FIELD OF THE INVENTION

The present invention relates to the field of chemical processing, for example mineral processing, such as hydrometallurgical processing of mineral slurries at high pressure and temperature, such as caustic leaching of aluminous ores, pressure oxidation of refractory metal ores and metal sulphide concentrates, metals such as gold, and high- pressure acid leach (HPAL) of lateritic metal ores, metals such as nickel, cobalt, copper, zinc, lead, platinum and palladium. However, the invention may be more widely applied including, without limitation, in the petrochemical industry and for treatment of organic waste. More particularly, the invention relates to a method and apparatus for performing such chemical processing.

BACKGROUND TO THE INVENTION

High temperature leaching of minerals such as pressure oxidation of refractory gold concentrate and acid leaching of lateritic nickel ore use agitated horizontal autoclaves Whereas caustic leaching of aluminous ores such as bauxite commenced use of vertical autoclaves or digesters as described in US Patent 3,1 12,994 by Don Donaldson. However, this vertical autoclave does not provide a separate means to vent the gasses that accumulate at the top of the vertical autoclave, either by regular venting through a manual valve by an operator or by some form of automatic control based on a level sensor. The design of the excess gas management system must be done carefully. In the case of the manual venting, it must be done safely due to the high temperatures and pressures involved, and the tendency for the vent lines to block due to precipitation of mineral deposits inside the vent pipe. In the case of automatic control, there are the same operational issues as for manual venting, but with the additional problem of poor reliability of instrumentation for level indication due to the severe operating conditions and the deposit of mineral scale through precipitation on the measuring instrument. Another issue with US Patent 3,1 12,994 is the requirement for slow moving agitator impellers or paddles to minimise vertical mixing. This does have the benefit of allowing the autoclave to operate close to the plug flow regime, but at the detriment of thicker boundary layers on the surface of the minerals hindering the leaching process; thus not allowing for the optimisation of the agitator speed to reduce surface boundary layer for chemical reaction and other design parameters such as length to diameter ratio, number of agitated cells with agitator impellers, such as Rushton turbines, and the number of successive vertical autoclaves. Another issue with this design is the need for an agitator to provide the agitation within the vertical autoclave. This requires a seal between the rotating shaft on the autoclave preventing the high pressure and temperature contents to escape to the environment.

The Zimmermann Process for the pressure oxidation of organic matter in liquid waste effluent and more specifically the complete oxidation of organic matter in waste sulphite liquor using compressed air, is described in US Patent 2,665,249. This patent teaches that a tower reactor is used and there are precipitates formed. The patent also discloses that the preferred application is a series of inter-connected tanks. The excess gas is vented with steam, but the patent does not disclose how this is done.

More recently has seen the advent of tube digestion technology as described in Australian Patents 676,920 and 697,381 by Dirk De Boer where the bauxite slurry is heated in horizontal tubes jacketed with steam to heat the slurry to the final digestion temperature. Invariably, this heating is followed by horizontal pipes to maintain the slurry at temperature to complete the chemical reactions for alumina leaching. Gasses that are generated or added such as air or oxygen during leaching are displaced along the tube to the next processing step, which is flash cooling. The advantage of incorporating the leaching of alumina within the heat recovery step is the considerable improvement in heat recovery due to the strongly endothermic nature of the dissolution of alumina as sodium aluminate. The drawback of this horizontal tubular leaching is the requirement of exceedingly long tubes designed to operate above the settling velocity of the slurry solids.

In the case of high-pressure leaching of metal ores such as nickel laterite, the current technology uses compartmented horizontal autoclaves; a typical example of this arrangement is described in US Patent 4,606,763 by Donald Weir. The commercial autoclaves are normally extremely large and difficult to construct and install in remote locations. Horizontal autoclaves have been as large as 5.7m diameter and about 36m long (1 ,008m ³ ) such as Taganito HPAL and 5.2m diameter and about 37m long (870m ³ ) at Ambatovy HPAL. The sheer size of these pressure vessels results in construction of these unique pressure vessels at special facilities, exceedingly long lead times for construction and delivery, especially to remote locations, and challenging installation of these mega pressure vessels. Often these mega pressure vessels are installed early in the construction phase and the rest of the refinery is constructed around them. Large nickel HPAL facilities in remote locations based on these large horizontal autoclaves require significant secondary process facilities resulting in long ramp-up times and thus reduced economics.

The hydrometallurgical industry has used horizontal autoclaves as a simple method for managing the excess gas for successive CSTRs. A series of CSTRs is desired to approximate plug flow to minimise reactor volume to achieve the desired degree of leaching. The simplest method available is to extend a horizontal autoclave and add a dividing wall. So, the slurry moves from one compartment to the next by gravity, while the excess gas for all compartments accumulates in the common empty space above the compartments. Thus, there is only one control point for gas flow and one control point for slurry flow for a series of CSTRs.

In the case of pressure oxidation leach of metal sulphide concentrates, such as for gold, the current technology uses horizontal autoclaves, sometimes compartmented as described in US Patent Nos. 3,961 ,908 by Freddie Touro, 4,738,718 by Nandkumar Bakshani and Peter Yu, and 6,395,063 by John Cole. The commercial autoclaves are normally very large and difficult to construct and install in remote locations; for instance, the Amursk refractory gold concentrate horizontal autoclave is 25m long, and 163 tonnes is a typical example. Again, as with high pressure leaching of metal ores like nickel laterite, the size of the horizontal autoclave requires construction at special pressure vessels construction facilities, and transportation to site on dedicated transportation facilities adapted to the unusually large size of the pressure vessel, special lifting cranes and equipment for installation. The cost, time and economic risk to commission these large horizontal pressure vessels, and manage excess gas, have held back many projects not just in the mineral processing industry but likely other chemical processing industries as well.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a chemical processing apparatus and method that provides an economic option for projects difficult and/or expensive to progress with current mineral processing apparatus noting the issues with excess gas management as described above. With this object in view, there is provided - in one aspect of the invention - a chemical processing apparatus comprising: at least one vessel comprising: an inlet for a liquid process stream; an outlet pipe for a liquid process stream to exit the vessel; and a zone at least partially occupied by a gas wherein the zone at least partially occupied by a gas communicates with the outlet liquid process stream pipe for removing accumulating gas from the zone to the outlet liquid process stream. In a further aspect, the present invention provides a chemical processing method comprising: conducting a chemical processing operation in at least one vessel comprising an inlet for a liquid process stream; an outlet pipe for a liquid process stream to exit the vessel; and a zone at least partially occupied by a gas and communicating with the outlet liquid process stream pipe; directing a liquid process stream entraining the gas to an inlet of a vessel, at least a portion of the gas being directed to and partially occupying a zone of the vessel; directing an exit liquid process stream through an outlet pipe; and removing accumulating gas from the zone to the outlet pipe.

Gas accumulating within the zone may be defined as excess gas and may include any gas naturally entrained into the vessel by the liquid process stream and generated by the liquid process stream in the vessel, such as by boiling, plus any gasses that may be injected into the vessel by the process, minus gasses that have been removed through dissolution or reaction with the liquid process stream and entrained in the outlet liquid process stream by the process. For instance, in the alumina industry, excess gas in the vessel would typically include gasses generated by the decomposition of bauxite organics in the vessel, whereas in pressure oxidation of mineral slurries excess gas would include the remains of the injected gas such as air or oxygen that has not dissolved and reacted with the mineral slurry. It will be understood that the foregoing is not intended to limit application of the method and apparatus to mineral processing. The methods may equally be applied in chemical processes in other industries including, without limitation, the petrochemical industry and organic waste treatment.

The zone may communicate with the outlet pipe for the liquid process stream through an excess gas port or duct, particularly where the apparatus includes two or more vessels in series. The outlet pipe, such as a dip tube, may extend through the interior of the vessel or externally to the vessel. If the outlet pipe extends through the vessel, it is disposed to avoid accessories internal to the vessel, such as agitators.

The feed inlet for the liquid process stream is most conveniently disposed at the top of the vessel, or in an upper portion of the vessel and the outlet for the liquid process stream is conveniently disposed at the bottom or lower portion of the vessel. However, liquid process stream flow direction (which may include a slurry flow regime) can be reversed, that is with the inlet for the liquid process stream being disposed at the bottom, or in a lower portion of the vessel and the outlet for the liquid process stream being disposed at the top or upper portion of the vessel while still achieving the desired excess gas management. The latter scenario can eliminate the requirement for an excess gas port or duct, but in this reverse regime there is a tendency for oversized particles and loosened scale to accumulate in the bottom of the vessel. This accumulation of oversized particles can be counteracted at least in part by increased agitation, indirectly through impellers or directly through increased baffling or velocity. In some cases, this reverse liquid process stream or slurry flow regime may be an advantage, such as the desire for solids retention in a fluidised bed of solid particles. This technique could be used, for instance, to increase available surface area for chemical interaction between gas, liquid and solids, such as in precipitation of a solute. If there is a tendency for excess gas to accumulate in the zone down into the lower part of the vessel in the reverse flow regime, due to low liquid velocity in a downward orientated outlet liquid process stream, then the excess gas port or duct is desirably sized for the volume of excess gas at high enough velocity to the subsequent appropriate lower pressure, downstream vessel, such as a subsequent reactor vessel or flash tank, to ensure that there is no accumulation of excess gas from the zone into the lower part of the vessel.

Optionally, the liquid process stream is a slurry of ore or concentrate and a process liquor such as a leachant. As stated above, and most conveniently, the feed inlet of the liquid process stream or slurry is disposed at the top or upper portion of the vessel and the outlet for the liquid process stream or slurry is disposed at the bottom, or in a lower portion of the vessel. A duct communicating with this outlet, whether internally or externally of the vessel, directs the slurry up to at least to the desired level of the excess gas and liquid process stream interface at which point there may be provided, and preferably is provided where the apparatus includes two or more vessels, an excess gas port in the case of an internal outlet slurry pipe, or an excess gas duct connected to the vessel in the case of an external outlet slurry pipe. An internal outlet slurry pipe may be a dip tube. The head loss from the slurry flowing through the vessel and the outlet slurry pipe between the excess gas connection points (excess gas port and excess gas duct) creates a pressure drop that will push excess gas and some slurry through the excess gas port or duct from the vessel to the outlet slurry pipe. Where provided, the size of the excess gas port or duct is selected, preferably to the minimum possible dimensions, to substantially remove, by venting, excess gas under design operating conditions. The excess gas port or duct allows any excess gas to escape to the lower pressure outlet slurry pipe until the level of slurry in the vessel submerges, due to displacement, the excess gas port or duct connected to the outlet slurry internal dip tube or external outlet slurry pipe. Thus, the excess gas volume in a zone at the top of the vessel is controlled without the need for manual or automatic intervention.

The vessel is desirably vertically disposed, though need not be so in all circumstances, (for example where the vessel results from a retrofit to a horizontally disposed vessel), and is conveniently an autoclave with one or more compartments, preferably a vertical cylindrical autoclave, that provides a simple means to manage excess gas such that the autoclave can be one or a series of pressure vessels or one or a series of vessels made of standard sized, off-the-shelf piping. Use of such a vertically disposed vessel may avoid the requirement, for horizontal tubular reactors, of exceedingly long tubes designed to operate above the settling velocity of the slurry solids. In the case of a series of vessels, excess gas duct(s) may bypass individual vessels or autoclaves if desired. The excess gas duct of the last of a series of autoclaves may be directed to a liquid/gas separation vessel, rather than to the vessel’s outlet pipe.

Under certain design conditions, use of a vertically disposed vessel or vertical autoclave would also eliminate the requirement of agitators. Although this technique to control excess gas can be applied to a horizontal autoclave, the preferred autoclave arrangement is vertical as this collects the excess gas in a manageable (typically small) volume at the top of the vertical autoclave. Vertical autoclaves do not require a motor and shaft for each compartment, but only one with agitator impellers located at each compartment in the autoclave. Vertical autoclaves are also preferred, when mechanical agitation is required, as several agitator impellers can be installed on one agitator shaft with one motor, instead of one motor and shaft for each agitator impeller in a horizontal autoclave.

Where employed, the excess gas duct can be connected to the subsequent vessel or autoclave and have topographically the same process impact as connecting to the external outlet slurry pipe. Although an excess gas duct connecting the top of the vessel or autoclave to the external outlet slurry pipe can be disposed horizontally, in other words connecting to the outlet slurry pipe at the same horizontal level, excess gas duct(s) may also be connected to the outlet slurry pipe at different levels of the vessel or autoclave. For instance, the connection to the outlet slurry pipe can be at a level higher than the connection point to the vessel or autoclave. Only excess gas flow from the top of the autoclave to the outlet slurry pipe can be achieved by a loop of the pipe rising above the height equivalent to the head loss of the mineral slurry through the vessel or autoclave and outlet slurry pipe. When the pipe diameter of the loop is sufficiently large to allow excess gas to disengage from the mineral slurry, then only gas will proceed to the outlet slurry pipe. This situation is sometimes referred to as an airlock. This is of an advantage for mineral processes (and other chemical processes involving slurries or suspensions) that are not scaling in nature as described below.

If, for example, as in the dissolution of alumina from bauxite into Bayer refinery liquor there is a propensity for a liquid process stream to scale, then there is an advantage to maintain a flow of this stream or slurry through the excess gas port or duct at a velocity such that the slurry erodes the scale formation, but not at a velocity high enough to erode the excess gas pipe. If erosion in an excess gas duct is an issue, then a flow restriction such as an orifice plate, manual valve or automated valve may preferably be installed sufficient to maintain the flow of excess gas and entrained liquid or slurry, and thus excess gas duct diameter can be increased to where erosion of the pipe is not an issue. If the pressure head between the top of the vessel or autoclave and the corresponding point in the outlet slurry pipe is insufficient to maintain an excess gas and slurry velocity to keep the pipe free of scale, then the excess gas duct of one vessel may be connected to the outlet slurry pipe of a subsequent vessel or autoclave or downstream process vessel where the head available is adequate to achieve the desired velocity to maintain the excess gas duct free of scale.

Net impact from any loss of process efficiency due to bypassing of slurry from the feed to the exit stream, may be much reduced or even negligible if the port or excess gas duct is dimensioned appropriately and a plurality of vessels or autoclaves, of lesser volume, are used in sequence rather than one relatively large vessel or autoclave. For instance, if there is a 10% bypass of slurry in one vertical autoclave, then after mixing only 10% of the 10% that was bypassed in the first autoclave will be bypassed in the second autoclave. So, with plural vertical vessels or autoclaves in series and each having excess gas managed as above described, the net impact on leaching efficiency should become negligible.

If, however, the bypass of slurry is not desired, for instance if there is significant natural variation in the excess gas load, then a control valve, operated with a suitable control strategy, can be provided in the excess gas duct, where selected. In order to have positive control of the interface at all times in such an embodiment, the diameter of the excess gas port or excess gas duct should be large enough to vent essentially all the excess gas at maximum generation rates. This means that at average excess gas generation conditions there could be a larger bypass of slurry than might be desired. So, in order to minimise the extent of slurry bypassing this control valve would be operated to control the slurry level in the vessel to a point below the excess gas duct. The excess gas could be vented to another location if so desired, but it is preferred to vent to the outlet slurry pipe as the duty for the valve will be easier to manage due to the relatively low pressure drop than potentially to an external system. If there is only a gradual minor variation in excess gas flow, say due to the nature of the orebody changing as it is mined, then a manual valve can be added to the excess gas duct to the outlet slurry pipe to minimise the flow of bypass slurry, or in the case of the port in the case of an internally disposed outlet dip tube, its size can be adjusted on the next maintenance cycle of the vertically disposed autoclave or vessel.

The vessel or autoclave may include baffles (preferably annular baffles) and/or agitator impellers (which may also have baffles, for example, of conical geometry, disposed relative to them) disposed along its height to provide a series of vertically disposed compartments that approximate continuous stirred tank reactors (CSTRs). However, the preferred arrangement is to have a series of desirably smaller vertical autoclaves rather than one large vertical autoclave thus making the individual vertical autoclaves easier to construct and deliver to a mineral processing site. Also, as the diameter of a vertical autoclave becomes smaller, it may be possible to remove the agitator impeller and replace it with inline mixers and, or agitation through gas injection such as through sparging. Also, as the diameter of the vertical autoclave reduces it becomes easier to design the vertical autoclave to handle higher operating temperatures, thus reducing overall vertical autoclave volume. With such smaller diameter vertical autoclaves, it is easier to design for operating temperatures of 250 to 300°C, and higher.

The internal design such as length, diameter and baffling of the autoclave or vessel can be adjusted to approach CSTR if required, such as when feed temperature is low and a minimum initial reaction temperature is required in an exothermic reaction such as the initial step in the pressure oxidation of a gold concentrate, or it can be more plug flow, once reaction conditions have been achieved, to minimise reactor volume.

The more vertical vessels or autoclaves are placed in series and, optionally, modular installations in parallel too, the smaller the vertical vessels or autoclaves need to be for a given overall throughput. Once the vertical vessels or autoclaves are small enough then, depending on mineral particle size distribution (PSD) and chemical reaction conditions, the vertical vessel or autoclave could be constructed from off-the- shelf piping of, say, for example 600mm as the shell thickness required is significantly less than in larger pressure vessels.

A vertical autoclave does not have the drawback of a horizontal tube reactor as the latter requires a minimum velocity to at least progress the slurry solids along the reactor thus making the reactor extremely long. In the case of a vertical autoclave, so long as there is adequate mixing by a mechanical agitator impeller, inline mixers or by some other means such as gas injection, such as by sparging, for the chemical reaction, the outlet dip tube or outlet port will be designed to have enough velocity to transport all the slurry solids to the next vertical autoclave or the next processing step. Thus, the length and mass of piping required can be significantly less than for a horizontal tube autoclave.

The last vertical autoclave in a series of vertical autoclaves typically feeds a flash tank via a control valve or pressure letdown station, a pressure drop that is higher than over one vertical autoclave. If under these higher pressure drop conditions, the wear in the pressure letdown station is too high due to high two-phase flow of the mineral slurry and excess gas, then the last vertical autoclave can have separate slurry and gas flow controls as in traditional horizontal autoclaves. If the subsequent process step after the last vertical autoclave is heat recovery by heat pipe heat exchanger, as described in the Applicant’s co-pending International Patent Application No. PCT/AU2018/050983, the contents of which are hereby incorporated herein by reference, then the pressure drop across a pressure letdown station will be less compared to feeding a flash tank.

Process streams other than a mineral slurry feed can be added at one or more points along the vessel or autoclave into one or more successive cells for various purposes, for instance and not limited to, steam for heating, slurry or liquids for cooling or as reagents, gasses as reagents or for agitation, pH modifiers and catalysts. If, for example, the vessel or vertical autoclave is being used for pressure oxidation, then oxygen or air is desirably injected at or near the bottom of the vertical autoclave. Further additions of oxygen or air can be made progressively up the vertical autoclave to maintain dissolved oxygen concentrations. The higher the velocity of injection, the greater the degree of agitation within the autoclave. In the case of gas injection, the higher the velocity of gas injection the higher the localised turbulence and the smaller the bubbles formed. In the case of air or oxygen, smaller bubbles will mean higher surface area and so speedier dissolution of oxygen for chemical reaction. The highest gas injection velocity will be at sonic velocities. Excess gas will accumulate or collect either in the zone at the top of vertical autoclave and be removed as described above to the outlet slurry pipe or will be entrained by the slurry flowing through the outlet slurry pipe. If the vertical autoclave is being primarily used for pH adjustment, for instance reduction of free acid through the injection of limestone slurry, then the limestone slurry is desirably injected with the autoclave feed, or at or near the top of the autoclave. A portion of the carbon dioxide that is liberated will rise to the top of the autoclave where the excess will join the outlet slurry pipe via the excess gas port or duct, while some of the carbon dioxide in the form of small bubbles will be entrained by the slurry in the vessel to the outlet slurry pipe at or near the bottom of the vertical autoclave.

It is possible to combine chemical reactions in one vertical autoclave, for instance, as in pressure oxidation of gold concentrate, oxygen or air is desirably injected at or near the bottom of the vertical autoclave and further additions of oxygen or air can be made at injection points progressively up the vertical autoclave, for example in a staggered arrangement, to maintain dissolved oxygen concentrations, while limestone slurry (or another alkali such as sodium hydroxide solution) can be injected with the autoclave feed, or at or near the top of the autoclave, and progressively down the autoclave to maintain a free acid that is high enough for the formation of ferric sulphate, but low enough to maximise haematite formation and minimise ferric hydroxy sulphate and jarosite formation, thus improving performance in the downstream gold cyanidation process. The reduction in the formation of jarosite has the added benefit of not tying up silver, if present, in the jarosite crystal. Generally, reagents and other process streams, whether in solid, liquid or gaseous state, may be introduced through suitable injection points, which may be arranged in a staggered configuration, directing the reagent or other process stream to determined compartment(s) of the vertical autoclave.

The above illustrates that in a series of two or more vertical autoclaves, each vertical autoclave can have a different chemical duty. For instance, one vertical autoclave can be dedicated to pressure oxidation, while a second vertical autoclave can be dedicated to free acid control, and yet another can be a combination of both. With multiple chemical duties conducted in one autoclave, the overall equipment cost would be lower, but the reagent for free acid control may be different to avoid dilution of the oxygen partial pressure. Such trade-off in capital versus operating cost would be evaluated on a case by case basis. A preferred embodiment does not preclude the addition of supplementary gas venting means, such as pipes, either to the outlet slurry line or to other destinations, operated manually or automatically. For instance, if an excessive amount of gas has been used or generated in the vessels or autoclaves, for example by boiling, then it might reduce downstream costs, both capital and operational, to separate the excess gas from the process stream. This could be achieved by extraction of the excess gas from the zone at the top of the autoclave or via a dedicated gas separation vessel. In some cases, and especially where the apparatus includes a single vessel or autoclave, such extraction of excess gas may foreclose requirement for an excess gas duct or port. Such an excess gas duct or port would typically be included where the apparatus includes two or more vessels or autoclaves, though this depends on the quantum of excess gas. Excess gas separation can be achieved in the zone by a combination of a small excess gas duct and a larger bulk gas vent located above the small excess gas duct, with a level control valve. The level control valve will control liquid level in the vessel at or around the level of the excess gas duct outlet from the vessel, thus ensuring substantially liquid free gas from the vessel in the bulk gas vent.

Mineral feed slurry densities will depend on the upstream processing but can be as high as 60% solids.

Temperature control of an autoclave may be implemented in a number of ways. For example, the peak temperature of the autoclave can also be controlled by allowing vapour to be vented. It is also possible to control autoclave temperature by injection of cold streams such as water injection at any required point or limestone slurry, and/or by controlling the pressure in the last vertical autoclave. The control of the pressure in the last vertical autoclave of a series of autoclaves can be achieved by a pressure letdown station on the outlet slurry pipe located after an excess gas injection point. With a series of two or more vertical autoclaves, there is another means of temperature control available that is not as readily available in a single large horizontal autoclave due the gravity flow from one chamber to the next, that is, installing a heat exchanger in the outlet slurry pipe from a vertical autoclave to a subsequent vertical autoclave in a series. Such heat exchanger allows cooling the slurry when the reaction in the autoclave is exothermic, or heating if the reaction is endothermic. The heat exchanger could be a flash tank in combination with either a splash tank (for example as shown in Figures 9 and 10) or shell and tube heat exchanger (not shown) or an indirect heat exchanger with elongated rectangular profile channels separated by elongated rectangular channels fed with a heat exchange medium, or an indirect heat exchanger with circular profile channels surrounded by a heat exchange medium using sensible heat as the means of heat transfer, or an indirect heat exchanger with circular profile channels surrounded by a heat exchange heat transfer fluid as an intermediate to transfer the heat to the heat exchange medium using the latent heat of evaporation of the heat transfer fluid as the principal means of heat transfer, sometimes referred to as a heat pipe heat exchanger as described in the Applicant’s co-pending Australian International Patent Application No. PCT/AU2018/050983, the contents of which are hereby incorporated herein by reference. In the latter embodiment, the heat exchanger transfers heat between a first process stream and a second process stream through the medium of a heat transfer fluid and comprises: at least one first process stream passage; at least one second process stream passage; and a shell enclosing said plurality of first and second process stream passages within a volume, said volume being, as a result of a heat transfer process, fully filled with both vapour and liquid phases of said heat transfer fluid wherein said at least one first process stream passage and said at least one second process stream passage are spaced by a disengagement zone enabling separation of said vapour and liquid phases and limiting accumulation of liquid phase heat transfer fluid about said at least one first process stream passage.

Preferred heat exchange equipment includes circular profile channels as they are less prone to erosion of the internals. In comparison of heat transfer to an external heat sink by sensible heat versus heat transfer principally by latent heat of evaporation, the equipment for heat transfer by sensible heat is simpler to design as the heat exchanger using heat transfer principally by latent heat of evaporation requires the additional steps of selecting the heat transfer fluid and determining the maximum flux based on the Souders-Brown equation as described in the Applicant’s co-pending Australian International Patent Application No. PCT/AU2018/050983 incorporated by reference. However, the heat pipe heat exchanger can achieve significantly higher heat transfer coefficients than heat exchangers based on heat transfer based on solely sensible heat. The heat pipe heat exchanger can more readily transfer heat between liquids that are scaling in nature and that are slurries such as may be found in a mineral processing facility. For a heat pipe heat exchanger operating above 140°C, the most appropriate heat transfer fluid is likely to be water. It will depend on the operating conditions which of the two types of heat exchangers are economically best suited to control the temperature of the intermediate process slurry between two vertical autoclaves.

The method and apparatus of the invention may be used for a range of chemical processing operations, such as oxidation processes. The method and apparatus is also useful in hydrometallurgy, including: recovery of nickel from lateritic or sulphidic ores; or recovery of gold from carbonaceous refractory ores, recovery of gold from sulphidic refractory ores, including those containing significant quantities of metal carbonates such as dolomite. Treating carbonaceous minerals may involve treating the ore at elevated temperature and oxygen overpressure under alkaline conditions. However, the same alkaline pressure oxidation process can be conducted in single vertical autoclave or series of vertical autoclaves with excess gas management.

DESCRIPTION OF PREFERRED EMBODIMENTS

The chemical processing apparatus and method may be more fully understood from the following description of preferred but non-limiting embodiments thereof made with reference to the accompanying drawings in which: Figure 1 is a schematic section view of an autoclave according to a first embodiment of the invention.

Figure 2 is a schematic section view of an autoclave according to a second embodiment of the invention

Figure 3 is a schematic section view of an autoclave according to a third embodiment of the invention.

Figure 4 shows a schematic section view of a series of connected autoclaves, each autoclave according to the second embodiment of the invention

Figure 5 is a schematic section view of an autoclave according to a fourth embodiment of the invention. Figure 6 is a schematic section view of an autoclave according to a fifth embodiment of the invention.

Figure 7 is a schematic section view of an autoclave according to a sixth embodiment of the invention. Figure 8 is a schematic section view of an autoclave according to a seventh embodiment of the invention.

Figure 9 is a process flow diagram for a laterite high pressure acid leaching process of the prior art.

Figure 10 is a process flow diagram for a laterite high pressure acid leaching process involving an autoclave according to the sixth embodiment of mineral process of the invention.

Figure 1 1 is a process flow diagram for a laterite high pressure acid leaching process involving an autoclave according to the sixth embodiment of mineral process of the invention with heat pipe heat exchangers for heat recovery. Figure 12 is a process flow diagram for a laterite high pressure acid leaching process involving an autoclave according to the fifth embodiment of mineral process of the invention with heat pipe heat exchangers for heat recovery.

Figure 13 shows a series of connected autoclaves with heat exchange according to a further embodiment of the present invention for an autothermal reaction such as is often found with sulphide concentrates.

Referring to Figures 1 to 8, there are shown vertically disposed autoclaves 10 acting as reactors for performing a chemical process, especially in hydrometallurgy. Each autoclave 10 includes an inlet pipe 12 for a liquid process stream; an outlet pipe 14 for a liquid process stream to exit the autoclave 10; and a zone 16 at least partially occupied by at least a portion of the gas. In all cases, the zone 16 at least partially occupied by a gas communicates with the outlet liquid process stream pipe 14 for removing gas from the zone 16 to the outlet liquid process stream pipe 14. In all cases too, the liquid process stream is a slurry of mineral ore or concentrate and leachant for conducting the hydrometallurgical process. Gas accumulating within the zone 16 is defined as excess gas and may include the gas naturally entrained into the vertical autoclave 10 by the liquid process stream and generated by the liquid process stream in the vessel, plus any gasses that may be injected into the vertical autoclave 10 by the process, minus gasses that have been removed through dissolution or reaction with the liquid process stream and entrained in the outlet liquid process stream by the process. For instance, in the alumina industry, excess gas in the vertical autoclave 10 would typically include gasses generated by the decomposition of bauxite organics in the vessel, whereas in pressure oxidation of mineral slurries excess gas would include the remains of the injected gas such as air or oxygen that has not dissolved and reacted with the mineral slurry.

The zone 16 may communicate with the outlet pipe 14 for the liquid process stream through an excess gas port 20 as shown in Figure 1 or an excess gas duct 25 as shown in Figures 2 to 8. The outlet pipe, such as an internally disposed dip tube 14A, may extend through the interior of the vessel or externally to the vessel as outlet pipe 14B. If the outlet pipe 14A extends through a vertical autoclave 10, it is disposed to avoid accessories internal to the vessel, such as agitator impellers 22. Each autoclave 10 is provided with a plurality of compartments 1 1. The size of the compartments 11 and their relative size depends on the chemistry and cost/benefit on the desired chemical reaction. In the case of a strongly exothermic reaction and little or no preheating, it is beneficial to have a larger first compartment in the first vessel or autoclave and either/or bypassing of the initial compartments.

The outlet pipe for the liquid process stream or slurry is disposed at the bottom 10a, of each autoclave 10. A duct 14A, 14B communicating with this outlet, whether internally or externally of the autoclave 10, directs the slurry up to at least to the desired level of the excess gas and liquid process stream interface 16a at which point there is conveniently an excess gas port 20 in the case of an internal outlet dip tube 14A or excess gas duct 25 connected to the autoclave 10 in the case of an external outlet slurry pipe 14B. The head loss from the slurry flowing through the autoclave 10 and the outlet slurry pipe 14A, 14B creates a pressure drop that will push excess gas and some slurry through the excess gas port 20 or duct 25 from the autoclave 10 to the outlet slurry pipe 14A, 14B. The size of the excess gas port 20 or excess gas duct 25 is just large enough to substantially remove, by venting, excess gas from zone 16 under design operating conditions. This excess gas port or duct 20, 25 allows any excess gas to escape to the lower pressure slurry outlet pipe 14A, 14B until the level of slurry in the vertical autoclave 10 submerges, due to displacement, the excess gas port or duct 20, 25 connected to the outlet slurry internal dip tube 14A or external outlet slurry pipe 14B. Thus, the excess gas volume in zone 16 at the top of the autoclave 10 is controlled without the need for manual or automatic intervention.

Under certain design conditions, use of a vertical autoclave 10 would also eliminate the requirement of agitator impellers as indicated by Figure 6. Vertical autoclaves do not require a motor and shaft for each compartment, but only one with agitator impellers 22 located at each compartment in the autoclave 10 with conical baffles disposed underneath as indicated by Figure 7. Vertical autoclaves are also preferred, when mechanical agitation is required, as several agitator impellers can, as shown, be installed on one agitator shaft with one motor, instead of one motor and shaft for each agitator impeller in a horizontal autoclave.

The excess gas duct 25 can be connected to the subsequent vessel or autoclave 10 and have topographically the same process impact as connecting to the external outlet slurry pipe 14B. Although an excess gas duct 25 connecting the top of the autoclave 10 to the external outlet slurry pipe 14B can be disposed horizontally as shown in Figures 2 and 4 to 8, in other words connecting to the outlet slurry pipe 14B at the same horizontal level, a plurality of excess gas ducts 25A may also be connected to the outlet slurry pipe 14B at different levels of the vertical autoclave 10. For instance, the connection to the outlet slurry pipe 14B can be at a level higher than the connection point to the vertical autoclave 10. Excess gas flow from the top of the autoclave 10 to the outlet slurry pipe 14B can be achieved by a loop 25A of the excess gas duct 25 rising above the height equivalent to the head loss of the mineral slurry through the vertical autoclave 10 and outlet slurry pipe 14B as shown in Figure 3. When the pipe diameter of the loop 25A is sufficiently large to allow excess gas to disengage from the mineral slurry, then only gas will proceed to the outlet slurry pipe 14B. This situation is sometimes referred to as an airlock. This is of an advantage for mineral processes (and other chemical processes) that are not scaling in nature as described below.

The above described vertically disposed autoclaves 10 are exemplary of autoclaves suitable for reactor systems conducting hydrometallurgical methods as described, for example in the following examples and which may be compared with the method and system of the comparative example.

Example 1

A ground sulphidic gold-containing ore which is refractory to conventional cyanidation and which contains sufficient dolomite to make conventional acidic pressure oxidation uneconomic, for instance >8%, is ground and has had its pH elevated by the addition of lime (CaO), caustic soda (NaOH) or soda ash (Na ₂ C0 ₃ ) is fed to a vertical autoclave or series of verticals. The economic residence time is expected to be >2 hours at 200°C to 260°C with an oxygen overpressure of 3 to 7 barg. The equipment does not need to be made of exotic steels or nickel as in acidic pressure oxidation but can be made of carbon steel. There are various methods for heating the feed to achieve autothermal reaction conditions. Direct steam injection may be used. However, with the heat pipe heat exchanger in the Applicant’s co-pending International Application PCT/AU2018/050983, the contents of which are hereby incorporated herein by reference, improves heat recovery from the outlet slurry of the autoclave(s) 10 to the feed of the autoclave(s) 10 to reduce or eliminate heat input by steam injection or by any other means. The heat recovery need not be only from the exit of the last vertical autoclave but can also be from the exit of a previous vertical autoclave in a series of autoclaves, thus allowing for a more consistent high reaction temperature throughout the series of vertical autoclaves, and so minimise the required residence time as in Figure 13. Also, for non-autothermal reactions such as sulphidic ores, the heat pipe heat exchanger can eliminate the need for the autoclave charge pump 125 operating at high temperatures and the temperature limitations these pump’s wet ends currently impose to the process design (Figures 9 and 10); this is achieved by locating prior to some or all the heat recovery conducted by the heat pipe heat exchanger (Figure 1 1 ). This heat recovery design also eliminates the requirement of large flash tank pressure vessels, and condensate systems if shell and tube heat exchangers are used in association with the flash tanks.

Alkalis such CaO, NaOH or Na ₂ C0 ₃ can be added to the autoclave to maintain the desired pH as acid is generated through the oxidation process and thus maximise the kinetics of the reaction. The advantage of an alkaline pressure oxidation process as compared to the acidic pressure oxidation process is the carbonate in the feed does not react with the added reagents and the equipment can be constructed of carbon steel. The disadvantage is the higher operating temperature range of 200 to 260°C for the alkaline process compared to up to 230°C for the acidic process and lower residence time. The use of smaller volume vertical autoclaves in series, as above described and especially in conjunction with the heat pipe heat exchanger described in the Applicant’s co-pending International Patent Application No. PCT/AU2018/050893, incorporated herein by reference makes it more practical to design for the alkaline process’s desired temperature and pressure (as indicated in Figure 1 1 ). It is expected to be able to practically design above 285°C, thus reducing the required residence time with some or all the equipment fabricated from off-the-shelf piping (as indicated in Figure 12). Thus, a practical approach for smaller, modular process equipment that minimises economic risk. Example 2

Recovery of gold from pressure oxidation of sulphide ore is conducted using a vertical autoclave 10 including a number of agitator impellers 22. Air or oxygen can be injected to just below each agitator impeller 22. In addition, conical baffles 23 can be installed to improve excess gas flow from the compartment 1 1 below each agitator impeller 22 as shown in Figure 7. A multi-compartment autoclave 10 is used. The initial compartment 1 1 A of the vertical autoclave 10 can be significantly larger than the subsequent compartments 1 1 B as defined by the location of the annular baffles 20. The first vertical autoclave 10 in a series of vertical autoclaves 10 can be designed as a CSTR, with a low length to diameter ratio. A portion of the slurry in a downstream vertical autoclave in a series of vertical autoclaves can be recycled to the feed of the first vertical autoclave 10. Vertical autoclaves may be installed in parallel to reduce the residence time in the first vertical autoclave to achieve autothermal reaction temperatures.

Example 3 Referring to Figure 8, a sulphide concentrate mineral slurry feed in a pressure oxidation can be split and fed to different compartments 1 1 A, 1 1 B in a vertical autoclave 10 similar to that described in the case of Figure 7, thus reducing the heat required to bring the first compartment 1 1 A to the minimum autothermal reaction temperature, and the excess heat generated in the first compartment 1 1 A can then be used to minimise or eliminate the heat required in the subsequent compartments 1 1 B so that, in the lower part 10a of the vertical autoclave 10, less cooling is required. The sulphide mineral slurry is split and fed to the first compartment 11 A of the vertical autoclave 10 through inlet pipes 12A, and then to subsequent compartments 1 1 B, and also potentially to subsequent vertical autoclaves 10 to achieve the same minimisation of heat injection and cooling to the autoclave or autoclaves if there are more than one in series as shown in Figure 13. This efficiency in heat reduction of the split feed technique will be dependent on various factors such as the sulphide concentration in the sulphide mineral slurry feed.

Comparative Example 1

The method and apparatus may be applied to the high-pressure acid leaching of nickel laterite. Normally there is not enough heat generated by the leaching process (of oxides, weathered sulphides and silicates) so heat recovery is an important part of the hydrometallurgical process, i.e. non-autothermal. Figure 9 is a schematic of a typical nickel laterite high pressure acid leach (FIPAL) facility with a horizontal autoclave 1 10. Laterite slurry is preheated in direct heaters 120 called Splash Tanks. Pumps 126 forward the slurry to the next Splash Tank 120. A high-pressure (HP) pump 125, normally a positive displacement pump, pumps the slurry into the first chamber 1 14 of the horizontal autoclave 1 10. The materials of construction of the diaphragm in the HP pump can limit the temperature of the slurry that can be pumped and hence the extent of heat recovery. The feed slurry to the horizontal autoclave 110 can be around 190°C. The chambers 1 14 in the horizontal autoclave 1 10 are agitated (not shown), and the slurry overflows from one chamber 114 to the next while air or oxygen can be injected into each chamber to convert insoluble nickel sulphides into soluble sulphates. The succession of chambers 1 14 maximises the plug flow nature of the reaction profile and so minimises the required residence time to achieve the desired level of leaching. Steam is injected into the slurry in the autoclave 1 10 to bring the temperature up to the desired operating temperature, which can be 220° to 230°C. The slurry from the last chamber passes through a pressure letdown station and into a series of flash tanks 130 where the slurry flashes and the generated steam is used to preheat the feed slurry in the splash tanks 120. The splash tanks 120 can optionally be shell and tube heat exchangers; not shown. Lastly, the mineral slurry passes into a blow-off tank 135 where any excess temperature above atmospheric boiling point is flashed to atmosphere.

Example 4 Figure 10 replaces the single large horizontal autoclave 1 10 of Figure 9 with 5 vertical autoclaves 210 each with 4 compartments 11 A, 1 1 B (each being as shown in Figure 7). The feed temperature to the first vertical autoclave would be the same, mainly because the heat recovery system is the same and the maximum operating temperature of the positive displacement pump would be the same too. If the reaction profile were to stay the same, and the five vertical autoclaves were identical, then the effective reaction volume of each vertical autoclave would be 20% of the original horizontal autoclave. Flowever, there are now 20 compartments in series, and thus a more plug flow regime, and so the actual reactor effective reactor volume required for each vertical autoclave would be approximately <20% of the original horizontal autoclave.

Example 5

Figure 1 1 replaces the Splash Tanks 120 and Flash Tanks 130 of Comparative Example 1 and Figure 10 with the Applicant’s Fleat Pipe Heat Exchangers 200. In contrast, several heat pipe heat exchangers are disposed in series to take the feed slurry up to close to autoclave operating temperature. Since the heat exchangers can be pressurised, the feed HP Pump can be located before the heat exchangers thus removing the temperature constraint on the materials of construction of the pump’s wet end. As evident from International Patent Application No. PCT/AU2018/050893, the heat exchangers 200 can provide a significant improvement of approach temperature to the autoclave exit temperature than the flash system it replaces. Thus, for ores with moderate sulphide contents, it is feasible for the reactor to operate without the requirement for steam injection with the appropriate heat transfer surface area, or at least a significant reduction in steam injection. Now that the HP Pump 125 is located prior to the heat pipe heat exchangers, or at least part way along the series of heat exchangers, it is now possible to design for higher autoclave temperatures. Also, as application of the present invention makes it practical for a large horizontal autoclave 1 10 to be divided into five smaller vertical autoclaves 210A-210E, it is possible to design for higher temperatures and further reduce total reactor volume.

Example 6

Figure 12 replaces the vertical autoclave pressure vessels 1 10 and 21 OA-210E, of the Comparative Example (Figure 9) and Example 5 (Figure 1 1 ) with large diameter pipe vertical autoclaves 410 that could have agitators (not shown), or agitation by gas injection or internal static mixers, or any combination thereof. The replacement of pressure vessels with vertical pipes arranged as vertical autoclaves 410 is a means of developing new projects with smaller, modular construction using lighter, off-the-shelf piping. The heat pipe heat exchangers 200, each as described in International Patent Application No. PCT/AU2018/050893, also lends itself to modular construction using lighter off-the-shelf piping. Seamless piping is readily available up to 600mm (24 inch) but can also be supplied up to 1.67m (66 inch). So, given the above it is now possible to initiate high-pressure hydrometallurgical projects with smaller, energy efficient, modular installations, thus allowing the ramp up to be achieved quicker, and at reduce capital risk before committing to larger installations to achieve the economy of scale once all the associated project issues have been resolved in the initial phase such as staff training, process optimisation, etc.

Example 7 A system of vertical autoclaves 510, each as above described, is in the pressure oxidation of a high sulphide concentrate, e.g. a gold sulphide concentrate, and an example flowsheet is depicted in Figure 13. The feed concentrate slurry is preheated in a series of heat pipe heat exchangers 610 using the excess heat from the exothermic heat of oxidation, in this example after the second autoclave 510A. This example also uses the technique of a larger first compartment 510a and splitting of the feed from inlet manifold pipe 512 through inlet pipes 512A between the first three compartments 510a-c. The lower of the two agitator impellers 522 in the first compartment 510a is an impeller that pumps the slurry up a draft tube to the topmost Rushton impeller. This arrangement will reduce the net heating in the early compartments 51 Oa-c and the net cooling required in the later compartments 51 Od-m. It is expected that under certain conditions, the need for net heating and cooling can be eliminated. There are many variations on these techniques applied in the above example, for instance only the feed to the first compartment 51 Oa is preheated in heat exchangers 610 for which purpose it is directed through loop 513, or the first and second, or recovery of heat from the exits of both the first and second autoclaves, etc. The optimum combination of the techniques depends on the local conditions and unit costs.

Advantages

• Simple means to manage excess gas between successive vertical autoclaves, so no expensive control systems for successive autoclaves.

• Ability to achieve required residence time with smaller, easier to construct and install autoclaves

• Ability to modularise installation, smaller units, progressive installation, thus lower capital risk.

• Smaller, modular units are easier to install in remote locations

• Shorter time to achieve revenue stream.

• For a given throughput can easily add successive autoclaves to the design of a train of vertical autoclaves without the need for additional pumps to the process design.

• Shorter length of piping required than for horizontal tube autoclaves for the same residence time.

• Excess gas port or duct are low cost to maintain.

• No expensive automation required to maintain a slurry level and gas pocket in the autoclave which is important to protect any mechanical seal on the agitator shaft if an agitator is employed.

• Able to design excess gas pipe so there is no slurry bypass in low/non-scaling conditions.

• Ability to design excess gas port or duct to control scaling through erosion by mineral slurry.

• Pressure drop across each vertical autoclave will be low, defined by the pressure drop for the slurry travelling through the autoclave and up through the outlet slurry pipe to the point where the excess gas is injected, so erosion of excess gas port or duct material of construction due to entrainment of slurry should not be excessive. • The excess gas ports and short excess gas ducts to the outlet slurry are easy and cheap to maintain if erosion does occur.

• The vertical autoclaves can be constructed from standard piping for ease of construction and installation.

• Injection of liquid or gas streams into the autoclave can contribute to agitation of the autoclave.

• Ability to dedicate one or more autoclaves to different chemical duties and secondary reactions, for instance the injection of limestone slurry to precipitate iron oxide and generate carbon dioxide gas.

• In combination with heat pipe heat exchanger, no stepwise depressurisation to cool the slurry eliminating the many problems associated with the very high velocities of abrasive slurries between flash stages and flash tank inlet pipes.

• In combination with heat pipe heat exchanger, can reduce heat requirement; no flash steam interchange allowing a closer approach to autoclave temperature, and therefore a lower usage of Boiler steam.

• In combination with heat pipe heat exchanger, no steam side scale or deposits on indirect heat exchangers, such as shell and tube heat exchangers.

• In combination with heat pipe heat exchanger, eliminate the requirement for inter-stage pumping; no direct use of flash steam injection heat exchangers with all the problems associated with series slurry pumps and level/pressure control equipment.

Modifications and variations to the chemical processing apparatus and method described herein may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.