Abstract:
A process for the recovery of minerals from the fine particle size fraction of froth flotation feed (slimes), which is conventionally discarded to waste because slimes interfere with efficient flotation. In the disclosed process, the slimes, which also may be termed fines, are scrubbed to clean the slimes, and are then subjected to conventional froth flotation. The scrubbing is done in the presence of an attrition media, in addition to chemical reagents for cleaning and dispersing fine particles. Thus an inert attrition media, of larger particle size than the slimes, is introduced into a scrubber. The process accordingly recovers a fine fraction of the flotation feed that is normally lost. The recovery process may be incorporated into an existing plant design, or as part of a new plant designed to recover material previously discarded in waste disposal areas.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to froth flotation for separating out minerals from ground-up ore and, more particularly, to the recovery of minerals from the relatively fine particle froth flotation feed, commonly known as “slimes,” which conventionally is discarded. 
     Froth flotation is a well-known process used to separate minerals, ground up into particles and suspended in or otherwise carried by a liquid, by attaching the mineral particles to gas bubbles to provide selective levitation of the solid particles into a froth. Conventionally the liquid is water. Selective levitation is accomplished by conditioning a flotation feed in the form of a slurry with various flotation reagents that selectively coat the particle surfaces of various minerals. The surface coating allows for either air bubble attachment to individual particles or prevents air bubble attachment, depending on the specific reagents used in conditioning and subsequent flotation. In some cases the desired mineral particles are carried upward into the froth and collected as product, leaving other material to settle as tails, which can be waste. In other cases, undesired particles are carried upward into the froth and discarded as waste, leaving desired mineral particles to settle as tails, which is collected as product. 
     For effective separation, it is essential that the particles be discrete particles of the individual minerals. To promote the most efficient and selective response to the flotation reagents utilized, it is also important that the particles have clean non-contaminated surfaces. (However, not all froth flotation facilities employ scrubbers.) 
     To produce discrete mineral particles, ore is crushed and ground to nominally 1 mm diameter and finer particle size for flotation feed. This crushing and grinding produce some material as fine as 0.001 mm. Normal flotation practices are performed over a particle size range of the feed determined to yield the most efficient, cost-effective and quality-acceptable flotation product. The following table lists the desired smallest size particle for flotation feed for various minerals, which may be viewed as a minimum particle size cut off point, as given by Crozier in  Flotation, Theory, Reagents and Ore Testing.   
     
       
         
               
               
               
             
           
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Mineral 
                 Minimum Particle Size 
               
               
                   
                   
               
             
             
               
                   
                 feldspar 
                 0.074 mm 
               
               
                   
                 phosphates 
                 0.105 mm 
               
               
                   
                 potash 
                 0.074 mm 
               
               
                   
                   
               
             
          
         
       
     
     These minerals are listed as examples only, and the list above is not all-inclusive. The majority of minerals recovered by froth flotation are currently processed at a minimum particle size cut-off point. 
     Relatively fine particles smaller than the minimum particle size, referred to as fines or slimes, interfere with efficient froth flotation. Under current practice slimes are therefore discarded, even though they contain significant quantities of usable minerals. For the minerals listed above, approximately 10%-20% of the flotation feed typically is finer than the minimum particle size cut-off point. 
     To provide mineral particles that have clean non-contaminated surfaces, scrubbing processes are employed in some froth flotation facilities. As an example, a conventional attrition scrubber takes the form of a tub into which a slurry is loaded. The slurry typically contains approximately 70% solids by weight in the form of particles to be cleaned, and is conditioned with cleaning reagents such as NaOH, H 2 SO 4 , sodium silicate, HCl and sodium hexametaphosphate, depending upon the particular minerals involved. Reagents serve cleaning, dispersion and conditioning functions. A rotating vertical shaft extends into the tub, and carries impellers which are angled so as to alternately push the slurry up and down. The particles rub against each other to effect cleaning, aided by the cleaning reagents. 
     SUMMARY OF THE INVENTION 
     It is therefore seen to be desirable to efficiently recover minerals from conventionally discarded fine froth flotation feed (slimes). 
     In an exemplary embodiment, the slimes are scrubbed in the presence of attrition media, and subsequently processed by froth flotation. The attrition media is removed either before or after froth flotation of the slimes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic flow diagram representing an embodiment of the invention; 
     FIG. 2 is a schematic flow diagram representing another embodiment of the invention; and 
     FIG. 3 is a partial schematic flow diagram representing the recovery of slimes from a waste dump. 
    
    
     DETAILED DESCRIPTION 
     The invention is based in part on a recognition that the relatively fine particles (slimes) interfere with efficient froth flotation because conventional scrubbing procedures do not produce the clean uncontaminated surfaces necessary for efficient flotation on the slimes particles. Clean surfaces and unagglomerated particles are essential for the selectivity of the flotation reagents. Embodiments of the invention employ scrubbing processes which clean the surfaces of the slimes particles, producing a flotation feed that reacts selectively and efficiently to subsequent flotation procedures. 
     With reference to FIG. 1, represented in schematic flow diagram form is a process embodying the invention, carried out in a froth flotation facility for separating minerals from ground-up ore. As examples, the ore may be spodumene containing iron minerals, mica, spodumene, feldspar and silica; or may be feldspar ore containing iron minerals, mica, feldspar and silica. In FIG. 1, ore and water are introduced at  10  and  12  into a conventional grinder  14  which produces a slurry including discrete mineral particles. The ore is ground to a desired particle size ranging from 40 mesh, down to 200 mesh, nominally 0.42 mm and finer in diameter. 
     However, at the same time, finer particles are produced, smaller than 200-mesh, some particles as fine as 0.001 mm in diameter. These relatively finer particles are referred to as fines or “slimes,” and interfere with the conventional forth flotation processes. For example, the slimes particles tend to non-selectively absorb froth flotation reagents, decreasing the overall efficiency of the process. In addition, slimes particles tend to stick to the desired mineral particles, resulting in contamination of the desired product. 
     Accordingly, a desliming stage  18  is conventionally employed, wherein slimes  20  are separated out from a stream  22  which becomes the froth flotation feed. Within the desliming stage  18 , any one of or a combination of conventional sizing processes such as screening, hydrocycloning, hydrosizing, settling, as examples, are employed. 
     After desliming, the remaining feed  22  may be cleaned in a scrubber  24  in the presence of appropriate reagents introduced at  26 , and is then delivered, as indicated by arrow  28 , as flotation feed to a conventional froth flotation process  30 , which includes conditioning with appropriate flotation reagents. Within the froth flotation process  30 , separation occurs into froth  32  and material  34  which settles, known as tails. The froth flotation process  30 , although shown as a single stage, may involve a number of successive flotations, as is well known. Thus, in the case of feldspar ore or spodumene ore, in a final flotation step, feldspar particles are floated as part of the froth  32 , while silica particles settle as tails  34 , both of which are recovered as products. 
     In conventional froth flotation facilities, the slimes  20  are discarded as waste, even though the slimes  20  in general contain significant quantities of the same desired minerals. 
     In the embodiment of the invention represented in FIG. 1, the slimes  20  are scrubbed in order to clean the slimes. In general, cleaning means to physically break apart agglomerated particles, and to clean the surfaces of the relatively fine slimes particles of, for example, oxidation or iron salts. 
     More particularly, the slimes  20  are delivered to a scrubber  40  in which the slimes are scrubbed in the presence of an attrition media introduced as represented at  42 , as well as in the presence of appropriate reagents for cleaning and dispersing fine particles. 
     The addition of the attrition media  42  facilitates effective scrubbing of fine particles (slimes). Requirements for the attrition media are that it be an inert material and of a particle size larger than the slimes being scrubbed. In this context, “inert” means that the attrition media does not react chemically with water or with reagents used during scrubbing and froth flotation. The attrition media is typically a sand having a particle size ranging from approximately 0.50 mm down to 0.177 mm (20-mesh sand) of any compatible mineral, usually silica, but may be any natural or synthetic grinding media of suitable size and mass to effect thorough cleansing of the surfaces of the slimes particles. The larger size facilitates efficient removal of the attrition media, which can be recycled. The attrition media gives the slimes mass, which aids in physically breaking apart agglomerated particles. The attrition media also cleans the surfaces of these fine particles. 
     A quantity of attrition media  42  is added so that attrition media  42  makes up approximately 40% to 70% by weight of the solids in the scrubber  40 . The percent of solids in the scrubber  40  (slimes and attrition media combined) is adjusted to approximately 70% to 75%, with the remainder being water. Scrubbing reagents  44  are added appropriate to the minerals present in the slimes  20 . Scrubbing reagents  44  can include, but are not limited to, NaOH, H 2 SO 4 , sodium silicate, HCl and sodium hexametaphosphate. Reagents serve cleaning, dispersion and conditioning functions. The time required for scrubbing is dependent on the makeup of the slimes, and can range from approximately one minute to approximately thirty minutes. 
     Following the scrubber  40 , feed  46  is directed to a sizing apparatus  48  wherein attrition media  50  is removed by sizing. The attrition media  50  is the coarsest fraction. The sizing apparatus  48  for example may comprise screens, a hydrocyclone, or hydrosizing apparatus, as examples. Preferably the removed attrition media  50  is recycled as at least part of the attrition media introduced at  42  into the scrubber  40 . 
     The feed then proceeds as indicated by arrow  52  to another sizing apparatus  54  wherein waste material  56  scrubbed from the surfaces of the slimes is removed, as the finest fraction, and is discarded as waste. The sizing apparatus  54  likewise may comprise screens, a hydrocyclone, or hyrdosizing apparatus, as examples. 
     Scrubbed slimes which remains, then serves as a flotation feed  58  which yields a selective and efficient float. Thus, the flotation feed  58  is delivered to a froth flotation stage  60 . Within the froth flotation stage  60 , the flotation feed  58  is conditioned with flotation reagents, and froth flotation is carried out in flotation cells to separate the mineral particles. 
     The froth flotation stage  60 , although shown as a single stage, may involve a number of successive flotations. Thus, in the case of feldspar ore, in a first flotation step, mica particles are floated as part of the froth, and can be recovered as a product, with remaining material settling as tails. In a second flotation step, iron mineral particles are floated as part of the froth, and can be recovered as product, with remaining material settling as tails. In a final flotation step, feldspar particles are floated as part of the froth  62 , and are recovered as product, while silica particles settle as tails  64 , and also may be recovered as product. In the case of spodumene ore, a similar sequence of flotation steps may be employed, with the addition of a flotation step, prior to the final step, during which spodumene particles are floated as part of the froth. 
     Depending upon the reaction of the minerals being treated, and the plant or facility flow design, flotation of the scrubbed slimes flotation feed  58  may be accomplished concurrently with and in the same cells as the conventional flotation feed  28 , or in a separate flotation circuit. 
     The following EXAMPLES show the results of slimes processing as described above with reference to FIG. 1, using silica sand as the attrition media, and NaOH and sodium silicate as scrubbing reagents. 
     EXAMPLES 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Slimes Processed 
                 Mineral Recovered 
                 % wt. of Slimes Recovered 
               
               
                   
               
             
             
               
                 1. Spodumene ore 
                 feldspar 
                 35%-40% 
               
               
                   
                 silica 
                 15%-20% 
               
               
                 2. Feldspar ore 
                 feldspar 
                 35-40% 
               
               
                   
                 silica 
                 10-15% 
               
               
                   
               
             
          
         
       
     
     For the foregoing EXAMPLES, flotation was performed according to conventional flotation procedures. The percent recovery was comparable to that achieved with the deslimed ore in the conventional flotation stage  30 . 
     With reference now to FIG. 2, represented is another embodiment of the invention, differing from FIG. 1 only in the processing following the scrubber  40  wherein the slimes  20  are scrubbed in the presence of attrition media  42 . 
     Rather than removing the attrition media for recycling at that point, as in FIG. 1, in FIG. 2 froth flotation is carried out prior to removing the attrition media. 
     Following the scrubber  40 , the feed  46  is directed to a sizing apparatus  70  wherein waste material  72  scrubbed from the surfaces of the slimes is removed, as the finest fraction, and is discarded as waste, as in the FIG. 1 sizing apparatus  54 . However, the attrition media remains. 
     Scrubbed slimes and attrition media combined then serves as flotation feed  74 , directed to a froth flotation stage  76 . The flotation stage  76  produces froth  78 , which necessarily contains the desired product in this embodiment, as well as tails  80 , which settles. As a particular example, in the FIG. 2 embodiment the slimes may contain ground-up particles of quartz and mica. In the flotation stage  76 , the mica floats as part of the froth  78 , and the quartz and attrition media stay behind as the tails  80 . 
     In FIG. 2, the flotation stage  76  is followed by a sizing apparatus  82  which separates out the relatively coarser attrition media  84  from the remaining tails  86 , which in this particular example comprise quartz. As in FIG. 1, in FIG. 2 the attrition media  84  is preferably recycled, to be introduced as the attrition media  42  into the scrubber  40 . 
     FIGS. 1 and 2 described hereinabove depict embodiments of the invention wherein minerals in the form of fine material are recovered from the slime streams in otherwise conventional froth flotation facilities, as part of the overall processing. 
     With reference to FIG. 3, embodiments of the invention are also useful in the recovery of minerals from fine flotation feed (slimes) that have previously been discarded to waste sites, such as pond storage, waste piles or land fill. Accordingly, embodiments of the invention permit the recovery of useful minerals from the waste. 
     Thus, in FIG. 3, a slime storage or disposal area is represented at  90 . Slimes  92 , either as a slurry or as powder or clumps, are delivered from the slime storage or disposal area  90  to a scrubber  94 , analagous to the scrubber  40  of the embodiments of FIGS. 1 and 2, to which attrition media  96 , cleaning reagents  98  and water  100  are added, as described hereinabove. Output  102  from the scrubber of FIG. 3 is then processed in the same manner described hereinabove as the output  46  from the scrubber  40  of FIG. 1, or the output  70  from the scrubber  40  of FIG.  2 . 
     Embodiments of the invention thus process slimes to produce a flotation feed which reacts selectively and efficiently to flotation procedures, either in existing flotation plants, intercepting a feed that would otherwise be discarded to waste, or processing previously-discarded fine flotation feed (slimes). 
     While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.