Abstract:
The temperature of a metallurgical slurry in a heater vessel is accurately controlled by heating a first portion of the slurry in the heater vessel by contact with a heating medium such as steam, and by adding to the heater vessel controlled portions of a slurry at lower temperature. The lower temperature slurry may be added to the heater vessel in an intermittent or continuous manner, with addition of the lower temperature slurry being regulated by a valve, the operation of which is controlled by a control device which receives temperature information from one or more temperature sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of International Application No. PCT/CA2002/001381, which has the international filing date of Sep. 12, 2002 and which was published under PCT Article 21(2) in English. 
   FIELD OF THE INVENTION 
   This invention relates to the heating of slurries in metallurgical processes. 
   BACKGROUND OF THE INVENTION 
   A number of metallurgical processes involve the treatment of aqueous slurries containing solid, metal-containing particles. In one such metallurgical process, an aqueous slurry of a mineral or metal-containing compound is subjected to leaching or oxidation at elevated temperatures and pressures in an autoclave. In order to maximize the efficiency of the leaching or oxidation process, it is preferred to preheat the slurry before feeding it into the autoclave. For this purpose, a heater vessel is provided upstream of the autoclave. The slurry is heated by contact with a heating medium inside the heater vessel. The heating medium typically comprises steam generated by an autoclave flashing process. 
   It is preferred that the temperature of the slurry be heated to within a narrow temperature range inside the heater vessel. The temperature of the discharge slurry is typically maintained by controlling the flash steam pressure in the apparatus (by venting). However, precise control of the slurry temperature is difficult to achieve in this manner as there are a number of other process variables such as autoclave throughput, heating medium composition, feed slurry specific heat capacity and boiling point elevation, which affect the quantity and quality of heating medium condensed by the process, and control of the vent gas flow is not always practical. 
   Accordingly, there is a need for improved apparatus and methods for heating slurries in a heater vessel. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the disadvantages of the prior art by providing an apparatus and method for heating a slurry to a predetermined temperature. According to the invention, a first portion of the slurry is heated in the heater vessel as in the prior art by contact with the heating medium. The heated slurry is combined with controlled portions of slurry at a lower temperature to accurately maintain the temperature of the heated slurry within the accepted range. The inventors have found that the method and apparatus for heating slurry according to the invention allows more accurate control of the temperature of the slurry heated within the heater vessel. 
   In one aspect, the present invention provides an apparatus for heating a slurry to a predetermined temperature, comprising: (a) a heater vessel having a slurry heating section and a slurry collection section downstream of the slurry heating section, wherein slurry heated in the heating section is collected in the collection section; (b) a heating medium inlet through which a heating medium for heating the slurry enters the heater vessel; (c) a first slurry inlet through which a first portion of the slurry enters the vessel and contacts the heating medium in the heating section; (d) a second slurry inlet through which a second portion of the slurry enters the vessel, the second slurry inlet being located relative to the heating section such that a temperature of the second portion of the slurry entering the collection section is lower than a temperature of the first portion of the slurry entering the collection section; (e) a slurry outlet adjacent the collection section; (f) temperature sensing means for sensing the temperature of the slurry in the collection section; and (g) a valve for controlling flow of the second portion of the slurry into the vessel through the second slurry inlet so as to control the temperature of the slurry in the collection section. 
   In another aspect, the present invention provides a method for heating a slurry to a predetermined temperature in a heater vessel having a slurry heating section and a slurry collection section downstream of the slurry heating section, the method comprising: (a) feeding a heating medium into the heater vessel such that the heating medium passes through the heating section; (b) feeding a first portion of the slurry into the heater vessel such that the first portion of the slurry passes through the heating section and is heated by the heating medium; (c) controllably feeding a second portion of the slurry into the heater vessel such that it bypasses at least a portion of the heating section; (d) collecting the first and second portions of the slurry in the collection section, wherein a temperature of the second portion of the slurry as it enters the collection section is lower than a temperature of the first portion of the slurry as it enters the collection section; and (e) monitoring a temperature of the slurry collected in the collection section; wherein the feeding of the second portion of the slurry into the heater vessel is controlled such that when the temperature of the slurry collected in the collection section exceeds the predetermined temperature, the second portion of the slurry is added into the heater vessel to lower the temperature of the slurry in the collection section, and when the temperature of the slurry in the collection section is less than the predetermined temperature, addition of the second portion of the slurry to the heater vessel is diminished or discontinued. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawing, in which: 
       FIG. 1  is a schematic view of a preferred apparatus according to the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A preferred method and apparatus according to the present invention will now be described with reference to  FIG. 1 . 
   In the preferred apparatus  10  illustrated in  FIG. 1 , a heater vessel  12  is provided for heating a slurry. The heater vessel  12  comprises a slurry heating section  14  and a slurry collection section  16  which is located downstream of the heating section  14 . In the specific embodiment shown in  FIG. 1 , the heating section  14  and collection section  16  are arranged vertically relative to one another with the heating section  14  being located in an upper portion of heater vessel  12  and the collection section  16  being located in a lower portion of heater vessel  12 . 
   The slurry heating section  14  preferably comprises a plurality of baffles  18  projecting inwardly from the walls of heater vessel  12 , and angled downwardly toward the collection section  16 . A heating medium for heating the slurry enters the heater vessel  12  through a heating medium inlet  20 . The heating medium is preferably a gas, most preferably steam which is generated by an autoclave flashing process. The steam flows in the direction of arrow A through a pipe  22  which carries the heating medium to the inlet  20 . As shown in the drawing, the heating medium is fed into the heater vessel  12  downstream of heating section  14 , preferably being located below the lowermost baffle  18 . The gaseous heating medium then flows upwardly through the heating vessel  12 , around baffles  18 , and exits the vessel  12  in the direction of arrow B through a vent pipe  24 . 
   A first portion of the slurry enters the heater vessel  12  through a first slurry inlet  26 . The first portion of the slurry is then pumped through a perforated pipe  28  within the heater vessel  12 , and is sprayed from the perforations in the pipe into the interior of the heater vessel  12 . The first portion of the slurry then passes downwardly by gravity through the heating section  14  of vessel  12 , passing down across the surfaces of the baffles  18  in the direction of the solid arrows shown inside the vessel  12 . As the first portion of the slurry passes downwardly through heater section  14 , it is brought into contact with the heating medium rising upwardly through the vessel  12 , thereby increasing the temperature of the slurry. 
   The first portion of the slurry, heated as described above, is collected in collection section  16 . The approximate level of the collected slurry inside section  16  is designated as numeral  30 . 
   A slurry outlet  32  is provided adjacent the slurry collection section  16 , and is preferably provided at the lower end thereof. From the outlet  32 , the heated slurry is pumped in the direction of arrow C through a pipe  34 , which may for example lead to an autoclave in which the slurry is subjected to high pressure, high temperature leaching. 
   The apparatus  10  further comprises a second slurry inlet  36  through which a second portion of the slurry enters the vessel  12 . The second slurry inlet is located relative to heating section  14  such that a temperature of the second portion of the slurry entering the collection section  16  is lower than a temperature at which the first portion of the slurry enters the collection section  16 . In other words, the second slurry inlet  36  is located such that the second portion of the slurry bypasses at least a portion of the heating section, thereby ensuring that the second portion of the slurry will be at a lower temperature than the first portion of the slurry when it enters the collection section  16 . Most preferably, as shown in  FIG. 1 , the second slurry inlet  36  is located downstream of the heating section  14 , below the lowermost baffle  18 , such that the second portion of the slurry flows directly into the collection section  16  without passing through the heating section  14 . 
   Thus, the first and second portions of the slurry are combined in the collection section  16 , with the relatively unheated second portion of the slurry cooling the heated first portion of the slurry. 
   In order to ensure that the temperature of the slurry in the collection section is accurately controlled, the flow of the second portion of the slurry through inlet  36  must be controlled. Accordingly, the apparatus  10  also includes at least one temperature sensor  38  for monitoring the temperature of the slurry in the collection section  16 . Temperature data collected by the at least one temperature sensor  38  is communicated to a control device  40 , which controls operation of a regulating valve  42 , which regulates flow of the second portion of the slurry through inlet  36 . This permits the pumping of the second portion of the slurry into heater vessel  12  to be controlled such that when the temperature of the slurry collected in the collection section  16  exceeds a predetermined temperature, the second portion of the slurry is added into heater vessel  12  to lower the temperature of the slurry in the collection section  16 . Conversely, when the temperature of the slurry in the collection section is less than the predetermined temperature, addition of the second portion of the slurry to the heater vessel  12  is diminished or discontinued. 
   The valve  42  is preferably of a type which is designed for handling abrasive slurries, and which is capable of adjusting the flow of slurry to the heater vessel  12 . That is, the flow of slurry through the valve is preferably controllable between a zero flow rate and a maximum flow rate, thereby providing precise control over the temperature of the slurry within heater vessel  12 . In fact, it may be preferred to maintain a continuous, variable flow of slurry through valve  42 . One preferred type of valve is the Valtek® Survivor™ control valve. Such valves have a sweep angle design to provide high flow capacity while minimizing impingement of particles on the valve body; ceramic trim to protect the valve from erosion; wiper rings to provide longer packing life; a clamped seat design to minimize erosion, simplify maintenance and provide high flow capacity; and an extended-venturi seat design to protect the valve body from erosive damage. 
   In a preferred embodiment of the invention, the apparatus  10  is operated continuously, with the first portion of the slurry  26  being pumped continuously into heater vessel  12  through inlet  26 , the heating medium being continuously fed into heater vessel  12  through inlet  20 , and the heated slurry collected in collection section  16  being continuously pumped from the heater vessel  12  through outlet  32 . The second portion of the slurry is added to the heater vessel intermittently in order to control the temperature of the slurry in collection section  16 . As shown in  FIG. 1 , the first and second portions of the slurry are delivered to the respective inlets  26  and  36  through pipes  44  and  46 , with the valve controlling flow of the slurry preferably being located in pipe  46  through which the second portion of the slurry is pumped. 
   Preferably, the first and second portions of the slurry originate from the same source, with the combined slurry preferably being pumped in the direction of arrow D through a pipe  48  having a branch at  50  which divides the flow of slurry into pipes  44  and  46 . 
   Although the present invention has been described with reference to a preferred embodiment, it will be appreciated that the present invention is not limited thereto. Rather, the invention includes all embodiments which may fall within the scope of the following claims.