Abstract:
A method is disclosed for recovering a metal from a metal containing material by autoclaving with an autoclave which includes an agitator that includes a first inlet for a recycled oxygen-containing gas, a second inlet for a fresh oxygen-containing gas, an impeller having a plurality of outlets to recirculate or introduce the oxygen-containing gases into the slurry and a set of mixing blades located below the outlets to radially disperse gas bubbles substantially uniformly throughout the slurry. The outlets for the gas can be located in an intermediate set of blades for thorough dispersion of the gas bubbles in the slurry.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to autoclaves and specifically to autoclaves having high rates of oxygen transfer to metal-containing solutions. 
     BACKGROUND OF THE INVENTION 
     To oxidize sulfide sulfur and thereby permit solubilization of metals compounded with the sulfide sulfur, base metal ores and concentrates, and refractory gold ores and Concentrates are commonly treated by pressure oxidation. Pressure oxidation is typically performed by passing a feed slurry of a metal-containing material through a sealed autoclave (operating at superatmospheric pressure) having multiple compartments. To provide for oxidation of the sulfide sulfur in the slurry, oxygen is typically fed continuously to the autoclave by means of a sparge tube located below the impeller. Commonly a large portion of the oxygen reacts with the sulfide sulfur, but there is a smaller significant portion that is vented from the autoclave and may be considered not effectively utilized. 
     In designing an autoclave, there are a number of considerations. By way of example, the autoclave should permit reaction of as much of the oxygen as possible with sulfide sulfur. If the oxygen is inefficiently reacted with the sulfide sulfur, the autoclave can have higher oxygen plant capital and operating costs. The autoclave should provide as short a residence time as possible for a given volume of slurry while realizing a high rate of recovery for the metal. Finally, the autoclave should vent inert gases that build up in the autoclave above the slurry to prevent rupturing of the autoclave from high pressure gas. Some oxygen gas is inevitably vented along with these inert gases. Other processes, which rely on efficient and effective gas/liquid transfer of oxygen and which are commonly carried out in autoclaves, include catalytic chemistry reactions, such as the conversion of ferrous to ferric ions, reoxidation of NO by oxygen, and cuprous amine conversion to cupric amine. 
     SUMMARY OF THE INVENTION 
     These and other design objectives are satisfied by the autoclave of the present invention. The autoclave includes a vessel for containing a feed slurry material, such as a metal sulfide-containing slurry, or a liquid comprising dissolved chemical compounds and an impeller attached to a rotatable shaft for agitating the feed slurry material. The shaft has a passage for an oxygen-containing gas and an outlet in communication with the passage for dispersing the oxygen-containing gas in the slurry. In one configuration, the passage passes along the length of the rotatable shaft, and the outlet is located at or close to the tip of the impeller. 
     The autoclave can realize relatively high oxygen transfer rates to the feed slurry material relative to conventional autoclaves through better oxygen gas dispersion in the feed slurry material. Commonly, the autoclave can yield an oxygen transfer rate of at least about 2 kg moles oxygen/cubic meter of slurry/hour. At such high oxygen transfer rates, a high rate of metal recovery can be realized in a relatively short residence time, and therefore lower capital and operating costs for the autoclave equipment can be realized relative to conventional pressure oxidation processes. 
     The autoclave is able to accomplish such high oxygen transfer rates without the use of a sparge tube. The sparge tube has proven to be an ongoing source of maintenance problems in existing pressure oxidation processes. 
     To consume as much oxygen as possible, the rotatable shaft can have an inlet for the oxygen containing gas located at an upper end of the shaft that is above the slurry surface yet is contained within the vessel. The inlet will provide a suction, drawing the atmosphere in the autoclave into the passage. After passing through the passage, the gas is dispersed into the feed slurry material. In this manner, the oxygen is continuously recycled during pressure oxidation to provide a high rate of oxygen utilization. By efficiently reacting the oxygen, the autoclave can have lower oxygen plant capital and operating costs than conventional autoclaves. 
     New oxygen can be supplied to the autoclave either directly through the rotatable shaft or through a separate conduit such as one having an outlet in close proximity to the impeller shaft gas inlet or above the feed slurry material. In the latter case, the shaft must include the inlet at the upper end of the shaft to permit oxygen escaping from the agitated feed slurry material into the autoclave atmosphere and/or supplied to the atmosphere to be drawn into the shaft and thereby entrained in the agitated feed slurry material. 
     Autoclaves can include a discharge control means for controllably removing the gas atmosphere from the sealed autoclave to prevent rupture of the autoclave from high pressure gases. The system includes: 
     (a) analyzing means (e.g., a gas analyzer) for analyzing a selected component (e.g., carbon dioxide and/or molecular oxygen) in the gas atmosphere inside the autoclave; 
     (b) an outlet for removing gas in the gas atmosphere from the autoclave interior; 
     (c) a controller (e.g., a computer) for receiving a signal from the gas analyzer and generating a control signal in response thereto; and 
     (c) a control means (e.g., a valve) for controlling the amount of gas removed in response to the control signal received from the controller. The control means vents the gas atmosphere when the amount of the component exceeds or falls below a threshold amount. In this manner, the autoclave can vent oxygen gas and other gases that build up in the autoclave above the slurry while maintaining the oxygen gas in the autoclave as long as possible for consumption in the oxidation of sulfide sulfur. 
     In operation, pressure oxidation using the autoclave follows the following steps: 
     (a) agitating a feed slurry material in the autoclave using the impeller, and 
     (b) during the agitating step (a), passing an oxygen-containing gas through the rotatable shaft and dispersing the gas radially outward from the shaft into the feed slurry material. In one autoclave configuration, the gas is passed through a blade of the impeller outwardly into the slurry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a side view of the interconnected impeller and the rotatable shaft of the agitation assembly, with certain parts of the agitation assembly being shown in cross-section; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 depicts the agitation assembly operating in an autoclave; and 
     FIG. 4 is a flow schematic depicting the discharge control system. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a sealed autoclave particularly useful for pressure oxidation of slurried ores and concentrates. Although the autoclave is discussed with reference to leaching processes, the autoclave is useful in numerous other applications including catalytic chemistry reactions. The autoclave includes an agitation assembly for discharging oxygen directly into the slurry. In this manner, the autoclave is able to realize relatively high rates of oxygen transfer into the slurry and, therefore, high oxidation rates and low residence times. The autoclave is particularly effective in the pressure oxidation of slurried metal sulfide-containing materials. The metal sulfides that can be effectively utilized include without limitation gold sulfides, iron sulfides, copper sulfides, zinc sulfides, nickel sulfides, and arsenic sulfides. 
     Referring to FIGS. 1 and 2, the agitation assembly  10  is depicted. The agitation assembly  10  includes a rotatable shaft  14 , a gas injecting impeller  18  and a mixing impeller  22  connected to the lower end of the shaft  14 , and a motor (not shown) connected to the upper end of the shaft  14  for rotating the shaft  14  during pressure oxidation. 
     The rotatable shaft  14  includes a gas inlet  26  in communication with a conduit  30  extending longitudinally along the shaft  14 . The conduit  30  is in communication with a number of conduits  34   a-d  in the gas injecting impeller  18  for dispersing the gas substantially uniformly throughout the slurry. A fresh oxygen-containing gas  37  from an oxygen supply plant or the ambient atmosphere can be introduced to the slurry via an inner conduit  25 , the conduit  30 , and finally radially outward through the conduits  34   a-d . An oxygen-containing gas  38  is recycled from the autoclave atmosphere via inlet  26  (which is open to the autoclave interior) because rotation of the impeller  18  creates a negative pressure at the tips  78   a-d  of the blades which draws the gas through the inlet  26 . The fresh oxygen-containing gas  37  mixes with the recycled oxygen-containing gas  38  downstream (or below) the outlet  39  of the inner conduit  25  and the mixed gas is outputted by the conduits  34   a-d.    
     The relative orientations and dimensions of the inlet  26  and shaft conduit  30  are important. The longitudinal axis  42  of the conduit  30  is substantially normal (i.e., transverse) to the longitudinal axis  46  of the inlet  26 . The conduit  30  and shaft  14  are coaxial and therefore have the same longitudinal axis  42 . The relationship between the cross-sectional area of the inlet  26  normal to the direction of flow (i.e., normal to the inlet longitudinal axis  46 ) depends upon a number of factors including the desired oxygen transfer rate, the compartment size of the autoclave, the operating oxygen partial pressure, the slurry viscosity, and the like. 
     The bottom  62  of the conduit  30  is may be conically shaped in a convex orientation to effectuate redirection of the gas into the conduits  34   a-d  of the impeller  18 . In this manner, eddies and other disturbances in the gas flow in response to the sudden change of direction are substantially minimized. 
     To facilitate dispersion of the gas in the slurry, the gas injecting impeller  18  has the outlet face  74   a-d  of each impeller blade  70   a-d  angled away from the direction of rotation of the gas injecting impeller  18  such that a shear zone exists at the tip  78   a-d  of each blade  70   a-d  to provide superior atomization and dispersion of the oxygen-containing gas (and therefore finer bubble formation). The outlet face  74   a-d  of each conduit  34   a-d  faces away from the direction of rotation while the longest side of the blade  70   a-d  faces in the direction of flow. The angle between the outlet face  74   a-d  and the tangent  82  of a circle defined by rotation of the tips  78   a-d  of the blades  70   a-d  is preferably about forty-five degrees. 
     The gas injecting impeller  18  is located at a depth in the autoclave slurry that maximizes effective gas transfer and dispersion. Locating the impeller below this optimum depth increases the hydraulic head that the impeller has to overcome to draw down the gas phase into the agitated slurry. This can significantly and unnecessarily increase the power required to maintain a given oxygen transfer rate. 
     The mixing impeller  22  is located below the gas injecting impeller  18  at a suitable depth to maintain in suspension the solid particles in the autoclave in the slurry and to assist in distribution of the entrained gas bubbles in the slurry. Typically, the concentration of gas bubbles in the upper portion of the slurry (which contains the gas-injecting impeller  18 ) is greater than the gas bubble concentration in the lower portion of the slurry (which contains the mixing impeller  22 ). 
     Referring to FIG. 3, the operation of the agitation assembly will be described. During introduction of the sulfide-containing slurry  86  into the autoclave  90 , the rotatable shaft  94  is rotated in a clockwise direction to induce turbulence in the slurry. Unlike the rotatable shaft  14  of FIGS. 1 and 2, the rotatable shaft  94  of FIG. 3 has a plurality of open inlets  98  that are open to the atmosphere in the autoclave  90  and an inner conduit extending the length of the shaft  94  to transport fresh oxygen gas from a source exterior to the autoclave. Rather, fresh oxygen  106  is introduced directly into the autoclave atmosphere via inlet  91  and drawn into the open inlets  98  and through a conduit (not shown) extending longitudinally along the shaft and finally through the blades and dispersed into the slurry. A vortex  102  forms where the shaft  94  is immersed in the slurry  86 . An oxygen gas  106  is introduced into the autoclave and mixed with recirculated gas  110  drawn into the shaft via the inlets  98 . The mixed gas  114  is dispersed radially outwardly, during rotation of the blades  70   a-d , in the slurry  86 . The mixing impeller  22 , which rotates at the same rate and in the same direction as the gas injecting impeller  18 , further assists in dispersing the gas bubbles  118  throughout the slurry  86 , maintains in suspension the solid particles in the slurry, and provides a turnover of slurry from the bottom to the top of the vessel on a continuous basis. 
     The autoclave  90  is able to realize high oxygen transfer rates into the slurry  86 . Typically, the oxygen transfer rate is at least about 2 kg moles and more typically at least about 4 kg moles and most typically ranges from about 2 kg moles to about 12 kg moles of molecular oxygen/cubic meter of slurry/hour. At such high transfer rates, the conversion of the metal sulfides into soluble metal salts or oxidized metal precipitates can be substantially completed (i.e., 90% or more) in residence times as short as about 60 minutes and more typically in as short as about 30 minutes. 
     FIG. 4 depicts a discharge control system for controllably removing the gas atmosphere from the autoclave  90  to prevent rupture of the autoclave  90  from high pressure gases. The system  130  includes a gas analyzer  134  for analyzing, either continuously or at suitable intervals of time, a selected component in the gas atmosphere in the autoclave  90 , a vent  138  for venting the gas in the atmosphere, a controller  142  to monitor the signal  144  from the gas analyzer  134  and generate a control signal  146  in response thereto, and a control device  150  for controlling the amount of gas discharged into the exterior atmosphere in response to the control signal  146 . 
     The selected component monitored by the gas analyzer  134  can be molecular oxygen, carbon dioxide, argon, and nitrogen, with molecular oxygen being most preferred. 
     When a threshold concentration, or partial pressure, of the selected component is reached, the controller  142  forwards a control signal to the control device  150  to open and release gas in the autoclave atmosphere. Preferably, the threshold is set such that the ratio of the partial pressure of oxygen to the partial pressure of nonoxygen compounds (e.g., carbon dioxide) ranges from about 1:4 to about 4:1 and more preferably from about 1:2 to about 2:1. Accordingly, when the partial pressure of oxygen drops below a certain level, i.e., when the ratio falls below the threshold, the control device  150  opens and the autoclave gas phase is vented to the atmosphere. Fresh “pure” oxygen is introduced at this time to maintain the autoclave operating pressure setpoint. The control device  150  closes either after the valve has been opened for a specified predetermined time or alternately, may be closed when the partial pressure of oxygen is restored to a specified setpoint. 
     While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims.