Abstract:
A method of transforming a molten material or melt into discrete solid particles comprises forming the melt into prills by passing the melt through a perforated member, dropping the prills onto a bed of powdered material, such as silica flour, to coat the prills with the material in the bed and permitting the prills to cool, thereby to solidify into discrete solid particles. In one embodiment, the melt comprises at least two components such as elemental sulphur and an olefinic hydrocarbon, which are subjected to a chemical reaction, such as the polymerization of the olefinic hydrocarbon with the sulphur to form a sulphur polymer stabilizer. A method of producing a sulphur concrete incorporating the sulphur polymer stabilizer is also provided.

Description:
FIELD OF THE INVENTION 
     This invention relates to a drop forming method for transforming a molten material or melt into discrete solid particles. It also relates to a method of producing a sulphur polymer stabilizer in particulate form and a method of producing a sulphur concrete. 
     BACKGROUND OF THE INVENTION 
     A sulphur concrete is a concrete mix in which sulphur is used as a binder instead of water and portland cement. Typically, a sulphur concrete comprises a mixture of sulphur with a filler material and an aggregate material. However, in order to preserve the strength characteristics of the sulphur binder it is necessary to introduce a stabilizer to prevent the transformation of monoclinic sulphur to orthorhombic sulphur and other crystallization effects. One such stabilizer comprises an olefinic hydrocarbon polymer which reacts with the sulphur to form a sulphur containing polymer, as described in U.S. Pat. Nos. 4,058,500 and 4,293,463, the entire contents of which is incorporated herein by reference. 
     One method of introducing the stabilizer is by reacting an olefinic hydrocarbon with elemental sulphur to produce a sulphur polymer stabilizer which is then used for producing the sulphur concrete. The stabilizer may be used immediately or at a later time. 
     A conventional process for producing the sulphur polymer stabilizer involves reacting the hydrocarbon with the sulphur in molten form and then pouring the resultant molten sulphur stabilizer onto a surface, allowing it to cool and solidify. Thereafter, it is broken up into pieces with machinery, such as a bobcat. The process creates a health hazard due to the dust produced. In addition, the product is subject to contamination. Furthermore, the resultant particles are inconsistent in size. This can result in a sulphur concrete mix of inconsistent quality. 
     It is accordingly an object of the present invention to provide a method for the production of a sulphur polymer stabilizer of improved quality with particles having a more uniform size. It is also an object of the invention to provide an improved sulphur concrete. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method of transforming a molten material or melt into discrete solid particles, comprising the steps of forming the melt into prills by passing the melt through a perforated member, dropping the prills onto a bed of powder material to coat the prills with the material in the bed and permitting the prills to cool, thereby to solidify into discrete solid particles. 
     The melt may comprise at least two components which are subjected to a chemical reaction prior to forming the melt into prills. In one embodiment, the two components comprise elemental sulphur and an olefinic hydrocarbon, the chemical reaction being the polymerization of the olefinic hydrocarbon with the sulphur to form a sulphur polymer stabilizer which constitutes the melt. 
     The chemical reaction may be carried out at a temperature in the range of from about 115° C. to about 150° C., preferably 140° C., and the melt may be cooled to a temperature in the range of from about 115° C. to about 125° C., preferably 120° C., prior to forming the melt into prills. 
     The polymerization may be permitted to proceed until the melt has a viscosity of about 30 to about 40 cp, preferably 35 cp, prior to cooling the melt for forming into prills. 
     The elemental sulphur and olefinic hydrocarbon may be combined at a ratio of about four parts elemental sulphur to 1 part of olefinic hydrocarbon by weight for effecting the polymerization, but other proportions are possible. 
     Also according to the invention there is provided a method of producing a sulphur concrete comprising the step of mixing aggregate material and filler material with elemental sulphur and a sulphur polymer stabilizer in a heated mixing vessel, wherein said stabilizer is in particulate form and prepared prior to said mixing by a method comprising the steps of subjecting elemental sulphur and an olefinic hydrocarbon to a polymerization reaction at an elevated temperature to produce a sulphur polymer in molten form, forming the molten polymer or melt into prills by passing the melt through a perforated member, dropping the prills onto a bed of powder material which is at a lower temperature than the prills to coat the prills with the material in the bed and permitting the prills to cool, thereby to solidify into discrete solid particles. 
     The powder material may comprise a filler material suitable for use in a sulphur concrete, such as silica flour. 
     Further objects and advantages of the invention will become apparent from the description of preferred embodiments of the invention below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatical representation of a process for producing a sulphur polymer stabilizer. 
     FIG. 2 is a schematical drawing showing a side view of drop forming apparatus for producing a sulphur polymer stabilizer in particulate form. 
     FIG. 3 is a schematical plan view of a forming tray which is part of the apparatus of FIG.  2 . 
     FIG. 4 is a schematical side view of the tray of FIG.  3 . 
     FIG. 5 is a flow diagram of a process for producing a sulphur concrete. 
     FIG. 6 is a schematical side view of apparatus for carrying out the process of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, apparatus for producing a sulphur polymer stabilizer in particulate form is generally indicated by reference numeral  10 . The apparatus  10  comprises a reaction vessel  12  provided with a stirrer  14  and drop forming apparatus  16  for transforming the stabilizer into particulate form. The reaction vessel  12  is also provided with heating and cooling means (not shown). 
     An olefinic hydrocarbon, such as an oligomer of cyclopentadiene, and molten elemental sulphur are introduced into the vessel  14  in a ratio of 1 to 4 by weight. The reaction which takes place in the vessel  14  is the polymerization of the olefinic hydrocarbon with sulphur. The reaction is carried out at approximately 140° C. for approximately 30 minutes. The rate of the reaction is dependent on the temperature at which the reaction is carried out. Generally, it is faster at higher temperatures. The reaction is terminated when the product has a viscosity of about 35 cp by reducing the temperature of the mixture to about 120° C. in a subsequent cooling stage  15 . 
     It is important to rapidly cool the reaction product in order to terminate the polymerization reaction. Such cooling should be effected within 5 minutes. Such cooling can be effected by various means, such as cooling coils or a jacket vessel using cooling water. The product is then maintained at the cooled temperature until it is transformed into particulate form. Transformation of the product into particulate form should be effected within 1 hour of cooling to counteract any further reaction taking place. 
     The drop forming apparatus  16  is shown in greater detail in FIG.  2 . It comprises an auger  18  for transferring powder from a powder reservoir  20  to a powder distributer  22 , which is in the form of a hopper provided with a sliding gate  24 . The powder distributer  22  has an outlet which is positioned above a conveyor  26  which is in the form of an endless belt. In the view shown in FIG. 2, the conveyor belt is rotated counter-clockwise. 
     The apparatus  16  also includes a drop forming tray  28  (FIGS. 3 and 4) which is located above the conveyor  26  downstream of the powder distributer  22 . The tray  28  is provided with holes  29  of about 1.6 mm (0.06″) spaced about 5 mm (0.2″) apart, on the underside. Only some of the holes  29  are shown in FIG.  3 . The perforated underside of the tray  28  is located at about 40 mm (1.6″) to about 60 mm (2.4″) above the conveyor  26 . 
     The powder used in the process preferably comprises a filler material which is suitable for use in a sulphur concrete, such as silica flour of less than 200 mesh, in the present example. The powder is poured from the distributer  22  onto the conveyor  26 . The outflow of powder is controlled by the gate  24  to form a bed of powder about 6 mm (0.24″) to 10 mm (0.4″) in depth on the conveyor  26 . The powder bed is at an ambient temperature depending on the location and time of year. Typically, this may be from about 5° C. to about 25° C., but the temperature may also be outside this range. The speed of the conveyor  26  is about 0.6 m/s (2 ft/s) to about 0.8 m/s (2.6 ft/s). 
     The molten stabilizer product at 120° C. is transferred to the forming tray  28 . In the present example the tray  28  is filled to an operating level of about 50 mm (about 2″) and the molten stabilizer product is maintained at this operating level in the tray  28 . 
     The stabilizer forms prills as the liquid stabilizer drops from the tray  28  to the powder bed on the conveyor  26 . In this particular example, the prille are formed in a temperature range of about 115° C. to 120° C. 
     The prills cool sufficiently during the free fall to form sticky, solid particles. When these solid particles fall onto the powder bed they are coated with the powder material, preventing the prills from agglomerating into a larger sticky mass. In the present example, the resulting prills are from about 1 mm (0.04″) to about 5 mm (0.2″) in diameter and contain about 6.75% of the powder material, but these values may vary to suit circumstances. Prills having a 5 mm (0.2″) diameter are preferred. 
     A vibratory screen deck  30  is located at the end of the conveyor  26  to receive the powder and coated prills dropping off from the conveyor belt  26 . The coated prills are retained by the screen  30  while the powder drops into the reservoir  20  for recycle to the conveyor  26 . 
     The resultant particulate polymer stabilizer is in a form suitable for later use. Typically, sulphur concrete is produced at a location distant from the production location of the sulphur polymer stabilizer, for economic reasons. For this reason, the particulate stabilizer is prepared for storage and transport. 
     A typical sulphur concrete composition is: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Weight Percent 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aggregate 
                 82.0 
               
               
                   
                 Elemental Sulphur 
                 11.5 
               
               
                   
                 Filler Material (&lt;200 mesh) 
                 5.3 
               
               
                   
                 Sulphur Polymer Stabilizer 
                 1.2 
               
               
                   
                   
               
             
          
         
       
     
     The above design mix is determined by first calculating the maximum density of the aggregate and filler and subsequent void space. The sulphur polymer stabilizer portion is typically one part sulphur polymer stabilizer to nine parts sulphur, but proportions may vary. The sulphur polymer stabilizer, sulphur and filler material are designed to fill the void space in the aggregate. The amount of filler material is typically about half of the weight of sulphur and sulphur polymer stabilizer. The composition is then varied on an iterative basis to obtain specific strength and workability for a specific application. 
     Sulphur concrete is produced by first selecting aggregates which are compatible with the application. For sulphur concrete, aggregates should be free of water-expansive material, compatible with sulphur and dense-graded. Sulphur required for the production of sulphur concrete can be either liquid or solid. Generally solid sulphur is easier to handle. The fine filler material should be graded to less than 200 mesh and be compatible with the environment in which the product will be used. 
     A process for the production of sulphur concrete will now be described. A flow diagram of the process is shown in FIG.  5  and apparatus is shown in FIG.  6 . 
     The aggregate is subjected to a preparation stage  31  which comprises drying and screening operations. Thus, the aggregate is dispensed from storage bins  32  and introduced into a direct fired rotary drum dryer  34  by means of a conveyor belt  36 . Product gases from the dryer  34  pass via a scrubber  38  to atmosphere. The dried aggregate is screened immediately after drying by screens  40  to separate the aggregate into four size fractions which are stored in hot bins  42 . 
     Sulphur and olefinic hydrocarbon are reacted in a preparation stage  44  to produce a sulphur polymer stabilizer. This production process has been described above with reference to FIGS. 1 and 2. The stabilizer can be produced immediately before use or ahead of time and stored until use. 
     The prepared aggregate, sulphur polymer stabilizer, additional sulphur and filler are then combined in a mixing stage  46 . 
     A heated mixer  48  is used. The different sized aggregates are proportioned as required and introduced into the mixer  48  by means of a conveyor belt  50 . The apparatus includes a weighing system (not shown) for effecting the proportioning. 
     The temperature in the mixer  48  is controlled in a range of 150° C. to 160° C. 
     The sulphur polymer stabilizer, filler and additional sulphur are introduced separately into the mixer  48 , as shown in FIG.  6 . 
     The temperature in the mixer  48  is approximately 150° C. when the additional sulphur is molten and approximately 160° when using solid sulphur. The elemental sulphur (solid or molten) is added to the mixer  48 , followed by the filler material, and the temperature controlled to a range of 130-140° C. The sulphur polymer stabilizer is then added and thoroughly mixed in the temperature range from 130-140° C. The sulphur concrete is then placed according to the application, typically within one hour. 
     While only preferred embodiments of the invention have been described herein in detail, the invention is not limited thereby and modifications can be made within the scope of the attached claims.