CA2031631A1

CA2031631A1 - Decyanation apparatus and process for purifying water from cyanide using the same

Info

Publication number: CA2031631A1
Application number: CA002031631A
Authority: CA
Inventors: Xinyu Yang
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-03-15
Filing date: 1990-12-06
Publication date: 1991-09-16
Also published as: JP2603760B2; ITMI910032A1; CN2063121U; IT1244362B; ITMI910032A0; CN1057245A; GB9101483D0; GB2241960A; AU7126491A; AU650252B2; JPH04222690A; GB2241960B; DE4040766A1; FR2659642B1; FR2659642A1

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a decyanation apparatus which can remove hypertoxic cyanide from the liquid industrial waste and therefore solve the pro-blems existed in the prior art apparatus. The present invention also provides a novel process for purifying liquid industrial waste using the apparatus in combi-nation with the principles of electrochemical reaction so as to remove cyanide through electrolytic purifi-cation. The gases exhausted from the process are re-purified, thus the problem of secondary pollution is solved.

Description

DECYANATION APPARATVS AND
PROCESS FOR PURIFYING WATER
FROM CYANIDE USING THE SAME

Technical Field The present invention relates to the purification of water, particularly to an apparatus for removing cyanide from liquid waste - a decyanation apparatus ; and a process using the same for purifying polluted -`- water.

, .i Background of the Invention , .
It is well known that hypertoxic cyanides exist in liquid waste from existing production processes of ore dressing, metallurgy, coking and electroplating etc. and methods universally adopted for removing cyanide such as ionic exchange, ozonization or direct electrolytic oxidation are not efficient. For example, in electrolytic oxidiation the current efficiency is 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 gases such a nitrogen trichloride, cyanhydrin and chloride acid escaped may cause secondary pollution.
In alkaline chlorine process, a chloric oxidant (chlorine, liquid chorihe sodium subchlorate, or bleaching power etc.) is added to the cyanide containing water so as to oxidize and decompose cyanides under 2os~l6~l alkaline condition. Since availibe chlorine may degrade during the storage of chloric oxidants, it reacts chemically with cyanic ions in the process, generating harmful gases such as cyanic acid and cyanogen chloride.
In addition, chloric oxidants are easy to leak out during the transportation, therefore, secondary pollution might be caused. The chlorinecyanogen equivalent wright is not easy to control in the process, which may produce excessive chlorine or cause the cyanogen content to exceed discharge standard. Meanwhile the cost is high.
Ionic exchange is usually used in the desalination of drinking water, and the processing of heavy metal ions and radioactive elements. It is effective in pro-cessing liquid waste containing less than 50PPM of cyanogen and not suitable to process liquid waste con-taining more than 200PPM of cyanogen.

Summary of the Invention The object of the present invention is to provide a decyanation apparatus which can remove hypertoxic cyanide from the liquid industrial waste and therefore solve the problems existed in the prior art apparatus.
Another object of the present invention is to provide a novel process for purifying liauid industrial waste using said apparatus in combination with the prin-ciples of electrochemical reaction so as to remove cyanide through electrolytic purification. The gases exhausted from the process are repurified, thus the problem of secondary pollution is solved.
The objects of the present invention are realized in a manner that three storage tanks containing HCl, NaOH and Nacl respectively supply said materials via electromagnetic valves and pipes to an electrolyser cell with their amount controlled by a flowmeter; the liauid waste is pemped into the cell of an electrolyser from a water collecting sump; the pH value in the electrolyser cell is controlled and displayed on a '~e.
controlling board via a pH sensor and an oxidation-reduction potentiometer (ORP) provided within the electrolyser; a fan outside the electrolyser blows air into the electrolyser cell for stirring which is further promoted by a three-dimentional eddy current generated by spray nozzles installed vertically and horizontally in the electrolyser; a set of electrode plates provided within the electrolyser cell reverse polarities under the control of a inverter; the CN bond of the cyanide is completely destroyed in the 7:, electrolysis and the gases exhausted from the elec-trolyser are sent to a purifying tower by a fan; there stored within the tower liquid alkali .. which is sprinkled by nozzles of a sprinkling unit; the gases prinkled with liquid alkali pass through a PN filler (Paul ring) layer in the middle section of the tower and are transformed into CO2 and N2 which are then exhausted. There is ~no secondary pollution casused by 2~3163~

gases that have been processed thus and so.

Brief Description of Drawing Fig. 1 is an embodiment of the decyanation apparatus according to the present invention;
Fig. 2 is the front view of an embodiment of the elec-trolyser;
Fig.3 is the front view of an embodiment of the electrode plate set;
Fig.4 is the contour of an electrode plate of the elec-trode plate sets in Fig.3;
Fig.6 is the sectional view of the wiring flange for the electrode plate set;
Fig.7 is the sectional view of the supporting frame of the electrode plate set;
Fig.8 is the front view of the purifying tower;
Fig.9 is the schematic diagram of the upper and lower supporting plates;
Fig.10 is the front view of an embodiment of the water-gas separating member of the purifying tower;
Fig.11 is the top view of the water-gas separating . ~
'~i member of the purifying tower;

Detailed Description of the Preferred Embodiment Fig.l shows an embodiment of the decyanation apparatus according to the present invention.
A pH acidimeter (14) and an oixdation-reduction potentiometer (ORP~ (15) are provided on an control board (13). A set of 6 keys on the right side of the board functions as follows: key (l) connected to the inlet pump of the electrolyser (93) controls the opera-tion of the pump; key (2) is connected to an electromagnetic valve (16) and a flowmeter of a hydrochlorlc acid tank (18); key (3) is connected to an electromagnetic valve (24) and a flowmeter of a alkali tank (19); key (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 t14), control the addition of materials in the acide, alkali: and salt tanks (18, l9, 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 electromagnetic valve (79) and a flowmeter (95) for controlling the flow of liguid alkali. into purifying tower; key (8) connected to a fan of the electrolyser (93) controls the fan to blow air into the electrolyser for stirring. Another set of 6 keys on the lower part 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 rectrifying 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 electrolysis; kev (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 exhausted from the electrolyser (93) to the purifying tower (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~ pump (78) of the purifying tower (94) and controls the pump to pump liquid alkali from the purifying tower (94) into a sprinkling pipe (72) on the upper section of the purifying tower, the liquid alkali. is then sprinkled via a number of nozzles, generating a purifying action on the waste gases filtered by a filtering layer; key (12) is connected to a agent pump (81) for tank (80) to control the pump to pump off acid, alkali. and salt .~ in tank (80).
In Fig. 2 the electrolyser of the apparatus according to the present invention comprises 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 forming 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).

203~631 On the cylindrical body (37), a blower (30) is provided between the body and an air inlet pipe (29) to the body, and connected with a plurality of horizontal and vertical blowing nozzles (31, 32) inside the body. A positive wiring means (33) and a negative wiring means (35) are fixed 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 of electrode plates defining an anode and a cathode is inserted into the lower part of the body defining a electrolyser cell.
The electrode plates are connected fixedly to each other by means of bolts (44), nuts (45) and washers (52) all painted with a layer of anticorrosive PTFE. The electrode plate set is supported by a frame connected integrally with the body and projected~ in ward in the body. The body has a contour at its bottom similar to cover (27). i.e. a pot-shaped contour with its central part projected downwardly forming a flange connected 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 within the body above the blowing nozzles. A
floating-ball fluviograph is installed on the body to control 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 20~163.~

plates (57) (cathode) are e~ual in number and arranged positive alternating with negative with spacings between each pair of plates being preferably 5mm and filled with insulation blocks (53). Each e]ectrode plate has two 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 stainless steel or other sort of steel painted with PTFE.
Fig. 4 is the lateral view of the electrode plate set. Two rectangular copper plates (one as positive and the other as negative) are spaced apart in opposition to each other. On each of which openings equal in number to the positive or negative plates are provided to receive positive or negative plates in a manner that positive electrode plates (56) are inserted 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.
S so that the possibility of coming into contact with two copper plates by a electrode plate is eliminated.
With the action of a phase-inverter, the polarities of the electrode plates reverse in regualar intervals, thus the inactivation of electrodes is effectively 2031~3~

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 plate set can be placed therein.
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 flange to fix the electrode plates (56, 57) with insulation blocks therebetween onto the flange.
Fig. 7 is the sectional view o~ the supporting frame (55). Supporting frame (55) is also in the form of a rectangular sleeve into which the electrode plate set is placed. At its upper and lower ends, holes are provided perpendicular to the rectangular opening of the sleeve so as to supportedly fix the electrode plates with insulation blocks there between into the frame.
~~ Now referring to Fig.ll. A water-gas separating member (28) is provided between the cover (27) and the body of the electrolyser, which is made up of two per-forated plastic plates sandwiched therebetween a th,n 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 ignited by high density hydrogen and back-flow 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 2031~31 tower (94). Purifyiny tower (94) is a cylindrical body constucted of three sections. The upper section is con-nected at is top with a taper cover forming a water-gas separating member (73) of the purifying tower; a flange formed thereon is connected with a flange (74) by bolts (59); through the central portion of flange (74) an exhaust pipe (58) for exhausting purified ron-toxic gases C~2 and N2 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 hermetically therethrough, so that a sprinkling chamber (70) is formed. The middle section of the cylindrical body is filled wlth polyhedral PN particles (Paul Ring) (68) forming a gas-liquid reaction chember (69) for changing the liquid phase of the waste gases and hydrolyzing and oxidizing the waste gases. A flange-like porous lower supporting plate (75) is provided between the lower saction and the middle section, a similar upper 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 pipe (64) having a flange at its end;
and the lower part forms a liquid alkali storage tank (65) 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 203163~

(65) 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 supporting plates (76, 75) a-e in the form of disc with a plurality of apertures for filtration.
The purifying process of the present invention is now described.
Firstly, the liquid 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 via electromagnetic valves into the electrolyser cell in which the pH value of the liquid waste is adjusted to 10.5 according to the display of acidimeter (1~) by adding acid or alkali into the cell. The adoption of pH acidimeter for automatically monitoring, controlling and adjusting the pH valve of the liquid waste in the electrolyser cell shortens the reaction process, increases current efficiency and reduces the operation cost. Next, Nacl of 180g/1 is added into the electrolyser cell under 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 liguid waste.
Then air is blown into the electrolyser cell by blower(30) to give a stir to t~le liquid therein. The introduction of air-stir in the electrolyser cell prevents the precipitation of metal cyanide and the formation of flocculus substance which may adsorb cyanic ions, hence reduces concentration difference so that the undissolved chlorine accelerates the decomposition of CN. Liquid NaOH is piped from the alkali tank via electromagnetic valve (24) into the storage tank within the purifuing tower and 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+HCN ~NaCN+H20 CN +C12+20H~ CNO +2Cl+H20 2CHO +3CLO +H20 Co2t+N2t+oN +3Cl CNCL+2NaOH ~NaCNO+NaCL+H20 2NaCNO+3NaCLO+H20 ~2C02t+N2~+2NaOH+3NaCl.
; The li~uid waste containing cyanide is electrolytically oxidized. The waste gas produced from electrolysis is decomposed and purified in the purifying tower, where the CN bond is destroyed completely, and the metal ions are separated out on the negative plates.
The reactions on anode in the electrolyser cell are ~-~ CN +20H -2e ~ CNO +H20 2CHO+40H -6e ~ 2Co2t+N2~+2H2o . _ The reaction on cathode is 2H +2e ~ H2~ with heavy ` metals are reduced and separated out; the secondary reaction is CNO +2H20 NH4 +C03 In order to accelerate the oxidlzation and decompo-2~31g31 sition of cyanide in the electrolyser cell, NaCl is added. The reaction on the anode is CL-e ~(C1) and secondary reactions are 2Cl-2e ~C12 20H +CL2- OCl +Cl +H20 CN +OCL +2H20--~CNCL+20H
CNCl+20H ~CNO +Cl+H20 2CNO +30Cl+H20 ~2Co2~+N2t+3CL +H20 and HOCl~ HCl+(O) ~' To overcome the problems existed in existing process of decyanide by electrolysis, i.e. instability of current efficiency, generation of harmful gases and `- high processing cost, experiments have been made by :~ the inventor on the condltion of electrolytical oxidization, current efficiency and the correlation 3 among the related quantities in the process. The prerequisite for a solution to the existing problems is found to be the materal of electrode. Hence, on the basis 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-out of nascent oxygen and nascent chlorine is promoted.
Furthermore, the current efficiency is increased.
The apparatus according to the present invention is provided with insoluble electrodes of same material in a manner that the posi~tive and negative electrodes 203~631 are equal in number and arranged in an assembly with small spacings therebetween. The electrodes used in the apparatus according to the present invention are capable of resisting high-density current and changing polarities automatically. Due to the salt adhered on the negative plates caused by the products on the electrodes, the deposite of calcium and magnesium ions on negative electrode, the concentration of products and electrolyte in the solution, the electrodes exhibit different overpotentials for different CL and H , hence the bath resistance in the electrolyser cell is increased resulting in boosting of bath voltage and reduction of current efficiency. The adoption of phase-reversion effectively prevents the inactlvation of electrodes, increases the conductivity, reduces voltage drop, therefore stabilizes the bath voltage and maintains a low overpotential for CL (a low EMF
for separating out chnorine). In the process, no sludge is procuded and secondary pollution is prevented accordingly.
The adoption of high density current ( 55A/dm2), small spacing between electrodes and phase-reversion at. regular intervals (every 3-8 minutes) facilitates decomposition of cyanide at both positive and negative electrodes; while the 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).

203163~

During the electrolysis, CN is continually destroyed and the complexing e~uillibrium of metal cyanide complexions is des-troyed forming indoluble metal cyanide which then precipitates, and at the same time forming flocculus substance which adsorbs a few cyanagen ions. The formation of insoluble metal cyonide and flocculus substance prevents oxidization of cyancgen by available chlorine. To solve this proble air-stirring is introdued, which results in the reduction of differential concentration and accelerates decomposition ' of CN in combination with undisselved chlorme. Comparing with mechanic stirring, air-stirring is advantageous to the dissociation of cyanide. In the electrolyser cell, sodium subchlorate and chlorine are generated because of the addition of NaCl thereinto. The reaction is :
NaCN+NaClO ~NaOCN+NaCl NaCN+Cl2~+2NaOH ~NaOCNt2NaCl+H20 It has been found through experiments that the reaction finishes instantaneously when pH712 and the critical 1 pH value is 10.5, but the primary product is hypertoxic cyanogen chloride no matter how much the pH value is NaCN+NaClO+H20 ~CNC12+2NaOH
For pHc10.5, the following hydrolytic process occurs:
CNCl+2NaOH -NaCl+NaCNO+H20 while NaOCN (cyanate) is completely oxidized into nitrogen, i.e.

2NaOCN+3C12t6NaOH ~2NaHC03+N2~+6NaCl+2H20 2~63~

where the critical pH is just the same as that in the process of cyanide transforming into cyanate, i.e. 10.5.
The oxide content in the process of reaction varies from a few milligrams per liter up to thousands of mil-ligrams per liter with industries. Therefore, a oxidizing reducing potentiometer (ORP) is employed to automatically monitor the oxide content so that electrolysis can be carried out without testing the oxygen content in the liquid waste separately. ORP is also used to monitor the equivalent potential of chlorine cyanogen. As the reading of ORP reaches 350mv tterminal potential) the liquid waste that has been purified is discharged. This significantly simplifies the procedure of lab testing of the content of chlorine and cyanogen and the feeding metering of chlorine and cyanogen and feeding metering of the reagents.
`i The apparatus according to the present invention is capable of treating 0.1-1000 tons of liquid waste a day with the concentration of the water to be purified being 1-8000mg/L .
J

-'

Claims

CLAIMS:
l. A process for removing cyanide from liquid waste by electrolysis comprising the steps of:
adding the liquid waste in an electrolyser cell;
adding HCl of 30% and NaOH of 16% in said electrolyser cell such that the pH value of the liquid waste in the cell is automatically maintained at 10.5;
adding NaCl of a concentration 180g/L in said electrolyser cell in a manner that Nacl/liquid waste is controlled to 0.5-3g/L;
introducing air into said electrolyser cell for stirring;
applying a low tension high density current (55A/dm2) to a set of positive and negative electrode plates defining an anode and a cathode respectively in said electrolyser cell;
on the anode:
CN-+2(OH)-2e?CNO-+H2O
2CHO-+4(OH)-6e-?2CO2?+N2?+2H2O
4(OH)-4e?2H2O+O2?
on the cathode: separating out heavy metals and releasing hydrogen;
waste gases CHN, CHCL produced being conveyed firstly into a purifying tower, where NaOH+HCN?NaCN+H2O
CN-+CL2+2OH-?CNO-+2Cl?+H2O
2CNO-+3CLO-+H2O?CO2?+N2?+ON-+3Cl-then through a layer of PN fillers becoming CO2.

and N2 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 claim 1, wherein the secondary reaction in the electrdyser cell is:
CNO-+2H2O?NH4?+CO32-
5. An decyanation apparatus 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:
a gas-water separating member (28);
a cylindrical body (37) connected with an air inlet pipe via a blowing means;
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;

a water inlet provided on said body;
an outlet provided on said body;
a chemical aditive inlet provided on said body above said water inlet and said outlet;
and wherein said gas pruifier comprises:
a purifying tower (94) constructed by a upper, a middle and a lower section a gas-water separting member (73) fixedly covering said tower an exhaust pipe (58) connected with said member;
a liquid alkaline sprinkling pipe (72) hermetically extending into said tower through said upper section forming a sprinkling chamber therein;
a porous upper supporting plate (76) provided between said upper section and said middle section;
a porous lower supporting plate (75) provided be-tween said middle section:
a reaction chamber filled up with polyhedral 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) connected 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 decyanation apparatus according to claims and 6, wherein said electrode plates are insoluble, highly oxidiable and reversible DSA electrode plates of same material.
8. A decyanation apparatus according to claims 5 and 6, wherein said positive and negative electrodes are controlled to reverse polarities.
9. A decyanation apparatus according to claim 6, wherein the spacing between a said positive electrode and a said negative electrode is 5mm.
10. A decyanation apparatus according to claims and 6, wherein said electrodes reverse their polarities every 3-8 minutes.
11. A decyanation apparatus according to claim 5, wherein a pH meter is provided to monitor pH value in said electrolyser cell.
12. A decyanation apparatus according to claim 5 comprises a ORP for monitoring whether the equivalent potential of chlorine-cyanogen in said electrolyser reaches terminal value (350mv).