Abstract:
A method is disclosed for controlling retention time in a reactor, such as an autoclave, having a plurality of compartments separated by dividers with underflow openings. A retention time of the reaction mixture is calculated and compared with an optimal retention time, and the volumes of the reaction mixture in the compartments are adjusted while maintaining the flow rate of the reaction mixture, so as to change the retention time to a value which is closer to the optimal retention time. The reactor may include a level sensor in the last compartment for generating volume data; a control valve for controlling the liquid level in the last compartment; and a controller which receives volume data from the level sensor and controls operation of the control valve.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CA2013/050629, filed on Aug. 15, 2013, the contents of which are hereby expressly incorporated by reference into the detailed description hereof. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to multi-compartment reactors and methods for their operation, and includes methods for controlling retention time in multi-compartment reactors. 
     BACKGROUND OF THE INVENTION 
     Multi-compartment reactors such as autoclaves are used in numerous chemical processes, and are commonly used in hydrometallurgical processes to recover metal values from aqueous slurries of ores or concentrates. For example, multi-compartment autoclaves are typically pressurized, cylindrical vessels having a plurality of fixed dividers separating adjacent compartments, with each compartment having an agitator and means for injecting an oxidizing gas into the stirred slurry. 
     As the slurry flows through the autoclave, it passes sequentially through each successive compartment until it reaches the last compartment, from which it is withdrawn as a product mixture for further processing. The slurry must have a sufficient residence or retention time in each compartment, and a sufficient overall retention time in the autoclave, to ensure that the chemical conversion is as complete as possible, in order to maximize metal recovery. The retention time in each individual compartment and the overall retention time in the autoclave are proportional to volume and inversely proportional to flow rate. 
     Autoclaves have traditionally been configured to permit cascading flow of slurry over the tops of the dividers. Autoclaves of this type are sometimes referred to herein as “overflow autoclaves”. The dividers in an overflow autoclave are progressively decreased in height throughout the length of the autoclave to provide a head drop between adjacent compartments. The slurry level and volume of each compartment except the last compartment are fixed by the heights of the dividers. The flow rate is largely determined by the height of the head drop between the compartments, with some limited variability in flow rate being caused by fluctuations in the feed rate of slurry entering the autoclave. Therefore, the flow rate is substantially constant or fixed. While the liquid level in the last compartment can be controlled by varying the rate at which slurry is withdrawn from the autoclave, this is largely done to compensate for fluctuations in the slurry feed rate. Therefore, the retention time in an overflow autoclave is largely determined by fixed parameters of the autoclave, and cannot be varied or controlled in any significant way. 
     Autoclaves are also known in which the dividers are provided with openings below the level of the slurry (referred to herein as “underflow openings”), in order to permit at least some of the slurry to flow through, rather than over, the dividers. For example, some overflow autoclaves are provided with relatively small openings in the lower portions of the dividers in order to permit movement of coarse particles through the autoclave and avoid buildup of solids within the compartments. For example, the provision of underflow openings is common practice in nickel laterite processing to avoid the buildup and growth of coarse alunite particles in the autoclave compartments, thereby ensuring that the liquid and solid components of the slurry have a similar retention time distribution (RTD). However, in many such autoclaves the majority of the slurry flows over the tops of the dividers, and they are subject to the same limitations in control of retention time as the overflow autoclaves discussed above. 
     It is also known to provide autoclaves in which most or all of the slurry flows through the underflow openings in the dividers. In this type of autoclave (referred to herein as an “underflow autoclave”), most or all of the dividers extend above the level of the slurry in the compartments. One example of an underflow autoclave in which all the slurry flows through underflow openings in the dividers is disclosed by Adams et al., in a paper entitled “Mixing Optimization of High Pressure Oxidation of Gold Ore Slurries”, presented at the 1998 Randol Gold &amp; Silver Forum. Another example of an underflow autoclave is disclosed in Ji et al., US 2007/0217285 A1, published on Sep. 20, 2007. In Ji et al., all of the dividers may be configured to permit only through-flow of slurry, or the last divider may be configured for overflow of slurry to compensate for fluctuations in feed rate, as in the overflow autoclaves discussed above. Although Ji et al. discuss the beneficial impact of underflow dividers on RTD, both Adams et al. and Ji et al are silent with regard to control of retention time in an underflow autoclave. 
     Despite the fact that control of retention time in a multi-compartment autoclave can provide significant benefits in terms of process optimization, the prior art is silent as to how retention time control can be achieved in an autoclave, and certain aspects of autoclave design are incompatible with retention time control. 
     SUMMARY OF THE INVENTION 
     In one aspect, there is provided a method for controlling retention time of an aqueous reaction mixture in a reactor, comprising steps (a) to (e). Step (a) comprises providing the reactor, wherein the reactor comprises a plurality of compartments including a first compartment and a last compartment, and wherein each adjacent pair of the compartments is separated by a divider having at least one opening for flow of the reaction mixture. Step (b) comprises passing the aqueous reaction mixture through the reactor at a first flow rate, wherein the reaction mixture flows through the reactor from the first compartment to the last compartment, wherein a liquid level is defined in each of the compartments, wherein most or all of the reaction mixture flows between each adjacent pair of compartments by passing through the at least one opening in the divider separating the adjacent compartments, and wherein the at least one opening is below the liquid levels in the adjacent compartments. Step (c) comprises calculating a first retention time of the reaction mixture based on the first flow rate and on volumes of the reaction mixture in the compartments. Step (d) comprises comparing the first retention time with an optimal retention time of the reaction mixture. Step (e) comprises adjusting the volumes of the reaction mixture in the compartments while maintaining the first flow rate, so as to change the retention time from the first retention time to a second retention time which is closer to the optimal retention time. 
     In another aspect, there is provided a reactor, such as an autoclave operating at elevated temperature and pressure, having retention time control. The reactor comprises: a plurality of compartments including a first compartment and a last compartment; one or more dividers, each separating an adjacent pair of compartments; an inlet in the first compartment; an outlet in the last compartment; a level sensor in the last compartment for generating volume data; a control valve for controlling the liquid level in the last compartment; and a controller adapted to receive the volume data from the level sensor and for controlling operation of the control valve. Each of the dividers has at least one opening for flow of a reaction mixture therethrough. Each opening is located below a minimum liquid level in each of the adjacent pair of compartments, and each divider has a height which is greater than a maximum liquid level in each of the adjacent pair of compartments. 
     In the reactor and the method described above, the reaction mixture may be withdrawn from the reactor through an outlet tube located in the last compartment; the reaction mixture may be agitated as it flows through the reactor; and an oxygen-containing gas or fluid reagent (acid, caustic, or oxidant) may be injected into the reaction mixture as it flows through the reactor. 
     In the method, the aqueous reaction mixture may be an aqueous slurry of an ore or an ore concentrate containing one or more metal values. The aqueous reaction mixture may include amounts of solid and/or liquid components of the aqueous slurry which have already passed through the reactor, for example where it is desired to recycle one or more components of the reacted slurry back to the reactor. An example of a process where solids are recycled in an oxidative pressure leach is disclosed in International publication no. WO 2007/143907 A1 by Dreisinger et al. It will be appreciated that the composition of the slurry, including the presence or absence of recycled components, will vary from one process to another, and is not critical to the operation of the processes and apparatus disclosed herein. 
     In the reactor, each divider may be provided with at least one opening below the liquid level in the compartments which are separated by that divider. For example, all the openings in all the dividers may be below the liquid levels in all the compartments. Each divider may have a height which is greater than the liquid levels in the compartments which are separated by that divider, such that all of the reaction mixture passes through the openings in the dividers. 
     In the reactor, the control valve may control the liquid levels in all compartments by controlling a rate of withdrawal of the reaction mixture from the last compartment. The operation of the control valve may be controlled by the controller, which receives information regarding the liquid level in at least one of the compartments, for example from the level sensor. 
     In the reactor, the total area of the opening(s) in each divider is sufficient to permit flow therethrough of all or substantially all of the reaction mixture at a desired flow rate. Furthermore, the total area of the opening(s) in each divider is substantially the same. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a partial cross section along a central longitudinal axis of a reactor according to an embodiment of the invention; 
         FIG. 2  is a transverse cross-sectional view of the reactor of  FIG. 1 ; and 
         FIG. 3  is a cross-sectional top plan view of the reactor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A multi-compartment reactor  10  and a method for controlling retention time in the reactor are now discussed below with reference to the drawings. In the following description, the reactor  10  is an autoclave for oxidative conversion of an ore or an ore concentrate containing one or more metal values, and the conversion may be exothermic or endothermic. An aqueous slurry of the ore/concentrate containing one or more metal values is referred to herein as the “reaction mixture”. Also, the raw reaction mixture entering the first compartment of reactor  10  is sometimes referred to herein as the “feed stream” and the processed reaction mixture withdrawn from the last compartment of the reactor  10  is referred to herein as the “product mixture”. 
     The reaction mixture may be alkaline or acidic and may be processed in reactor  10  at elevated temperature and pressure. It will be appreciated, however, that the reactor and method according to the invention can be applied to numerous chemical processes where a liquid reaction mixture, which may or may not include a solid component, is passed through a multi-compartment reactor, and wherein the reactor and/or the reaction mixture are maintained at elevated, ambient or low temperature and/or pressure. 
     Although the specific identity of the ore/concentrate, the metal values, the slurry composition and the reaction conditions is unimportant for the purpose of describing the invention, the reactor  10  is described below as a multi-compartment autoclave operating at elevated temperature and pressure, containing an acidic aqueous slurry of an ore and/or concentrate. The solid particles in the ore and/or concentrate may contain values of base metals, platinum group or precious metals in various forms. Specific base metal values include copper, nickel, cobalt, zinc, molybdenum and vanadium, which may be present in the ore/concentrate as either sulfides or oxides. Nickel ores processable in reactor  10  include nickel laterite ores. Specific precious metal values include gold, silver, platinum, palladium, niobium, and tellurium, which may be present in the ore/concentrate locked up within sulphide complexes or other base metal matrices. 
     Reactor  10  includes an outer wall  12 , typically made of steel, which defines an elongate interior space. The inner surface of wall  12  may be provided with a refractory lining  13  which may be comprised of refractory bricks. The refractory lining  13  generally follows the shape of the outer wall  12 , the lining  13  being provided to resist heat and corrosion. In the reactor  10  shown in the drawings, the outer wall  12  has an elongate, cylindrical shape with rounded ends which is typical of an autoclave for use at elevated temperature and pressure. However, the exact shape of the reactor  10  is variable, and is at least partly dependent on the process for which it is used. The reactor wall  12  defines a longitudinal axis A, which is shown in  FIG. 1  as being collinear with the section line for  FIG. 3 , and which is parallel to the direction in which the slurry flows through the reactor  10 . 
     The interior space of reactor  10  is divided into a plurality of compartments. In reactor  10  shown in the drawings, five compartments are defined, and these are labelled as  14 ,  16 ,  18 ,  20  and  22 . Compartment  14  is the first compartment of reactor  10 , into which the slurry is introduced as a feed stream through an inlet  46 . Compartment  22  is the last compartment, and the reacted slurry is withdrawn from this compartment as a reaction product through an outlet  48 . The remaining compartments  16 ,  18  and  20  are intermediate compartments located between the first and last compartments  14 ,  22 . 
     Each adjacent pair of compartments in reactor  10  is separated by a divider, and therefore reactor  10  includes four dividers, and these are labelled as  24 ,  26 ,  28  and  30  in the drawings. The first divider  24  separates the first compartment  14  from the second compartment  16 , second divider  26  separates the second compartment  16  from the third compartment  18 , third divider  28  separates the third compartment  18  from the fourth compartment  20 , and fourth divider  30  separates the fourth compartment  20  from the last compartment  22 . Although reactor  10  includes a total of five compartments and four dividers, it will be appreciated that reactor  10  may comprise fewer or more compartments and dividers than are described herein. 
     Each of the dividers is in the form of a wall extending transversely across the interior of reactor, and having edges which are in sealed contact with the interior surface and/or inner lining  13  of wall  12 . The dividers may comprise generally flat, planar metal plates having outer edges which follow the contours of the wall  12  and lining  13 . The tops of the dividers are flat and spaced from the upper portion of inner lining  13  to provide a continuous head space  32  within the reactor  10 . 
     A gap  21  is provided at the bottom of each divider  24 ,  26 ,  28 ,  30 . The gaps  21  define underflow openings  44  through which solid and liquid components of the reaction mixture can flow between the compartments. In reactor  10  all of the dividers are provided with underflow openings  44 . The underflow openings  44  in  FIGS. 1 and 2  are centrally located in the dividers but, as discussed further below, this may not be the case with all embodiments of the invention. 
     Each of the dividers has a height, measured from the bottom of gap  21  to the flat top of the divider. The heights of the dividers are related to the liquid levels, i.e. the level of the reaction mixture, in the compartments. For reasons which are discussed below, the liquid levels in the compartments are different, and therefore they are identified herein by distinct reference numerals. In this regard, the liquid levels in compartments  14 ,  16 ,  18 ,  20  and  22  are identified by reference numerals  34 ,  36 ,  38 ,  40  and  42 , respectively. 
     Reactor  10  illustrated in the drawings is configured as a 100% underflow autoclave, meaning that all of the reaction mixture flowing through reactor  10  passes through the underflow openings  44  between adjacent dividers, and none of the reaction mixture flows over the dividers during normal operation of the reactor  10 . Therefore, the liquid levels  34 ,  36 ,  38 ,  40 ,  42  shown in the drawings represent maximum liquid levels, and the height of each divider, measured from the bottom of lining  13  to the flat top of the divider, is greater than the maximum liquid levels in the compartments which are separated by that divider. In this regard, the first divider  24  has a height which is greater than the maximum liquid levels  34 ,  36  in the respective first and second compartments  14 ,  16 . The second divider  26  has a height which is greater than the maximum liquid levels  36 ,  38  in the respective second and third compartments  16 ,  18 . The third divider  28  has a height which is greater than the maximum liquid levels  38 ,  40  in the respective third and fourth compartments  18 ,  20 . Lastly, the fourth divider  30  has a height which is greater than the maximum liquid level  40 ,  42  in the respective fourth and fifth compartments  20 ,  22 . 
     For simplicity, all of the dividers may have the same height, in which case each divider will have a height which is greater than the maximum liquid level in all of the compartments. However, in the illustrated embodiment, the dividers are of different heights, each having a height which is slightly greater than the maximum liquid level in the compartment which is immediately upstream of that divider. In this regard, the height of first divider  24  is slightly greater than the maximum liquid level in the first compartment  14 , the height of second divider  26  is slightly greater than the maximum liquid level in the second compartment  16 , the height of third divider  28  is slightly greater than the maximum liquid level in the third compartment  18 , and the height of fourth divider  30  is slightly greater than the maximum liquid level in the fourth compartment  20 . 
     As mentioned above, the liquid level in each compartment is different, and the liquid level in each compartment decreases from the first compartment  14  to the last compartment  22 , as does the height of the dividers in the illustrated embodiment. The difference in liquid level between adjacent compartments is referred to herein as the head drop, and is related to the areas of the underflow openings  44 . In particular, large underflow openings  44  are associated with a high flow rate and a low head drop, whereas small underflow openings  44  are associated with a larger head drop. According to the present invention, the head drop is desirably in the range from about 50 to about 150 mm, or from about 2 to about 6 inches. In one embodiment of the invention the head drop is from about 75 to about 150 mm, or from about 3 to about 6 inches. The head drop must be great enough to prevent backflow of the reaction mixture in reactor  10 , but is not so great that the reaction mixture overflows the dividers. The inventors have found that a sufficient head drop between adjacent compartments can be achieved by providing all the dividers with underflow openings  44  having the same total area. 
     Where reactor  10  is a 100% underflow autoclave, all the openings  44  in the dividers are located below the minimum liquid levels in the compartments and must be of sufficient area to permit the reaction mixture to flow therethrough at a desired flow rate. Although the drawings show the openings  44  in the dividers being located proximate to the bottom of wall  12 , and each of the dividers is provided with one opening  44 , it will be appreciated that this is not necessarily the case. Rather, the dividers may have more than one opening  44 , so long as the total area of the openings  44  in each divider is sufficient to permit the reaction mixture to flow therethrough at the desired flow rate. Also, the openings  44  can be located above the bottom of the compartment, so long as they are located below the minimum liquid levels in the compartments separated by the divider. For example, the openings  44  can be located from about 12 to about 24 inches above the bottom of the compartment. 
     Furthermore, although the underflow openings are shown in the drawings as being centrally located in the dividers this is not necessarily the case. For example, the underflow openings  44  may be displaced away from the centre of the dividers, and may for example be displaced toward the leeward side of the impellers  54  in order to minimize short-circuiting of flow in reactor  10 . 
     Each compartment of reactor  10  is provided with at least one agitator  52  having an impeller rotating on a vertical shaft  56 . The reaction mixture may be vigorously agitated so as to retain solid particles in suspension. The drawings show one agitator  52  per compartment, but it may be desired to two or more agitators in at least one of the compartments. The direction in which the impellers  54  rotate in adjacent compartments may be the same, or the direction of rotation may be reversed from compartment to compartment. 
     Each compartment may further be provided with sparger pipes (not shown) through which a gaseous or liquid reagent is injected into the reaction mixture. In the context of a process for oxidative conversion of an ore or an ore concentrate containing one or more metal values, the sparger pipes will inject a molecular oxygen-containing gas or an oxidizing liquid such as hydrogen peroxide into each compartment, for oxidation of metal-containing compounds in the reaction mixture. 
     As mentioned above, the reactor  10  has an inlet  46  through which the feed stream is introduced into the first compartment  14 , and an outlet  48  at the opposite end of reactor  10  through which the product is withdrawn from the last compartment  22 . Where the outlet  48  is provided in an upper portion of the reactor  10 , the outlet  48  may be provided with a dip tube  50  which extends from the outlet  48  to below the liquid level  42  in the last compartment  22 , to enable the reaction product to be withdrawn from the last compartment  22 . 
     The feed stream may be introduced into the first compartment  14  on a continuous basis, and the product may be removed from the last compartment  22  on a continuous basis, thereby providing a continuous flow of slurry through the reactor  10 . The rate of introduction of the feed stream and the rate of withdrawal of the reaction product are typically maintained as consistent as possible, subject to minor fluctuations in the feed rate, so as to maintain a substantially constant flow rate of slurry through the reactor  10 . 
     The reactor  10  further comprises a control valve  60  to control the liquid levels in the compartments of reactor  10 . The control valve  60  permits controlled withdrawal of a portion of the reaction mixture from reactor  10  so as to provide control over the liquid levels. In this regard, it can be seen that withdrawing a portion of the reaction mixture from one of the compartments in underflow reactor  10  will not only reduce the liquid level in the compartment from which the liquid is withdrawn, but will also bring about a reduction in the liquid levels in the other compartments. Therefore, by controlling the control valve  60 , the liquid level within all of the compartments can be controlled. 
     The control valve  60  is in direct flow communication with one of the compartments of reactor  10 . While control over the liquid level can be achieved by using valve  60  to control withdrawal of reaction mixture from any of the compartments, it is preferred that valve  60  is used to control the rate of withdrawal of the reaction mixture from the last compartment, to ensure that the liquid being removed has undergone sufficient reaction in reactor  10 . Since an outlet  48  and dip tube  50  are already provided for withdrawal of the product mixture from the last compartment  22 , the control valve  60  may conveniently be made to communicate with outlet  48  and dip tube  50 , so as to permit variability in the rate at which the product mixture is withdrawn from the last compartment  22  through dip tube  50  and outlet  48 . The control valve  60  may either be located inside or outside reactor  10 , although it may be preferred to locate valve  60  outside of reactor  10  to avoid exposure of valve  60  to the corrosive atmosphere inside reactor  10 . In an embodiment of the invention, the valve  60  is located in a flash tank (not shown) which is located downstream of reactor  10 . 
     As will be appreciated, the volume of slurry in each compartment is determined by the liquid level in that compartment. Therefore, it is readily apparent that the use of control valve  60  to simultaneously vary the liquid levels in the compartments will have a direct impact on the volume of the reaction mixture in each compartment, and throughout the reactor  10  as a whole. 
     The retention time of reaction mixture in reactor  10  is a critical parameter which is desirably maintained as close as possible to an optimal retention time. For example, the retention time must be sufficient to allow the reaction mixture to react as completely as possible as it is converted from a feed stream to a product mixture, so as to maximize the recovery of metal values. The optimal retention time will vary somewhat during operation of reactor  10 , depending on a number of factors, such as composition of the feed stream entering reactor  10 , the rate of agitation, variations in the volumes of other components added to the reactor  10  such as quench water and/or recycled slurry. For each set of reaction conditions there will be an optimal retention time. 
     The inventors have recognized that it is beneficial to control the retention time of the slurry within reactor  10 . This is distinct from the control of the retention time distribution (RTD) discussed in the above-mentioned patent application of Ji et al. As is already well known in the processing of nickel laterites, autoclaves with underflow openings provide better (i.e. narrower) RTDs than autoclaves with only overflow openings as they allow solid components of a slurry to flow through the autoclave at about the same rate as the liquid components. In contrast, controlling the retention time has the effect of increasing or reducing the overall or average retention time, without necessarily changing the RTD. 
     The inventors have also recognized that the retention time can be controlled independently of fixed reactor parameters such as reactor size and the area of openings  44 . Thus, the inventors have discovered that it is possible to adjust the retention time without changing any of the reactor parameters. In particular, as discussed above, the retention time is proportional to the volume of slurry in reactor  10 , and is inversely proportional to the flow rate of the slurry through reactor  10 . Furthermore, since the level of slurry in all compartments of an underflow reactor can be controlled by varying the liquid level in one of the compartments, it is possible to control the volume, and therefore the retention time, by controlling the liquid level in one of the compartments. Since the flow rate is substantially constant and is typically varied only by external factors, such as fluctuations in the feed rate, and since the size of the autoclave and its compartments is pre-determined and fixed, variation in the liquid level within one of the compartments has a direct and predictable effect on the retention time. 
     In order to permit automatic adjustment of the liquid levels in the compartments, the reactor  10  may be provided with a liquid level sensor  62  located in one of the compartments. The sensor  62  generates information regarding the liquid level in one of the compartments, for example the last compartment  22 , and transmits the information to valve  60 . A controller  64  may also be provided to receive the level information from the sensor  62 , calculate the retention time of reactor  10  and compare it with an optimal retention time, and then adjust operation of valve  60  to vary the rate at which the product mixture is withdrawn from the last compartment  22 , and thereby modify the retention time to be closer to the optimal value. 
     In any given reactor the liquid levels in the compartments will be variable between a maximum level (eg. levels  34 ,  36 ,  38 ,  40 ,  42  in reactor  10 ) and a minimum level. The minimum level is determined, at least in part, by the heights of the impellers  54  and by the heights of the openings  44 , i.e. the agitators must remain in contact with the reaction mixture and the openings  44  must be below the minimum liquid level to prevent short circuiting of the flow. 
     The reactor  10  shown in the drawings is configured for 100% underflow, i.e. all of the reaction mixture flows through the underflow openings  44  in the dividers as it passes from the first compartment  14  to the last compartment  22 . It will be appreciated that the reactor  10  described herein may not necessarily have all dividers and compartments configured for 100% underflow. For example, the reactor  10  may be configured as a combination underflow/overflow reactor, in which a portion of the reaction mixture is permitted to flow over at least one of the dividers. However, regardless of whether or not reactor  10  is configured for 100% overflow, each of the dividers of reactor  10  is provided with underflow openings  44  which having sufficient total area such that a portion of the flow passes through the openings  44  in each the divider, and the areas of the openings  44  in all the dividers are such that a major portion of the flow through the reactor  10  is through openings  44  rather than over the tops of the dividers. For example, at least 80%, or at least 85%, or at least 90% by volume of the flow passes through the underflow openings  44  of reactor  10 . 
     Although the invention has been described in connection with certain embodiments, it is not intended to be limited thereto. Rather, the invention includes all embodiments which may fall within the scope of the claims.