Abstract:
A continuous preparation of calcium hydroxide slurry can be made on a mobile apparatus that uses an eductor to initially combine metered flows of calcium oxide and water. The calcium hydroxide slurry is retained in a deaerating sump and then pumped into a serpentine centrifugal mixer so that the reaction of the calcium oxide and the water is safely complete. Because the apparatus allows the calcium hydroxide slurry to be recirculated into the deaerating sump, the slaking process can continue even when it is necessary to pause the discharge of slurry to change transport tankers.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for the preparation of calcium hydroxide, in particular for a mobile system that facilitates the continuous preparation of calcium hydroxide slurry from calcium oxide and water at a jobsite location. 
     BACKGROUND OF THE INVENTION 
     The use of calcium hydroxide slurries to stabilize clay soils in highway and general construction projects is well established. 
     When preparing calcium hydroxide slurry by mixing calcium oxide and water together, a highly exothermic reaction takes place. Generally apparatus designed for this process have been of a batch nature, or a limited capacity continuous process. 
     What is therefore needed is a slaking and mixing apparatus that is mobile. 
     What is also needed is a slaking and mixing apparatus capable of processing high hourly volumes and servicing a fleet of mobile slurry spreading tankers through high volume intermittent output from the apparatus. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an apparatus to continuously process high volumes of calcium oxide up to a rate approximating 30 tons per hour. 
     Another object of the invention is to provide a continuous slurrying/slaking process which has provision to discharge either into a static slurry storage tank, or to discharge intermittently into a fleet of mobile tankers for spreading the product at the jobsite 
     Another object of the invention is to provide an apparatus that allows for accurate and infinitely variable consistency of the product slurry as measured in percentage solids. 
     Another object of the invention is to provide an apparatus that can be mounted on a trailer, easily moved to a job site and set up in a minimal time. 
     Another object of the invention is to provide a lime slaker that requires only a source of water, a commercial lime delivery, and slurry distribution trucks or a storage tank for production. In this way, the slaker can be used economically on smaller jobs where current methods cannot be justified because of the high set-up costs. 
     Another object of the invention is to provide an apparatus that permits accurate blending of calcium oxide and water in a continuous process that can produce limited quantities of lime slurry necessary for small job site, or larger quantities of lime slurry for larger job sites. 
     Another object of the invention is to provide an apparatus capable of producing high shear and turbulent mixing of calcium oxide and water through the use of an eductor designed to promote initial mixing, agitation in the system tanks created by high velocity streams from the system pumps directed through specifically designed flow devices to achieve maxium turbulence, high shear mixing within the slurry pump and differential velocity mixing in the serpentine centrifugal mixer. 
     According to one aspect of the invention, the equipment is fully mobile, self-contained and designed to be pulled by a small pick-up truck requiring only delivery of the two chemical components, calcium oxide and water, at the jobsite. 
     According to one aspect of the invention, the apparatus includes variable and accurate dosing of the chemical components, thus ensuring the required slurry consistency. 
     According to one aspect of the invention, the apparatus includes an eductor to provide intimate mixing of the two chemical components. 
     According to one aspect of the invention, a serpentine centrifugal mixer is used to achieve a high shear rate between the components of the slurry, thus ensuring full conversion of the calcium oxide to calcium hydroxide. 
     According to another aspect of the invention, the calcium oxide and water mixture is subjected to pressure during the later stages of hydration, forcing water into any remaining unhydrated particles, assuring complete. hydration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, advantages, features and characteristics of the present invention, as well as methods, operation and functions of related elements of structure, and the combination of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein: 
     FIG. 1 is a process flow diagram in accordance with the invention. 
     FIGS. 2 and 2B is a plan view, and a side elevation view of the equipment in its field operation arrangement, in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     In accordance with the preferred embodiment, the mobile lime slaker is transported to or near a jobsite. Calcium oxide is delivered to the mobile lime slaker in dry powder or pebble form, generally in a bulk road tanker (not shown). 
     The calcium oxide is discharged from the bulk road tanker by blowing the dry powder or pebbles through a cyclone inlet  111  into a cyclone  110 , which separates the solid particles from the conveying air. The cyclone inlet  111  is fitted with a pressure relief valve  113  to prevent system over-pressures by the conveying air. The cyclone  110  deposits the calcium oxide into a surge bin  120 . The surge bin  120  stockpiles the calcium oxide so that the flow of calcium oxide can be regulated and continuous. 
     For safety reasons, the level of calcium oxide present in the surge bin  120  is monitored with a high-level alarm  121  to alert the operator when the level of calcium oxide in the surge bin  120  exceeds a predetermined level so that the operator can stop or adjust the flow into the surge bin  120 . 
     As the calcium oxide is discharged through the cyclone  110 , air is discharged from the cyclone  110  through a cyclone outlet  112  into a passive dust collector  140 . The passive dust collector  140  removes dust particles and deposits them through a rotary feeder  141  into a metering feeder  150  capable of maintaining a constant-bulk density of the calcium oxide, preferably a vibrating screw feeder. The air is thereby left substantially free of dust particles, and so can be discharged into the atmosphere. 
     Calcium oxide from the surge bin  120  is discharged into the vibrating screw feeder  150  where it combines with calcium oxide from the dust collector  140  to completely fill the vibrating screw feeder  150 . The vibrating screw feeder  150  may be electrically driven. Furthermore, the vibrating screw feeder  150  may have variable speed control. An electrically driven vibrating screw feeder  150  with a variable speed control advantageously allows the feed rate of calcium oxide to be varied accurately so that the proper slurry mixture can be consistently delivered. 
     A low-level measuring device  122  in the surge bin  120  automatically stops both the vibrating screw feeder  150  and the water supply through the solenoid valve  161  when the level of calcium oxide solids in the surge bin  120  reaches the preset minimum level. A timer is incorporated so that the minimum level must be detected continuously for an adjustible period of time before the feeder and water supply are stopped. The timer prevents transient conditions from shutting down the process. 
     Calcium oxide is mixed with water by means of an eductor  170 . The eductor  170  is a device that uses high-pressure water supplied through the solenoid valve  161  to create suction. This suction draws calcium oxide from the vibrating screw feeder  150  into the line where the calcium oxide is mixed with water to form calcium hydroxide. 
     Water for the eductor  170  is supplied from a reservoir or surge tank  160  that allows a controlled supply to the water pump  162 . The water could be transported to the jobsite, or drawn from a nearby hydrant, creek, pond or similar source. Water is drawn from the reservoir tank  160  and pressure is developed through a water pump  162 . Accurate metering of the water is provided by a turbine flow meter  163 . The turbine flow meter  163  controls a metering valve  164 . The turbine flow meter  163  and metering valve  164  allow an operator to determine and control the flow of water being mixed with the calcium oxide in the eductor  170 . In this way, the percentage of calcium oxide and water in the final solids composition of the slurry can be controlled. 
     Discharge from the eductor  170  flows by means of a pipeline  171  into the deaerating sump  180 , where further mixing of the calcium oxide and water allows the slaking reaction, which transforms the calcium oxide into calcium hydroxide during a controlled retention period. The deaerating sump  180  is a double-walled tank to maximize the retention time for the slurry while the slaking reaction proceeds. Slurry from the inner tank  182  overflows into the outer tank  184 , and the retention time created by the overflow process is sufficient to allow the slaking reaction to be significantly advanced. 
     Good mixing of the solids and water are required to allow the slaking reaction to proceed rapidly and this is achieved through a conical-shaped valve  172  at the base of the discharge pipeline  171  in the inner tank  182  of sump  180 . The conical-shaped valve  172  creates a circular motion of the fluids due to the shape of the valve. This motion, volume and pressure of the incoming mixture impinging upon the valve produces a high degree of agitation. 
     Further agitation of the slurry is provided by a positive slurry feedback circuit  190 . The flow control valves  192  and  194  recirculate a proportion of the slurry that has been discharged back into deaerating sump  180 . The slurry pump  200  forces the recirculated slurry under pressure into the inner tank  182  through flow control valve  192  and into the outer tank through flow control valve  194 . This recirculation causes further intimate mixing to take place while assisting the safe dispersion of the heat generated in the exothermic reaction by the sump tank  180  being open to the atmosphere. 
     Slurry discharged from the deaerating sump  180  moves through a sump outlet  186  in the base of the outer tank  184 . The slurry is pulled under suction into a slurry pump  200  which develops pressure in order to force the discharge of slurry from the slaking operation. A proportion of the slurry exiting the pump  200  is diverted through flow control valves  192  and  194 , back into the deaerating sump  180  to provide the agitation and mixing of the materials without the requirement for mechanical agitation equipment in this chemically aggressive and difficult environment. 
     The slurry pump  200  forces the slurry, which is still reacting to some degree, into a serpentine centrifugal mixer  210  consisting of a tube  212  turned into a number of coils  214  around a fixed form  216 . Forcing the slurry through a series of turns creates high shear mixing under pressure that further breaks down any still reacting particles in the slurry and ensures continuation of the slaking reaction. 
     The slurry exits the serpentine centrifugal mixer  210  through an air-operated diaphragm valve  220 . The valve  220  controls the flow of slurry to maintain a pre-set level in the outer tank  184 . The slurry is finally carried out of the mobile lime slaker by means of a discharge hose  222  attached to a jib crane  224  that can be turned to either side of the mobile lime slaker or put into a position to recycle the slurry in a holding pattern within the system. 
     To achieve full slurry recirculation, the air-operated diaphragm valve  220  is temporarily closed and the discharge hose  222  attached to a jib crane  224  is rotated to recirculate the slurry into the deaerating sump  180  at which point diaphragm valve  220  is reopened and the slurry goes in a continuous loop without being discharged from the equipment. When the next receiving tank  240  is in place, the diaphragm valve  220  is again closed while the discharge hose  222  is placed in position to discharge into the tanker  240  and valve  220  is reopened to allow continuation of the discharge process. 
     Most typically, slurry is taken away from the mobile lime slaker  100  in road delivery tankers  240  to the jobsite and there is a period of time between the departure of one vehicle and the setting up of the next when it is not possible to discharge slurry, but in view of the dangerous nature of the exothermic reaction, it is not advisable to stop the flow of material. 
     A pressure relief valve  230  is provided should a malfunction cause the slurry pump  200  to stop while the lines are full. 
     Control for the process is preferably provided through electrical sensors and speed controls, solenoid valves and instrumentation operated by a generating set which is mounted as an integral part of the apparatus and allows a single operator to maintain control of all aspects of the equipment from a central control panel  250  mounted upon the apparatus  100 . Electrical power for the system&#39;s electrical components is preferably provided by a diesel generator  270 . 
     The entire unit can be mounted on a trailer. In this way the unit can be transported to a job site, and set up in a minimal amount of time. Operation of the slaker in accordance with the present invention requires only a source of water, a commercial lime delivery and slurry distribution trucks, or a storage tank to begin production. On larger jobs, a storage tank may be incorporated at the site to allow for continuous production. The unit can, thus, be used economically on small jobs where current methods cannot be justified because of high set-up costs. 
     Following use, it is essential to drain-down and wash out all the equipment to prevent a build-up of lime scale. The achieve this, the conical mixing valve  172  can be raised by means of an air cylinder  260  exposing a discharge opening  262  in the base of the inner tank  182  of the deaerating sump  180  through which slurry and wash-down water can access the slurry pump  200  and be discharged. 
     Although the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. The invention is limited only by the following claims and their equivalents.