the negative plates baused by the products on the electrodes, the deposite of calcium and magnesium ions on negative electrode, the concentration of products and electrolvte in the solution, the electrodes exhibit different overpotentials for different CL and H hence the bath resistance in the electrolyser cell is _reased resulting in boostina of bath voltace and reduct4on of current e f 10 1 c -4 e n cv. The adoi)t-on O.L effectively prevents the inactivation the phase-reversion e-l--czrc--;es, increases conducz-4vitv, reduces h -c',;= c e drcn, s-.a"---4-1-izes the ba--. -L-_ -MF n.; ma4Lnta-Lns a _Low ever.Loten-c-;a.1 fcr CL (a!c,.,; E JEcr senaratinc out chnorine). Tn the process, no sl L udae -ceccndar., accordingly.

The adoption of high density current 55A/dm 2 small spacing between electrodes and phase-reversion at regular intervals (every 3-8 minutes) facilitates decomDosition of cyanide at both positive and negative elect-rodes; while t h e addition of sodium chloride (0.5-3g/L) results in the generation of sodium subchlorate with remaining cyanogen being oxidized by chlorine at a certain pH value (10.5).

1 During the electrolysis, destroved and the c=plexing cyanide complexions is destroyed c1 'de L. 1 T, an - wl- ch 'hen CN is continually equillibrium' of metal forming indcluble metal an_ at the sarre t--.-.e.=c--...ing fIccculus substance which adsorbs a few cyanacen ions. The formation 'of insoluble metal cyonide and flocculus substance prevents ox-.dization of cyanogen by ava-i'.-n'-^,le chlorine. To solve this proble air-stir_ring is inz:rcdued, whic,"i' results in the reduction of dif---=enr.ial ccncentration and accelerates decomposi-l-- 'cn c 2N in combination with undi-eselved chlorme. Comparing mechanic stirring, ai-s-L-4----ing is advanrageous the dissociation of c,,--=.--4de. In the elect=olysar Cell, sodium subchlo--rate and chlorine are generated because of the addition ?JaCl Lhereinto. The NaCN-NaC 10 NaOC:-.7aC! NaCN+Cl 2 t-2NaOH "NaCCN+"4NaC!-H 2 0 It has been found through experiments that the reaction finishes instantaneously when pH>12 and the critical pH value is 10.5, but the primary product is hypertoxic cyanogen chloride no matter how much the pH value is NaCN+Na=+H 2 O-C-CDJC1 2 +2NaOH For pH.4.10.5, the following hydrolytic process occurs:

WC1+2NaOH-ONaCI+Na=+H 2 0 while NaOCN (cyanate) is completely oxidized into nitrogen, i.e.

2NaWN+3C1 2 +6NaOH---.>2NaHCO 3 +N 2 f+6NaCl+2H 2 0 - where the critIcalPH is just the same as that in the process of cyanide transforming into cyanate, i.e. 10.5.

The oxide content in the 1Drocess of reaction varies a:E=-..,r mill-arams per liter up --j -Lhousan,-4-- 1-2lar-;%ms per liter with indjst=--es. There.Ecra, a oxidizing reducing potentiometer (ORP) is employed to automatically monitor the oxide content so that electro-lv-!--is can be carried ouz without testing the oxygen cc-itenz in thd liquid w=--=ze separately. ORP is alsc used to monitor the equivalent potential of cl-jrine cyancien. As the rea,--'-4ng of ORP reaches 350mv --arminal potential) the 1-,:-:----4d waste that has been --urijlied is discharzed. Th--'s sianificantly simr-lifies tl,-Le procedure of lab testing of the content of chl--r-4ne and cvancaen -=nd the fee,74na metering of and c,,-anocen and meT:er--'.--- -- : -the reacenzs.

The apparatus according to t-he p-resenr inven----f--n is canable of treStina 0.1-1000 tons of liau--'dwaste a _ a=v th-e ---2 n c e-- t_he -,;;-= =er t being 1-8000mg/L 16 1

Claims

CLAIMS:

1. A process for removing cyanide from liquid waste bv electrolysis comprising the steps cf:

add-.'ng the liquid waste in an e--i-,ztrolyser cell; addi.-g HCl o úf 30% and NaW of. 16% in said electrolyser cell suctC that the pH value of the liquid waste in the cell is automatically maintained at 10.5; NaCl of a concent--ation 180a/L in said elect:.:: --.,.-.=er cell in a manner -:,hat Nacl/licruid waste is -:ntrolled t:) 0.5-3g/L; introducing air into said electrolyser cell for irrina; appl.ving a low tens--'cn high density current cL 2) (55A/ m to a set of pos--,----ve and negative electrode zi-ptes definina an anode and a cathode respect--.-4.-elv salc elect-7-2-'vc-er ce-11, on the anode:

WN0- +4(0H)- -6e 2C02,1-+N2 l+2H20 4(0H)_ -4e---r2H 2 0+0 2 t on the cathode: separating out heavy metals and releasing hydrogen; waste gases CHN, CHCL produced being conveyed firstlv into a purifying tower, where NaOH+HW -NaCN+H..O CN- +CL2+20H-A> W0-+2Cl_+H20 2WO-+3C1 0_+H2 0 --!;p C02'+N2.+OH-+3Clthen through a layer of PN fillers becoming CO 2 and N 2 and being exhausted.

2. A process according to. claim 1, wherein said positive and negative electrode plates are controlled to reverse polarities.

3. A process according to claim 2., wherein the of the said electrode' plates reverse. polarities every 3-8 minutes.

4. A process according to any preceding claim wherein the secondary reaction in the electrolyser cell is:

2- -NO +2H 2 O-INH 4 +CO 3

5. A decyanation aDmaratus comprising an electrolyser and a gas purifier, wherein said electrolyser comprises:

covering means (27) having a pH sensor connection means (50) and an ORP sensor connection means (51) thereon and connected with an exhaust pipe; gas-water separating member (28); cylindrical body (37) connected with an air inlet pipe via a blowing me- ans; a set of positive and negative electrode plates defining an anode and a cathode provided within the lower part of said body defining an electrolyser cell and connected therethrough with a pair electrode wiring means; a pH sensor means (42) connected heremetically with said pH sensor connection means (50):

an OR sensor means connected hermetically with said ORP sensor connection means; 18 -1 -1 a water inlet provided on said body; an outlet provided an saidbody; a chemical aditive inlet provided on said body a'ove said water inlet and sa.-'-d oucle:; and wherein said gas prui-fier c,-,-,iprises: a purifying tower (94) constructed by a upper, a middle and a lower section a =-ns- water separting membe:: (73) fixedly covering said 7o-ger =n exhaust pipe (58) connected with said member; a liquid alkaline sprinkl--ng pipe (72) hermetically ---end4ng into said tower t--=ouc.i said upper L -orTning a sprinkling chamber therein; a porous upper suppirting plate (76) prOvided between said unrer section and said middle sec-,.4cn; a zorcus lower s.L:zzor-.,-4g p-i-%te (75)!--e- tween said middle secz--'on:

a reaction cham.ber filled uj with -oo",,,-hedral PN fillers in said middle section between said plates (75, 76); a liquid alkaline storage tank formed within said lower section and connected with a liquid alkaline pump via a pipe (66); a gas inlet pipe (64) c--nnected with said lower section above said storage tank.

6. A decyanation apparatus according to claim 5 wherein said positive electrodes (56) and negative electrodes (57) are equal in number and arranged positive alternating with negative and spaced by insulating material (53).

7. A dec-vanation apparatus according to claim or 6, wherein said electrode plates are -inso-'.u"c-le, highly oxidisable- and reversible DSA electrode plates of same material.

8. A decyanation apparatus according to claim 5, 6 or 7 wherein said positive and negative electrodes are controlled to reverse polarities.

9. A decyanation apparatus according to claim 6--.7 or 8 wherein the spacing between a said positive electrode and a said negative electrode is 5mm.

10. A decyanation apparatus according to claim 8 wherein said electrodes reverse their nolarities every 3-8 minutes.

11. A deevanation anparatus acco-rding to any of clainis 5-10..;herein a cH meter is provided to monitor cH %-alue -Ln sa4Ld elect--olyser cell.

1 - 'Co any of claims 2. L- A decyanation apparatus accoraing 5-11 corrprising amORP for monitoring whether the equivalent potential of chlorine-cyanogen in said electrolyser reaches terminal value (350mv).

13. A process for removing cyanide from a liquid waste by electrolysis comprising the steps of:

introducing the liquid waste into an electrolyser cell; supplying HCl and NaOH to the electrolyser cell to maintain the pH value of the liquid waste therein at about 10.5; adding NaCl to the electrolyser cell to maintain the r 1 atio of NaCl/liquid waste in the range 0.5-3g/,; stirring the liquid waste..

passing a current between a set of electrode plates located within the electrolyser cell; and passing gases produced in the electrolyser cell Into a purifying tower.

14. A process according to claim 13 wherein the liquid waste is stirred by introducing air into the electrolyser cell;

15. A process according to claim 13 or 14 wh erein the cell employs a high current density, at least 30A/dm2 and preferably about 55A/dm2.

16. A process according to claim 13, 14, or 15 wherein the purifying tower is adapted to remove HW and CNCZ.

17. A process according to any of claims 13-16 wherein, In the cell, W- Is oxidised at the anode to C02 and N2; and at the cathode, heavy metals (if present) are 21 removed from the liquid waste, and hydrogen is released.

18. A process for removing cyanide from liquid waste substantially as herein described with reference to the accompanying drawings.

19. A decyanation apparatus comprising:

first tank mean for containing acid; second tank means for containing alkali; third tank means for containing salt; gas source for supplying pressurised gas; an electrolyser cell connected to a waste liquid source; pipe and valve means for connecting the first, second and third tank means and the gas source to the electrolyser cell; a pH sensor means, an OR sensor means and a set of electrode plates arranged inside the electrolyser cell; a gas purifier connected to the electrolyser cell by an exhaust pipe; and control means connected to the valve means, pH sensor means, and OR sensor means; whereby the pH value of the waste liquid in the electolyser cell is sensed and controlled by selectively adding the acid and/or the alkali.

20. Apparatus according to claim 19 adapted to perform a process according to any of claims 1-4 or 1318.

22 1

21. A decyanation apparatus stantially as herein described with reference to and as Illustrated in the accompanying drawings.

1 'i 23 Published 1991 at The Patent O1rice. Concept House. Cardifr Road. Newport. Gwent NP9 I RH. FurLhercopies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwnifelinfach. Cross Keys. Newport. NP) 7HZ. Printed by Muluplex techniques ltd. St Mar - v Cray. Kent.

..... - -Y menrisques sic. bi mary cray. Kent.