WO1990007979A1

WO1990007979A1 - Regeneration of carbon

Info

Publication number: WO1990007979A1
Application number: PCT/AU1990/000015
Authority: WO
Inventors: Peter Gerald Lanigan
Original assignee: The Broken Hill Proprietary Company Limited
Priority date: 1989-01-19
Filing date: 1990-01-18
Publication date: 1990-07-26
Also published as: ZA90367B

Abstract

A process and apparatus for regenerating granules of carbon used in the recovery of gold from ores by cyanidation treatment. The process comprises heating the granules in a kiln by means of microwave energy to a temperature sufficient to regenerate the carbon. The apparatus comprises (i) a kiln (3) having an inlet end (9) and an outlet end (11) for receiving therethrough granules of carbon, the kiln being rotatable about an inclined axis, and (ii) a source (13) of microwave energy coupled to the kiln (3) by means of a waveguide (15) operable to direct microwave energy into the kiln.

Description

REGENERATION OF CARBON The present invention relates to a process and an apparatus for regenerating carbon. The present invention relates particularly to a process and an apparatus for regenerating carbon used in cyanidation treatment to extract gold from ores. Cyanidation treatment is an important technique for extracting gold from ores. Typically, crushed ores are leached with an alkaline cyanide solution to form a gold-cyanide complex. Gold may then be recovered by adsorbing the gold-cyanide complex onto granules of carbon, separating the loaded carbon from the leach solution, and stripping the gold from the carbon. ^"After the stripping step the carbon is regenerated prior to reuse in the adsorption step. The regeneration is necessary since the carbon tends to lose its gold-cyanide complex adsorption capacity. Specifically, the purpose of the regeneration is to:

(a) restore the surface of the granules to near original condition;

(b) remove organic contaminants adsorbed onto granules in the leach step that inhibit gold-cyanide complex adsorption; and

(c) remove inorganic foulants (particles and coatings) on granules, such as lime salts, siliceous and clay minerals.

The granules of carbon may be regenerated by washing, such as acid and/or organic solvent washing.

However, usually, the granules of carbon are regenerated by heat treatment in the absence of air at temperatures in the range of 500°C to 800°C. The heat treatment is carried out in a range of different kilns, including inclined rotary kilns with heated shells, vertical pipe kilns with internal heating, conveyor belt fed horizontal kilns, and electric resistance kilns in which the carbon is the resistance- el-emeϊiifc.- ⁱ -~

There are a number^{' "}of

as-sbciated with the use of the known kilns.

In this regard, for example, in the case of rotary kilns, the energy requirements tend to be relatively high since it is necessary to run the kilns at an external, i.e. shell, temperature which is much higher than the temperature required for regeneration in order to generate the required regeneration temperature in the kiln. Furthermore, in the case of static kilns, such as conveyor belt fed horizontal kilns and electric resistance kilns, it is difficult to produce homogeneously regenerated carbon. In addition, the known kilns tend to be fed with wet carbon which results in some undesirable steam oxidation of the carbon. Furthermore, regeneration in the known kilns is not always successful in removing inorganic foulants that accumulate when processing some types of ores and tend to harden to an adhering scale.

It is an object of the present invention to alleviate the disadvantages described in the preceding paragraph.

According to the present invention there is provided a process for regenerating granules of carbon comprising, heating the granules in a kiln by means of microwave energy to a temperature sufficient to regenerate the carbon.

The invention, as described above, is based on the realisation that carbon in granular form is a strong absorber of microwave energy at a wide frequency range and heating of granules of carbon can occur by Joule heating from currents induced in the granules. One consequence of the use of microwaves is that the currents are induced substantially in the shell of each granule and the core is largely unaffected for appropriate carbon particles and microwave frequencies. As a consequence, the principal heating caused by microwave energy is in the shell. This is an advantage, in terms of optimising energy consumption, since the gold-cyanide complex adsorption capacity of the granules is largely a function of the surface properties of the granules, and thus this is the region of the granules which requires regeneration. It is preferred that the kiln is a rotary kiln.

In this regard, it has been found that tumbling action of the granules in a rotary kiln ensures uniformity and completeness of the heating of the granules. In addition, it has been found that inter-granular current flows are interrupted as the granules tumble, and this results in numerous microwave frequency arcs being established briefly between granules. The rapid and purely surface heating that results from such arcs has the beneficial effect of shattering or abrading brittle deposits of inorganic foulant on the granules.

It is preferred that the granules of carbon are dried before entry into the kiln.

It is also preferred that the atmosphere in the kiln is controlled. Typically, the controlled atmosphere may comprise nitrogen. The controlled atmosphere may also comprise nitrogen and steam.

According to the present invention there is also provided an apparatus for regenerating granules of carbon comprising: (a) a kiln having an inlet end and an outlet end for receiving therethrough granules of carbon; and

(b) a source of microwave energy coupled to the kiln and operable to heat granules of carbon in the kiln to a temperature sufficient to regenerate the carbon.

It is preferred that the kiln is a rotary kiln. It is preferred that^"-the apparatus further comprises a drye-r-

of A..? inlet end of the kiln operable to dry granules of carbon prior to entry into the kiln.

It is preferred that the apparatus further comprises an attenuation throat at the outlet end of the kiln to restrict the passage of microwaves through the outlet end.

Additionally, it is preferred that the apparatus further comprises microwave chokes and absorbing material at the inlet end of the kiln to limit microwave energy leakage from junctions between stationary and rotary parts of the kiln. The method and apparatus described above may comprise part of a plant for recovering gold from ores by cyanidation treatment.

In this regard, according to the present invention there is provided a process for recovering gold from ores by cyanidation treatment comprising the regeneration step as described above.

Furthermore, according to the present invention there is also provided an apparatus for recovering gold from ores by cyanidation treatment comprising the regeneration apparatus as described above.

The invention is now described with reference to the accompanying drawings in which:

Fig. 1 is a vertical section through a preferred embodiment of a regenerator in accordance with the invention; and

Fig. 2 is a section along the line 2-2 of Fig. 1.

The regenerator shown in the drawing may form part of a plant for recovering gold from ores by cyanidation treatment. In such a plant, the regenerator receives granules of carbon which have been used to adsorb gold-cyanide complexes, subsequently stripped of the gold values, and then washed an drained, and restores the granules to an acceptable gold-cyanide complex adsorption capacity. The' restored granules are then reused in the plant.

The regenerator comprises a kiln 3 having an inlet end 9 and an outlet end 11 supported by roller bearings 5 for rotation by means (not shown) about a kiln axis 7 which has a small slope to the horizontal.

The regenerator further comprises a source 13 of microwave energy coupled to the kiln 3 by means of a waveguides 15 operable to direct microwave energy into the kiln 3. In use, the microwave energy heats granules of carbon in the kiln 3 to a temperature sufficient to restore the granules to an acceptable gold-cyanide complex adsorption capacity. In any given situation the actual temperature required will depend on a number of factors, such as the level of organic contaminants and inorganic foulants on the gold bearing ore. Typically, the temperature will be in the range of 600°C to 750°.

The regenerator further comprises means (not shown) to introduce into and to maintain in the kiln 3 a controlled atmosphere to optimise regeneration of the granules. Typically, the controlled atmosphere will comprise nitrogen. In some situations the controlled atmosphere may comprise nitrogen and steam. The granules are fed into the^' kiln 3 by means of a screw feeder 19 or other suitable form of positive displacement device.

A dryer section 21 is located intermediate to the screw feeder 19 and the inlet end 9 of the kiln 3 and is operable to dry the washed and drained granules prior to entry into the kiln 3. The dryer section 21 comprises a conduit 23 and a plurality of inlets 25 along the length thereof for introducing heated air to dry the granules in the conduit 23. The regenerator further comprises an attenuating throat 27 at the outlet end 11 of the kiln 3 to restrict the passage of microwaves through the outlet end 11 of the kiln 3.

The regenerator further comprises a heat exchange section 29 for cooling the heated granules which pass from the kiln 3 through the outlet end 11 and the attenuating throat 27 and for heating a stream of air which is then directed along a conduit 31 to the inlets 25 of the dryer section 21. The stream of air is also heated by energy generated by the attenuation of the microwaves in the attenuation throat 27. The heat exchange section 29 comprises, a conduit 33 through which the granules pass, an outer sleeve 35 which defines an annular chamber 37 which encloses the conduit 33 and the attenuation throat 27, and an air fan 39 to direct the stream of air through the annular chamber 37 and subsequently through the conduit 31 to the dryer section 21. The heat exchange section 29 further comprises a plurality of baffles 41 in the annular chamber 37 to force the stream of air to move along a tortuous path through the annular chamber 37 thereby to improve the heat exchange to the stream of air.

The microwave energy induces currents substantially in the outer shell of the granules, and the core of the granules remains largely unaffected. As a consequence, the heating of the granules caused by the induced currents can be restricted substantially to the outer shell. The concentration of the induced heating in the outer shell is an advantage since this is the region responsible for gold-cyanide complex adsorption and thus is the region which requires regeneration. As a consequence, the use of microwave energy enables energy consumption to be optimized for a given throughput of granules, since there is only minimal use of energy to heat the cores of the granules.

In addition, the rotation of the kiln 3 ensures uniformity and completeness of regeneration by causing the granules to tumble within the kiln 3, and as a consequence intergranular current flows are interrupted resulting in numerous microwave frequency arcs being briefly established between the granules. The rapid, purely superficial heating that results shatters and abrades brittle inorganic foulants by heating and arc radiation. Additionally, UV and visible light and heat from the arcing help degrade and disperse organic