AU2002300290B2 - Process and apparatus for producing aqueous solution of sodium cyanide - Google Patents

Description

Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

Name of Applicant: Orica Australia Pty Ltd Actual Inventor(s): Michael James Sparrow Address for Service: DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, Victoria 3000.

Invention Title: "Process and apparatus for producing aqueous solution of sodium cyanide" Details of Associated Provisional Application(s): No(s): PR6664/01 The following statement is a full description of this invention, including the best method of performing it known to me/us: P:\OPERJccSPCIFCATONSQ255562 comple.do-26/07/02 -1- PROCESS AND APPARATUS FOR PRODUCING AQUEOUS SOLUTION OF SODIUM CYANIDE The present invention relates to a process for producing an aqueous solution of sodium cyanide at a predetermined concentration. The invention also relates to an apparatus for use in the process. Furthermore, the invention relates to uses of an aqueous solution of sodium cyanide as produced by the process of the invention.

Certain industrial processes, for example gold extraction, require the use of sodium cyanide liquors at specific concentrations. The liquor is usually prepared by dissolution of solid sodium cyanide in water and this is typically done in bulk by flushing a large tank (usually termed an "isotank") filled with solid sodium cyanide with an appropriate volume of water in order to dissolve the sodium cyanide and achieve the desired concentration of sodium cyanide in solution. For example, using an isotank containing about 22 tonnes of solid sodium cyanide (as is typical with the bulk process), to produce a sodium cyanide solution having a concentration of approximately 22% would require sparging the isotank with approximately 60,000 litres of water. The sodium cyanide solution produced is then usually transferred to a large on-site storage facility. This facility must be supplied with bunding, in case of spillage, and appropriate pumps, piping, and instrumentation in order to manage distribution and meterage of the solution. However, as such large scale production per run necessitates the use of correspondingly large scale equipment, this process is economical only for sites which have a high sodium cyanide consumption. The process is not economically viable for sites which do not use large volumes of sodium cyanide per annum.

For sites where the cyanide consumption is lower, small quantities of solid sodium cyanide, such as 1 tonne, are dissolved in water as needed. However, this kind of production run is labour intensive and very much a "hands on" procedure. This may expose personnel to hazards, such as sodium cyanide dust and hydrogen cyanide gas.

It would be desirable to provide a process for producing an aqueous solution of sodium 18-04-'08 12:45 FROM-Davies Collison Cave +61392542770 -0 0427FIl T-102 P004/007 F-116 RPERV2CcKrCFIcATOJfl)$%$ 4L SPA tP 'S04 Ct doc-1?ICJI2H 00 o -2cyanide having a predetermined concentration in variable volumes and, desirably, on an as <1 need basis. Such a process would be attractive for use on sites where the consumption of 00 sodium cyanide solution is not sufficiently high to merit the kind of' bulk production described above and would negate the need to use the kind of large scale storage and o metering equipment associated with such bulk production. This would be desirable from ci economic and practical perspectives. It would also be desirable to provide a process which o iminimises exposure of operating personnel to hazardous materials.

o Accordingly, the present invention provides an industrial process for producing an aqueous solution of sodiumn cyanide at a predetermined concentration, which process consists of: feeding water into a tank containing solid sodium cyanide thereby dissolving a portion of the sodium cyanide to produce a sodium cyanide solution; removing the sodium cyanide solution from the tank and determining the concentration of sodium cyanide in solution; and if necessary, adjusting the concentration of sodium cyanide in the solution to achieve the predetermined concentration, wherein the tank initially contains an amount of solid sodium cyanide sufficient to last at least two process runs.

Initially, the tank is loaded with a sufficient amount of solid sodium cyanide to give the desired concentration solution taking into account the volume of water being used and solubility parameters, The tank is also sized taking these faictors into account. The volume of water used is less than that required to dissolve the amount of solid sodium cyanide in the tank. To minimise handling of the solid sodium cyanide the tank is usually loaded with an excess of solid sodium cyanide so that re-filling of the tank is minimised. Tfhus it is envisaged that the amount of solid sodium cyanide put into the tank will be sufficient to last a number of process runs. The tank may be replenished with solid sodium cyanide as necessary. The tank is typically sized to contain up to 5 tonnes, for example about 2 tonnes, of solid sodium cyanide. The sodium cyanide is typically provided in the form of briquettes. The briquettes are usually pillow-shaped having a weight of about 30 grams, about 45 mm long, about 35 mm wide and about 20 num thick.

Water is fed into the tank in order to dissolve the solid sodium cyanide and produce a COMS ID No: ARCS-187353 Received by IP Australia: Time (I-Pm) 10:47 Date 2008-04-18 P:\OPER\Jc\SPECIFICATIONS\2555562 complactdo-26/07/2 -3solution. Typically, the water is pumped into the tank under pressure and sprayed onto the solid sodium cyanide through a series of nozzles provided inside the tank. The water is usually sprayed at ambient temperature. This process by which a sodium cyanide solution is produced is termed "sparging". By sparging the tank with relatively small volumes of water, such as 5,000 litre lots, significant savings are possible by reducing the size of the equipment the sparging pump) which is needed. Preferably, the solid sodium cyanide is supported on a mesh under which there is a reservoir or channel. Sodium cyanide solution collects in this reservoir/channel. Separation of the solution produced from the solid sodium cyanide allows the concentration of the solution to be controlled more accurately. If the solution produced is in contact with the solid sodium cyanide, its concentration will be influenced by the sparging time.

Typically, as soon as the chosen volume of water has been fed into the tank, the aqueous sodium solution produced is removed, typically by pumping. If the tank has a reservoir/channel the solution is simply pumped from this.

After removal the concentration of sodium cyanide in solution is determined. Typically, this is done by density measurement. Usually, the sodium cyanide solution removed from the tank is pumped into a separate tank where the solution is sampled in order to determine its concentration. Subsequent processing of the sodium cyanide solution will depend upon its concentration.

If the sodium cyanide solution sampled is at the predetermined concentration it may used as is or, alternatively, it may be pumped to a storage container.

If the concentration of the sodium cyanide solution is too dilute, i.e. less than the predetermined concentration, its concentration may be increased by using the solution instead of water to sparge the solid sodium cyanide. Following this sparge, the concentration of sodium cyanide present in the (second pass) solution is measured again and this recirculation/concentration measurement repeated until the predetermined concentration of sodium cyanide is achieved. The solution may then be used or stored.

11-04-'08 09:39 FROM'-Davies Collison Cave +61392542770 T03 P0/1 -7 T-073 P006/012 F-073 P:'OAUC'SPEC?)CA ~N 3rd JSPA hP 11 04S6o.iOM6AOOS 00 If the concentration of the sodium cyanide solution extracted from the tank is in excess of the predetermined concentration, water may be added to the solution to achieve the predetermined concentration. The volume of water needed to achieve the predetermined concentration may be calculated based upon the concentration of the sodium cyanide solution removed from the tank. It will be appreciated that following recirculation of o sodium cyanide solution as described above, the resultant concentration may be in excess en of the predetermined concentration. In that case, the solution may be diluted with water as o described.

The process of the present invention is based on concentration control and, if necessary, concentration adjustment of the sodium cyanide solution. This results in efficient and economic use of the solid sodium cyanide source and, as the volume of solution produced is relatively small, the process avoids the need to use large scale storage vessels for the sodium cyanide solution and heavy duty pumps and ancillary equipment. The result of this is that the process of the invention is available for the wider sodium cyanide user market.

The essential difference between the process of the present invention and the bulk process described above is that in the present invention, smaller quantities of cyanide solution may be produced on an as required basis thereby minimising the need to store large quantities of cyanide liquor. Also, in the present invention the total amount of solid sodium cyanide in the tank is not dissolved in one go. The process of the present invention also provides accurate control of the sodium cyanide solution concentration and this may be managed more accurately when compared with much larger volumes of solution.

The present invention also provides apparatus for carrying out the process of the invention.

The apparatus consists essentially of a holding tank and a sparging, tank. The holding tank comprises means for supplying water thereto, means for supplying the contents of the holding tank to the sparging tank and means for removing the contents of the holding tank.

The sparging tank includes means for returning liquid from the sparging tank to the holding tank. In practice, the various means described comprise pipes, pumps and valves which, in operation, are used to control the flow of liquid components into and out of the holding and COMS ID No: ARCS-186452 Received by IP Australia: Time 09:42 Date 2008-04-11 P:\OPERXJcc\SPECIFICATIONS\2555362 conplietedoo-26/07JO2 sparging tanks. Preferably, the sparging tank includes a reservoir/channel portion at the base of the tank above which is provided a mesh. The purpose of the mesh is to separate solid sodium cyanide from sodium cyanide solution which is produced. In practice of the invention the sparging tank is charged with a suitable quantity of solid ammonium cyanide, such as sodium cyanide briquettes. Typically, the amount will be in the region of 5 tonnes.

The holding tank is supplied with a volume of water with which it is intended to sparge the solid sodium cyanide. The various supply means of the apparatus are adjusted as necessary and water is then pumped from the holding tank to the sparging tank where it is used to sparge the solid sodium cyanide, for example by being pumped through a number of spray nozzles provided around the solid sodium cyanide. This causes dissolution of a quantity of the solid sodium cyanide resulting in a solution of aqueous sodium cyanide being formed. When the contents of the holding tank have been used for sparging, the various supply means are adjusted and the aqueous sodium cyanide solution is pumped from the sparging tank back to the holding tank. At this point sodium cyanide solution is sampled and its concentration is determined.

If the concentration is as required the sodium cyanide solution may be removed from the holding tank. The solution may be used immediately or transferred to a storage vessel. If the concentration of the sodium cyanide solution is in excess of the predetermined concentration, water may be introduced into the holding tank via the water supply means in order to dilute the sodium cyanide solution to the predetermined concentration. The sodium cyanide solution can then be removed from the holding tank as described. If the concentration of sodium cyanide is less than the predetermined concentration, the solution present in the holding tank is fed to the sparging tank where it is used to sparge the solid sodium cyanide present there. This results in an increase in the concentration of the sodium cyanide solution. The resulting, more concentrated sodium cyanide solution is then returned to the holding tank where its concentration is determined once more. If necessary, the concentration of the sodium cyanide solution present then in the holding tank may be diluted with water or increased further by use in the sparging tank. In this way the concentration of the sodium cyanide solution may be adjusted to the predetermined concentration, after which the solution may be removed from the holding P:\OPER\JccSPECIFICATIONS\2555562 cplacdo-2607/2 -6tank.

It will be appreciated that instead of returning sodium cyanide solution to the holding tank, the solution may be pumped to a separate further tank where sampling takes place. The solution may then be removed from this further tank for use or storage, diluted with water, or returned for use in the sparging tank, as required.

In practice, caustic soda may be added to the sparged water prior to sparging in order to minimise hydrogen cyanide egress. This may not be necessary if the process is carried out in a well vented area.

In a preferred embodiment, the nature of the invention enables the holding tank, sparging tank and associated supply means to be sized so as to be portable. Thus, the apparatus may be arranged on a portable skid, which may be transported on the back of a truck. This allows the apparatus to be moved between location on a single site or between separate sites.

Sodium cyanide solution produced in accordance with the invention may be used in a variety of industrial processes, such as in gold extraction.

In a separate embodiment, the process of the present invention may be applied to provide solutions of sodium cyanide for use in applications where an approximate concentration of sodium cyanide is required but where concentration variations are not of particular concern. Such applications include heap leach and vat leach operations or systems in which sodium cyanide is added from batch titrations.

This embodiment involves pumping water into a tank containing solid sodium cyanide, allowing the water to remain in contact with the solid sodium cyanide for a period of time sufficient to allow dissolution of a quantity of sodium cyanide to produce a sodium cyanide solution having an approximate predetermined concentration, and pumping the sodium cyanide solution out of the tank. In this embodiment, the residence time of water P:\OPERJccWSPECI'ICATIONS\2555562 conipl-cAdoc-6/07/2 -7in contact with the solid sodium cyanide will obviously have an impact on the concentration of the sodium cyanide solution that is produced. Other factors such as the rate at which water is pumped into the tank, the temperature of the water and the amount and form of solid sodium cyanide in the tank will also be influential. In practice the various factors will be manipulated to achieve the desired concentration of sodium cyanide solution. One skilled in the art would have no difficulty in doing this, for example by trial and error and by measuring the concentration of sodium cyanide solution produced for a given set of operating conditions. In practice however it is an advantage of this embodiment that the desired approximate concentration of sodium cyanide solution may be produced without the need to carry out any concentration measurement. This simplifies the process. It will be appreciated that this embodiment is not appropriate for use in situations where the concentration of sodium cyanide solution is required to be controlled to within strict tolerances.

Typical applications where this embodiment may be useful require a sodium cyanide concentration of approximately 25% by weight. In practice this may be produced by pumping water at a rate of 40-50m 3 /hr into a tank containing 22 tonnes of solid sodium cyanide. The water is at ambient temperature and is allowed to remain in contact with the solid sodium cyanide for a period of 10-20 minutes. The sodium cyanide solution is pumped out of the tank at a rate of 40-50 m 3 /hr. The ambient water temperature may obviously vary and the other process parameters may be altered as necessary to achieve the approximate concentration of sodium cyanide solution that is desired. Also, due to dissolution of the solid sodium cyanide, after a number of batch runs it may be necessary to change the operating parameters to maintain the approximate concentration of sodium cyanide solution. Again, this is something that may be factored into practice of the process. Desirably the operating conditions of the process may take the form of a program or schedule which takes into account such things as ambient water temperature and effects due to batch-wise operation.

Typically, in this embodiment, the tank is a bulk storage container used for the transportation of solid sodium cyanide. Such containers are so-called "isotainers". They P.\OPER\Jcc\SPECIFICATIONS\2555562 compdct.dmc26/07'O2 -8generally have a capacity for 22 tonnes of solid sodium cyanide.

The volume of water pumped into the tank may vary depending amongst other things on the volume of the tank and the volume of sodium cyanide solution it is desired to produce.

As the sodium cyanide solution produced is often held in a storage tank prior to use, the volume capacity of the storage tank may well be a further consideration. These things considered, the volume of water used per batch is usually from 1 to 5 m 3 Obviously, the tank is equipped with the necessary pipes, valves and pumps to facilitate operation of the process.

An embodiment of the invention is illustrated in Figures 1-5 which are schematics showing how the invention may be put into practice.

In Figure 1 the holding tank 1 has a capacity of 8m 3 The tank 1 is connected via series of pipes (of 80mm diameter) to a sparging tank 2 (not shown in detail), to the mine site or storage tank 11 and to a centrifugal pump 12. Valves 3, 4, 5, 6, 7, 8, 9 and 10 enable flow of liquid in the system to be controlled as desired.

Figure 2 illustrates addition of water to the holding tank 1 by opening of valve 3 connected to a water supply. The remaining valves are closed, and the pump 12 is turned off. The valve 3 is kept open until the holding tank contains sufficient water, for example 5, 000 litres.

Figure 3 illustrates transfer of the contents of the holding tank 1 to the sparging tank 2.

This is achieved by opening valves 4, 5, 7 and 8. The rest of the valves are closed. The pump 12 is turned on. Sparging of solid sodium cyanide in the sparging tank then takes place.

Figure 4 illustrates return of the sodium cyanide solution produced after sparging to the holding tank 1. Valves 8, 9, 5 and 6 are open and the remaining valves closed. The pump 12 is turned on (and operated in reverse when compared with Figure After the solution P:\OPER\JccSPECIFICATIONS'2555562 complc.do-26/O7/02 -9is transferred to the holding tank 1 it is sampled to determined its concentration. If the solution is too dilute it is recirculated to the sparging tank as per Figure 3 and then back to the holding tank as per Figure 4 for concentration measurement. If the solution returned to the holding tank is too concentrated, additional water may be added as per Figure 1.

When the sodium cyanide solution is at the required concentration it may be removed from the apparatus for use or storage, 11. This is achieved by opening valves 4, 5, 7 and closing the remaining valves and operating the pump. This is illustrated in Figure One skilled in the art would be able to select suitable materials from which to construct the apparatus. For instance, the holding tank may be made of carbon steel and may be provided with an elevated vent to the atmosphere and have an overflow to a bunded lute.

The tank may also have a removable inspection cover. The lines used to transfer sodium cyanide solution may be made of steel.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims (1)

18-04-' 08 10:45 FROM-Davies Collison Cave +61392542770 T12 P0/0 T-102 P005/007 F-116 "MPXRCO~fiCtCAT1UGU&24thSPA 0 IU.4ac-flOJOfl 00 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 00 I Anx industrial process for producing an aqueous solution of sodium cyanide at a o predetermined concentration, which process consists of: feeding water into a tank containing solid sodium cyanide thereby dissolving a portion of the sodium cyanide to produce a sodium cyanide solution; en removing the sodium cyanide solution from the tank and determining the concentration of O sodium cyanide in solution; and ci if necessary, adjusting the concentration of sodium cyanide in the solution to achieve the predetermined concentration, wherein the tank initially contains an amnount of solid sodium cyanide sufficient to last at least two process runs. 2. A process according to claim 1, wherein water is pumped into the tank under pressure and sprayed onto the solid sodium cyanide through a series of nozzles provided inside the tank. 3. A process according to claim 1 or 2, wherein per process run from 1 to 5m3 water are fed into the tank. 4. A process according to any one of the preceding claims, wherein if the concentration of the sodium cyanide solution produced is less than the predetermined concentration, its concentration is increased by repeating the steps of the process using the solution instead of water until the predetermined concentration of sodium cyanide is achieved. A process according to claim 4, wherein if following repetition of the steps of the process the concentration of the sodium cyanide solution is in excess of the predetermined concentration, the predetermined concentration is achieved by dilution with water. 6. A process according to any one of clams 1 to 3, wherein if the concentration of the sodium cyanide solution produced is in excess of the predetermined concentration, water is COMS ID No: ARCS-187353 Received by IP Australia: Time 10:47 Date 2008-04-18 11-04-'08 09:40 FROM-Davies Collison Cave +51392542770 T-073 P008/012 F-073 Pn.UPCVC" a CNCA'iO*SUA2 dIr SPA KP 1104 OAc-oMvz 00 ci added to the solution to achieve the predetermined concentration. 7. A process according to any one of claims I to 3, which comprises pumping water into a tank containing solid sodium cyanide, allowing the water to remain in contact with the solid sodium cyanide for a period of time sufficient to allow dissolution of a quantity of osodium cyanide to produce a sodium cyanide solution having an approximate enpredetenmined concentration, and pumping the sodium cyanide solution out of the tank. ci 8. A process according to claim 7, wherein the tank is a bulk storage container used for the transportation of solid sodium cyanide. 9. A process according to any one of the preceding claims, wherein the sodium cyanide solution produced is used immediately or transferred to a storage vessel. 10. A process according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings. 11. An apparatus suitable for carrying out the process claimed in claim 1, which apparatus consists essentially of a holding tank and a sparging tank, the holding tank comprising means for supplying water thereto, means for supplying the contents of the holding tank to the sparging tank and means for removing the contents of the holding tank and the sparging tank comprising means for returning liquid from the sparging tank to the holding tank 12. An apparatus according to claim 11, wherein the sparging tank includes a reservoir/channel portion at the base of the tank above which is provided a mesh, the mesh separating solid sodium cyanide from sodium cyanide solution which is produced. 13, An apparatus according to claim 11 or 12, wherein the holding tank, sparging tank and associated supply means are sized so as to be portable. COMS ID No: ARCS-186452 Received by IP Australia: Time 09:42 Date 2008-04-11