Abstract:
A system  10  is provided to recycle a waste gas stream  32  of an autoclave  20  to re-use oxygen in the waste gas stream  32  to compliment an oxygen feed stream  19  to the autoclave  20 , thereby recycling the waste gas stream  31  to avoid the necessity of having to continuously purge the waste stream  32  of contaminants, while also reducing the amount of oxygen necessary in the feed stream  19  to the autoclave  20 . The waste gas stream  32  is also pressurized to be substantially similar in pressure to that of the feed stream  19  to the autoclave  20.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to systems wherein a waste gas stream containing unused oxygen is recycled to an autoclave inlet to be mixed with oxygen input to meet the oxygen demand in the autoclave and in particular, to autoclave waste gas streams associated with nickel leaching processes.  
           [0003]    2. Description of the Prior Art  
           [0004]    In a nickel leaching process, such as that pertaining to acid leach or ammonia leach, oxygen is supplied in a pressurized gas stream to an autoclave. Only a portion of the oxygen contained in the stream is consumed (typically from about 40-60%), while the remainder of the oxygen is rejected or exhausted in an oxygen depleted exhaust gas stream. The exhaust stream also contains ammonia (and sometimes carbon dioxide) which must be recovered downstream before the gas is vented external to the process or into the environment.  
           [0005]    Depending upon the nickel leaching process being used, the exhaust stream may be at an elevated pressure.  
           [0006]    When air is supplied to an autoclave, a portion of the oxygen contained in the air is consumed by the autoclave. The oxygen depleted exhaust stream exits the autoclave to be passed to a scrubbing system where the ammonia in the system is recovered from the leach system.  
           [0007]    The above systems therefore have the disadvantage that a considerable portion of the oxygen in the feed air to the autoclave is exhausted i.e., wasted during the process. In addition, the exhaust stream must be scrubbed of harmful products and chemicals before it is vented to the environment.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0008]    The autoclave waste gas recycle system according to the present invention recompresses and recycles the waste gas stream (“waste stream”), for example, from a Nickel Leach Autoclave. The waste stream having an oxygen content is mixed with feed oxygen to meet the autoclave oxygen demand. The system of the present invention recirculates the oxygen remaining in the waste stream to be re-used and thereby reduce the amount of air or oxygen required to meet the oxygen demand at the feed for the autoclave.  
           [0009]    In particular, there is provided a system for recycling a waste stream of an autoclave which includes an autoclave having an inlet for receiving a feed stream at a first pressure and with a first oxygen content, an outlet for exhausting a waste stream at a second pressure and with a second oxygen content less than the first oxygen content; and circulating means interconnecting the inlet and the outlet of the autoclave for circulating the waste stream to the inlet at a pressure greater than the second pressure for being mixed with the first oxygen content of the feed stream.  
           [0010]    The present invention also provides a method of recycling oxygen for use in an autoclave and includes the steps of providing a feed stream with a first oxygen content to an autoclave; exhausting a waste stream with a second oxygen content from the autoclave; pressurizing the waste stream exhausted from the autoclave; circulating the pressurized waste stream to the feed stream; and mixing the pressurized waste stream with the feed stream for the second oxygen content to be combined with the first oxygen content.  
           [0011]    The oxygen concentration in the feed gas stream to the autoclave can be adjusted, such as enriched, to meet the demand of the inventive process, and the recycling feature enables this to be done without additional oxygen loss.  
           [0012]    The recycling of the autoclave waste stream substantially reduces the requirement for continuous downstream ammonia recovery thereby reducing operating costs and losses.  
           [0013]    In the present invention, the waste stream exiting the autoclave is also recompressed or pressurized for delivery at the feed pressure, mixed with the feed oxygen to restore the concentration necessary for introduction at the feed inlet and delivered to the autoclave. Accordingly, the oxygen that would have been vented from the waste stream as an unused by-product is now re-used and consumed in the process of the present invention and thus, a reduced amount of oxygen (as air or pure oxygen) needs to be supplied at the autoclave inlet.  
           [0014]    The amount of oxygen in the feed gas stream can be adjusted to a concentration best suited to the process conditions and to take into account the introduction of the additional oxygen that will be provided from the recycled waste stream.  
           [0015]    The present invention can be applied to Oxide ore concentrates, hydroxide precipitates (all laterite mineral derived), as well as sulfide concentrates and mattes.  
           [0016]    Objects and Advantages of the System of the Present Invention Include:  
           [0017]    1. Improved oxygen utilization. Nearly complete oxygen utilization is achieved, thus reducing oxygen supply costs.  
           [0018]    2. Reduced power consumption. For a high pressure leach system (e.g., one operating at 10 bar), there are significant savings in power consumption. The energy to recompress the waste stream from 7 bar at the autoclave exhaust to 10 bar at the autoclave feed inlet, combined with the energy to provide the make-up oxygen at 10 bar, is considerably less than that associated with a single pass air system.  
           [0019]    3. Eliminate ammonia recovery/losses. The waste stream from the autoclave contains a significant amount of gaseous ammonia. This has to be recovered for economic and environmental reasons before the stream can be vented. Venting requires a scrubbing system to dissolve the ammonia in water and a stripping system to concentrate the ammonia solution for re-use. By recycling the waste stream, instead of immediately exhausting the waste stream from the system, these requirements are substantially reduced if not eliminated, giving both capital benefits and operational savings in the utilities required (steam and cooling water).  
           [0020]    4. Eliminate carbon dioxide recovery losses. Where carbon dioxide is dissolved as part of the process, recycling the waste stream reduces the losses.  
           [0021]    5. Ability to control oxygen concentration. This will increase the efficiency/throughput of the leach system by increasing the oxygen concentration above that of an air based system. Recycling of the waste stream provides for an economic option, since the present invention eliminates the oxygen losses that would otherwise have ensued from this approach. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    For a more complete understanding of the present invention, reference may be had to the following detailed description taken in conjunction with the accompanying drawings, of which:  
         [0023]    [0023]FIG. 1 is a diagram of a preferred embodiment of the waste gas recycle system according to the present invention; and  
         [0024]    [0024]FIG. 2 is a diagram of another preferred embodiment of a feed stream for the system of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring to FIG. 1, there is shown an air based reactor, such as an autoclave, retrofitted to provide an oxygen based recycle system for the autoclave according to the present invention shown generally at  10 .  
         [0026]    Air  12  introduced to a process air compressor  14  is subsequently provided at an increased pressure to an air separation unit (ASU)  16 . The compressor  14 , instead of providing air to the autoclave, now provides air for the ASU  16 .  
         [0027]    The ASU  16  provides a first exhaust stream  18  containing oxygen at for example, 10 bar(g) to an autoclave  20 , as well as secondary exhaust streams  22 A-C of liquid products, for example liquid oxygen, liquid nitrogen, crude liquid argon, respectively, for the merchant market and gaseous nitrogen for other site users. The ASU  16  can be an “internal compression cycle” type.  
         [0028]    Some of the air  12  provided by the process air compressor  14  can be further compressed in a booster air compressor  24  to provide a high pressure air stream  26  (25-60 bar) of which part is used to vaporize a pumped liquid oxygen stream to provide the 10 bar(g) oxygen (see 18 below), while a further portion is expanded to provide the refrigeration for liquid production as discussed hereinafter.  
         [0029]    The first exhaust stream  18  becomes a feed stream  19  having oxygen from the exhaust stream  18 . The feed stream  19  for the autoclave  20  is mixed with a recycled waste gas stream  28  to achieve a combined feed stream oxygen purity of around 21%. The mixing of streams  19 ,  28  preferably occurs external to the autoclave  20  at a pre-mixer apparatus  23  as shown in FIG. 2, so that a single feed stream  25  enters the autoclave  20 . Alternatively, the streams  19 ,  28  can be introduced separately as in FIG. 1 for mixing within the autoclave  20 . This mixing as would result from either of the embodiments of FIGS. 1 and 2 maintains the same operating conditions in the autoclave  20  as occurs with the original air fed operation. The recycled stream  28  has an oxygen content of about 10%, while the feed stream  19  has an oxygen purity of around 99.5%, with Argon being its main impurity. High purity oxygen is used to minimize the buildup of inerts in the system  10 . A lower oxygen purity can be used, such as 95%, but this would result in a higher loss at a purge stream  30  or  30 A of the system  10 .  
         [0030]    The combined streams (recycle waste  28  plus feed  19 ) are provided to the autoclave  20  as in either of FIGS.  1  or  2 , where a significant portion of the contained oxygen is consumed in the autoclave  20 . A waste gas stream  32  (“waste stream”) exits the autoclave  20  at about 7 bar(g) and is depleted in oxygen relative to the feed stream  19 . A typical oxygen content of approximately 10% can be expected in the waste stream  32 .  
         [0031]    The waste stream  32  also contains significant quantities of ammonia, one of the reactants in the process. The ammonia would have to be removed in a scrubbing and recovery system if the waste stream  32  were to be vented to the atmosphere. Instead, the waste stream  32  is recompressed (pressurized) and recycled at a recompressor device  34  to 10 bar(g) to provide the waste stream  28  to the feed stream  19  of the process. In a preferred embodiment, the recompressor device  34  includes components constructed and arranged to provide an integral unit to both recirculate the waste stream  32  and pressurize the waste stream  32 . Accordingly, both the unused oxygen and the ammonia in the waste stream  32  are recycled in the process.  
         [0032]    The energy required to recompress the waste stream  32  and to provide the make-up or additional oxygen (i.e., enhance the oxygen introduced into the autoclave  20 ) is considerably less than that required to provide air only to the process. The costs of recovering the ammonia are also substantially reduced, if not eliminated.  
         [0033]    Sensing means is also provided for the system  10 . Sensor  21 A is provided to be in communication with the streams  18  and  19 , to thereby sense oxygen content and pressure of the streams  18 ,  19 . Sensor  21 B is in communication with the waste stream  32 , to thereby sense oxygen content and pressure of the stream  32 . Sensor  21 C is in communication with the recycle waste stream  28  to thereby sense the oxygen content and pressure of the stream  28 . Monitoring of the sensors  21 A-C is to be aware of and control the oxygen content and pressure of the streams  18  and  19 ,  28  and  32  so that the stream  32  is properly pressurized at recompressor  34  for recycling the stream  28 , depending upon the oxygen content sensed in the stream  32 . Monitoring of the sensors  21 A-C enables controlling of the streams  18  and  19 ,  28  and  32  and also determines when the stream  32  must be purged at  30 .  
         [0034]    In an alternative embodiment, the sensors  21 A-C will also sense the flow rate of the respective streams  18  and  19 ,  32  and  28  to facilitate controlling the flow rate.  
         [0035]    Continuous or batch purging of inerts or ammonia in the streams  28 ,  32  can be used.  
         [0036]    The stream  30  is provided to purge the waste gas stream  32  of the accumulation of inerts in the process gas. The principal source of the inerts is the 0.5% Argon content in the feed stream  19 . Purging of the waste stream  28  can also occur at purge steam  30 A, which would not impact greatly the pressure or power demand to control the stream  28 .  
         [0037]    The process system  10  of the present invention can also, for example, provide specific requirements for the site in which the system is provided for operation. Referring to FIG. 1, if there is a demand for a high pressure nitrogen flow  36  for refrigeration for example, this demand can be met by compressing a low pressure nitrogen flow  38  from the ASU  16  which is providing the oxygen for the autoclave  20 . The flow  38  is compressed at compressor  40  to provide the high pressure nitrogen flow  36 . It is also possible to withdraw a portion of the nitrogen at the ASU  16  as a medium pressure nitrogen flow  42  at approximately 5 bar(g) and feed the medium pressure nitrogen flow  42  to the compressor  40  which permits a reduction in the power necessary at the compressor  40  to provide the high pressure flow  36 .  
         [0038]    Parameters of the features/elements of the present invention are provided below by way of example:  
                                                       12 air   38,675 nm 3/h           14 process air compressor   2950 kW           18 first exhaust stream   210 tons/day gaseous oxygen           20 autoclave   65% oxygen consumption           22A liquid oxygen   60 tons/day           22B liquid nitrogen   14 tons/day           22C crude liquid argon   3 tons/day           24 booster air compressor   1600 kW           28 recycled waste gas stream   33,300 nm3/h; 10% oxygen           34 recompressor device   540 kW           36 high pressure nitrogen flow   25 bar(g)           38 low pressure nitrogen flow   13,000 nm 3/h; 0.05 bar(g)           40 compressor   1830 kW                      
 
         [0039]    It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.