Patent Publication Number: US-2023140807-A1

Title: Method and Apparatus for Producing Core-Shell Calcium Hydroxide-Calcium Carbonate Particles

Description:
FIELD OF INVENTION 
     This present disclosure relates to the production of fillers, and more specifically, the production of core-shell calcium hydroxide-calcium carbonate particles. 
     BACKGROUND OF THE INVENTION 
     The use of fillers in polymer compositions, paints, and coatings is well known and established in literature. Fillers usually impart enhanced properties to the final product, including mechanical, optical, physical as well as fire retardancy properties. U.S. Pat. No. 9,493,658 and U.S. Pat. No. 6,310,129 provide suitable techniques for the use of fillers. Different commercial inorganic powdered fillers such as calcium carbonates, talcs, clays, gypsum, barytes, feldspar and silicates are currently widely in use. Nevertheless, the application of ground mineral fillers is limited by their relatively large sizes, as indicated in U.S. Pat. No. 6,310,129. Thus, micro-size fillers are usually synthesized chemically, which makes them much more costly. 
     Synthetic Ca(OH) 2  filler in polyvinyl chloride (PVC) neutralizes the toxic chlorine gas produced in the event of PVC combustion. The fire retardancy of Ca(OH) 2 , on the other hand, is questionable, since Ca(OH) 2  reacts exothermically at relatively low temperature with CO 2  in presence of air yielding CaCO 3 , rather than decomposing endothermically to its oxide upon heating in the presence of air as indicated in U.S. Pat. No. 6,310,129. However, Ca(OH) 2  additive slowly reacts with atmospheric CO 2  to yield CaCO 3 , which may limit functionality of Ca(OH) 2 . Nevertheless, commercial Ca(OH) 2  is still used as an additive for different thermosetting resins to improve the tracking resistance of electrical/optical instruments, as indicated by U.S. Pat. No. 9,493,658, U.S. Pat. No. 6,310,129 and U.S. Pat. No. 7,883,681. 
     Carbide lime coproduced during acetylene manufacturing consists of 70 -85% wt/wt Ca(OH) 2  and 5 - 25% wt/wt CaCO 3  in the form of shell onto the Ca(OH) 2  grains as indicated in U.S. Pat. No. 7,883,681. Carbide lime has been found an effective filler in many products owing to its multifold properties. Carbide lime is used for waste acid neutralization, gas scrubbing and desulphurization, pH control in sewage and water treatment plants, production of building blocks and paving material, dehalogenation as well as the manufacturing of calcium magnesium acetate and calcium hypochlorite. These uses for carbide lime are indicated in U.S. Pat. No. 6,310,129 and U.S. Pat. No. 5,997,883 and F.A. Cardoso et al. “Carbide lime and industrial hydrated lime characterization”, Powder Technol., 2009, doi: 10.1016/j.powtec.2009.05.017. Carbide lime is also an effective antibacterial, anti-viral, and anti- fungal agent as described in U.S. Pat. No. 6,310,129 and U.S. Pat. 7,883,681. Carbide lime is ground and screened to collect particles of desired sizes for a given application. 
     However, the use of carbide lime is limited by its greyish color due to the coke used during the acetylene gas production. Thus, all resin molded products utilizing the processed raw carbide lime have dark colors as taught in U.S. Pat. No. 7,883,681. Therefore, synthetic core-shell Ca(OH) 2  — CaCO 3  particles (also referred to herein as calcium carbonate-coated calcium hydroxide particles) have been prepared. 
     One method of preparing the calcium carbonate-coated calcium hydroxide particles is through blowing CO 2 -containing gas, e.g., flue gas, into a bed containing Ca(OH) 2  particles as described by Meade in U.S. Pat. No. 7,883,681. In addition, exposure time helps controlling the thickness of the CaCO 3  coating, however, the process is poorly reproducible, mainly due to particle collision. Collision deteriorates part of the coating, blocks particles from reacting, and contributes to major particle aggregation as shown in U.S. Pat. No. 7,883,681. To overcome this limitation, U.S. Pat. No. 9,493,658 teaches the preparation of calcium carbonate-coated calcium hydroxide particles upon reacting finely ground commercial Ca(OH) 2  particles with dry ice. In one design, Ca(OH) 2  particles and dry ice are added to a silo from two separate ports. Mixing between the reactants is enabled by gravity settling of the particles in sublimating dry ice. Another design allows for a very brief mixing of the reactants in a Hobart mixer prior to introducing the reactants to the silo from a single port. This approach addressed the limitations reported previously and was successful in producing a more consistent and even particles having 70 - 95 wt% Ca(OH) 2  and 5 - 30 wt% CaCO 3  surface coating as shown in U.S. Pat. No. 9,493,658. 
     However, it is noted that gravity settling within the silo may not be ideal to control the thickness of the CaCO 3  coating. While residence time depends on the height of the silo and Ca(OH) 2  particle size as indicated in H. Scott Fogler, “Elements of chemical reaction engineering”  Chem. Eng. Sci. , 1987, doi: 10.1016/0009-2509(87)80130-.6, less control over the calcium carbonate-coated calcium hydroxide particle size distribution is achieved within a silo. Moreover, the calcium carbonate-coated calcium hydroxide particle size is mainly controlled through selecting the size of the Ca(OH) 2  reactant particles. For example, reacting ~ 44 µm Ca(OH) 2  particles with dry ice having a mesh size of minus 12 - plus 18 in the form of flakes produces calcium carbonate-coated calcium hydroxide particles of 0.1 - 75 µm. For a given silo, the mass ratio of the reactants is used to control the calcium carbonate-coated calcium hydroxide product specifications as indicated in U.S. Pat. No. 9,493,658. Lastly, mixing of the solid reactants prior to introducing the reactants to the silo may lead to Ca(OH) 2  particle aggregation, especially given the particle small size and the corresponding surface energy, as taught in M. Husein, “Preparation of nanoscale organosols and hydrosols via the phase transfer route”,  Journal of Nanoparticle Research . 2017, doi: 10.1007/s11051-017-4095-0. 
     Modeled after carbide lime, the calcium carbonate-coated calcium hydroxide particles produced from reacting Ca(OH) 2  with dry ice also have proven antibacterial, antifungal, and antiviral attributes as well as significant pH adjustment property as indicated in U.S. Pat. No. 9,493,658. These attributes make these synthetic calcium carbonate-coated calcium hydroxide particles an ideal filler for different commercial products. Preliminary testing in the Southwest Research Institute, “Final Report of Southwest Research Institute (SwRI) Project 20637 (Proposal No. 01-72445) “Mold Resistance Efficacy Testing of Paint with ZeroMold Additive,” 2015 and Southwest Research Institute, “Laboratory Testing Results,” 2014 showed that resin-molded products and paints mixed with calcium carbonate-coated calcium hydroxide particles impart significant sterilizing properties, including bactericidal, fungicidal, and virucidal attributes. These antimicrobial attributes are induced by the high Ca(OH) 2  content and is expected to last for up to a hundred years, per the accelerated aging testing as is indicated in U.S. Pat. No. 9,493,658. 
     In addition, in Vance et al. (2015) ‘Direct Carbonation of Ca(OH)( 2 ) Using Liquid and Supercritical CO 2 : Implications for Carbon-Neutral Cementation’, Industrial &amp; engineering chemistry research, 54(36), pp. 8908-8918. doi:10.1021/acs.iecr.5b02356, the carbonation of Ca(OH) 2  upon placing in liquid CO 2  was investigated. An isothermal process to vent out the liquid CO 2  was used. Analysis of the reaction kinetics showed that Ca(OH) 2  reaction in liquid CO 2  is rapid (~ 80% conversion in 2 h). This suggests that the product CaCO 3  layer is non-passivating. The pressure and temperature had little effect on the carbonation rate. Furthermore, scanning electron microscope (SEM) images for the carbonated Ca(OH) 2  have indicated the formation of calcite layers on the surfaces of Ca(OH) 2  grains. Irregular growth, nonuniform morphological structure, and exfoliation of the initially formed CaCO 3  surface layers (terracing effect) are the main reasons behind the non-passivating calcite layer formed on top of Ca(OH) 2 . The materials produced using the procedure of Vance et al. were tested for biocidal activity and the results showed low effective biocidal activity. This was due to the high extent of particle agglomeration as well as inconsistent CaCO 3  film. The drawbacks in morphology contributed to less effective biocidal particles. 
     Furthermore, according to Dheilly, R.M, J Tudo, Y Sebaïbi, and M Queneudec. “Influence of Storage Conditions on the Carbonation of Powdered Ca(OH) 2  .” Construction &amp; building materials 16, no. 3 (2002): 155-161, a drawback of the reaction of Ca(OH) 2  with gaseous CO 2  is that it occurs slowly at the temperatures associated with the throttling process, especially in absence of moisture. 
     SUMMARY OF THE INVENTION 
     According to various aspects to the present invention, there is provided a method for preparing calcium carbonate (CaCO 3 )-coated calcium hydroxide (Ca(OH) 2 ) particles. The method includes introducing liquid carbon dioxide into a reaction vessel, introducing calcium hydroxide particles into the reaction vessel, and effectively mixing the calcium hydroxide particles into the liquid carbon dioxide. The method further includes inducing a phase change in the liquid carbon dioxide so as to coat the calcium hydroxide in dry ice. In addition, the method includes sublimating the dry ice after a predetermined residence time to control the thickness of the calcium carbonate coating on the calcium hydroxide particles. 
     The method may include the liquid carbon dioxide being introduced into the reaction vessel at a pressure of 8 MPa and a temperature of —25° C. 
     Alternatively, the method may include the liquid carbon dioxide being introduced into the reaction vessel at a pressure range of 0.518 MPa to 16 MPa and a temperature range of -56.56° C. to 30.98° C. 
     The introduction of calcium hydroxide particles into the reaction vessel may include feeding the calcium hydroxide particles into an auxiliary chamber, flushing the calcium hydroxide particles in the auxiliary chamber with the liquid carbon dioxide and introducing the mixture into the reaction vessel to be further mixed with the already present liquid carbon dioxide. 
     Alternatively, the calcium hydroxide particles may be introduced into the reaction vessel prior to the liquid carbon dioxide being introduced into the reaction vessel. 
     The method may include a high-pressure reactor as the reaction vessel, the high-pressure reactor including a stirrer for mixing. 
     Alternatively, the method may include an inline mixer as the reaction vessel. 
     Inducing the phase change in the liquid carbon dioxide may be performed using a throttle valve to flash the liquid carbon dioxide into dry ice. 
     The throttle valve may flash at a pressure of 0.1 MPa to create dry ice. 
     Alternatively, the throttle valve may flash at a pressure range of 0.01 MPa to 0.518 MPa and a temperature lower than —56.56° C. 
     Controlling the thickness of the calcium carbonate coating on the calcium hydroxide particles occurs over the predetermined residence time in a separator vessel at a pressure of less than or equal to 0.518 MPa. 
     The method may further include collecting gaseous carbon dioxide from the sublimation of the dry ice and inducing a phase change in the gaseous carbon dioxide to provide liquid carbon dioxide to be introduced into the reaction vessel. 
     According to various aspects to the present invention, there is provided a system for producing calcium carbonate (CaCO 3 )-coated calcium hydroxide (Ca(OH) 2 ) particles. The system includes a reaction vessel for receiving liquid carbon dioxide and calcium hydroxide particles. The system further includes a stirrer to effectively mix the liquid carbon dioxide and calcium hydroxide particles, and a throttle valve for inducing a phase change to liquid carbon dioxide to coat the calcium hydroxide particles in dry ice. In addition, the system includes a separator vessel for sublimating the dry ice after a predetermined residence time to control the thickness of the calcium carbonate coating on the calcium hydroxide particles. 
     The system may include the liquid carbon dioxide being received by the reaction vessel at a pressure of 8 MPa and a temperature of —25° C. 
     Alternatively, the system may include the liquid carbon dioxide being received by the reaction vessel at a pressure range of 0.518 MPa to 16 MPa and a temperature range of -56.56° C. to 30.98° C. 
     The calcium hydroxide particles may be received by the reaction vessel through flushing the calcium hydroxide particles in an auxiliary chamber with the liquid carbon dioxide and introducing the mixture into the reaction vessel to be mixed with the already present liquid carbon dioxide. 
     Alternatively, the calcium hydroxide particles may be received by the reaction vessel prior to the liquid carbon dioxide being received by the reaction vessel. 
     Alternatively, the liquid carbon dioxide is received by the reaction vessel prior to the calcium hydroxide particles being received by the reaction vessel. 
     The system may include a high-pressure reactor as the reaction vessel and the high-pressure reactor including a stirrer for mixing. 
     Alternatively, the system may include an inline mixer as the reaction vessel. 
     The throttle valve of the system may induce a phase change by flashing the liquid carbon dioxide at a pressure range of 0.01 MPa to 0.518 MPa and a temperature lower than -56.56° C. 
     Alternatively, the throttle valve of the system may induce the phase change by flashing the liquid carbon dioxide to a pressure of 0.1 MPa. 
     Controlling the thickness of the calcium carbonate coating on the calcium hydroxide particles occurs over the predetermined residence time in a separator vessel at a pressure of less than or equal to 0.518 MPa. 
     The system may further include a gaseous carbon dioxide outlet connected to the separator vessel, where the gaseous carbon dioxide outlet collects gaseous carbon dioxide from the sublimation of the dry ice in the separator vessel. The system may also include a return line with an in-line pressurization system connecting the gaseous carbon dioxide outlet and the reaction vessel, where the return line with the in-line pressurization system may be configured to induce a phase change to the gaseous carbon dioxide to provide liquid carbon dioxide to be introduced into the reaction vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The embodiments of the present invention shall be more clearly understood with reference to the following detailed description of the embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    depicts a system for producing calcium carbonate-coated calcium hydroxide entrapped into dry ice in accordance with an embodiment of the invention; 
         FIG.  2    depicts a method of producing calcium carbonate-coated calcium hydroxide particles in accordance with a first production line of the example system in  FIG.  1   ; 
         FIG.  3    depicts a method of producing calcium carbonate-coated calcium hydroxide particles in accordance with a second production line of the example system in  FIG.  1   ; 
         FIG.  4    depicts a system for producing calcium carbonate-coated calcium hydroxide in accordance with another embodiment of the invention, where the end product gaseous CO 2  is recirculated into the system for further use as liquid CO 2   
         FIG.  5    depicts a method of producing calcium carbonate-coated calcium hydroxide particles in accordance with a first production line of the system shown in  FIG.  4   ; 
         FIG.  6    depicts a phase diagram of carbon dioxide (CO 2 ) showing the stability fields of the solid, liquid and vapor states; 
         FIG.  7    depicts a phase diagram of carbon dioxide (CO 2 ) showing the regions of thermodynamically stable state(s) of CO 2  (i.e. solid, liquid and vapor states) at different values of pressure (psia) and specific enthalpy H (Btu/lb m ), where pressure is provided along the Y-axis on the left side of the phase diagram, specific volume ν (ft 3 /lb m ) is provided along the Y-axis on the right side of the phase diagram and specific enthalpy is provided along the X-axis of the phase diagram and other thermodynamic properties corresponding to given values of pressure and specific enthalpy, specific entropy S (Btu/(1b m )(R)), temperature (°F) and χ, weight fraction vapor, are also shown. The reference state is saturated liquid CO 2  at -40° F., where specific enthalpy H=0 and specific entropy S=0; 
         FIG.  8    depicts a system for producing calcium carbonate-coated calcium hydroxide particles in accordance with yet another embodiment of the invention, where the calcium hydroxide particles induce heterogeneous nucleation of dry ice in an exit stream of the throttling valve; and 
         FIG.  9    depicts a method of producing calcium carbonate-coated calcium hydroxide particles in accordance with the example system in  FIG.  8   . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     The description, which follows, and the embodiments described therein are provided by way of illustration of an example, or examples of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals. 
     By way of general overview, there is provided a method for preparing calcium carbonate-coated calcium hydroxide particles in accordance with a preferred embodiment of the invention. The method generally involves dispersing calcium hydroxide (Ca(OH) 2 ) particles in liquid carbon dioxide (CO 2 ) and then flashing/throttling the particles to induce a phase change in the liquid carbon dioxide so it becomes dry ice. The resultant dry ice entraps the Ca(OH) 2  particles within its solid structure achieving enhanced coating of the particles with dry ice. 
     In contrast to existing methods, method  200  is advantageous in that it allows improved carbonation and enhanced control over the level of carbonation of the particles. This is achieved through solid-liquid mixing, which tends to achieve a more thorough mixing than solid-solid mixing. Major carbonation reaction, however, proceeds between the dry ice and the entrapped Ca(OH) 2  particles, thereby allowing for more uniform coating of the particles. In this method, carbonation proceeds at the same rate in all directions, including the radial direction. As discussed below, the thickness of the CaCO 3  shell can be controlled by selectively reducing or increasing the residence time during which the carbon dioxide remains as a dry ice coating on the calcium hydroxide core, prior to sublimating the dry ice. This, in turn, permits better customization of the structural properties of the resultant calcium carbonate-coated calcium hydroxide particles such that the particles can be used in a wider range of products or have a wider range of applications, such as use in fillers for plastics, papers, cement and drywall. In addition, the resultant calcium carbonate-coated calcium hydroxide particles also have a higher biocidal activity, leading to an increased number of uses, especially in environments where a biocidal effect is advantageous. Method  200  further differs from existing production methods in that phase changes are induced in the carbon dioxide from liquid phase to solid and gas phases, as opposed to existing methods where the carbon dioxide changes from the solid phase to the gas phase. In addition, method  200  is advantageous over existing production methods in that it has a significantly faster production time. 
       FIG.  1    depicts a system  100  for producing calcium carbonate-coated calcium hydroxide particles having two production lines  160  and  160 A. The first production line  160  is depicted above the dotted line in system  100  and the second production line  160 A is depicted below the dotted line in system  100 . The first production line  160  and the second production line  160 A may run in parallel and share certain components that will be discussed further below. 
     The first production line  160  includes an insulated pressurized liquid CO 2  storage tank  104  (hereinafter referred to as storage tank  104 ) connected to an insulated high pressure reactor  124  through a CO 2  feed line  108 , allowing liquid CO 2  to be sent from storage tank  104  to high pressure reactor  124 . Gate valve  112  and in-line pressurization system  116  may be positioned along CO 2  feed line  108 . In addition to receiving liquid CO 2  from CO 2  feed line, high pressure reactor  124  also receives Ca(OH) 2  particles from Ca(OH) 2  feed line. High pressure reactor  124  includes stirrer  128  for mixing the liquid CO 2  and the Ca(OH) 2  particles. High pressure reactor  124  is connected to separator vessel  136  via throttle valve  132 , where throttle valve  132  flashes the liquid CO 2  surrounding the Ca(OH) 2  particles and separator vessel  136  receives the resultant dry ice. The dry ice then sublimates in separator vessel  136  where the produced gaseous CO 2  is discharged through the connected gaseous CO 2  outlet  140  and the produced calcium carbonate coated hydroxide particles are discharged via the connected calcium carbonate coated calcium hydroxide product particle outlet  144 . 
     Insulated pressurized liquid CO 2  storage tank  104  stores liquid CO 2 , and is readily available through commercial means. Storage tank  104  may be of any size, and in this current embodiment may be the standardized 50 tonne storage tank that is typically supplied by tanker trucks. Typically, the storage tank  104  installation and associated piping and controls is part of vendor’s supply and service agreement. 
     Liquid CO 2  feed line  108  leads from storage tank  104  to high pressure reactor  124 , allowing liquid CO 2  to be sent from storage tank  104  to high pressure reactor  124  (also known as a reaction vessel), where it is received by high pressure reactor  124  through a leak-proof port. In the current embodiment, CO 2  feed line  108  includes going through gate valve  112 , and in-line pressurization system  116 . Gate valve  112  allows control of the flow of liquid CO 2  from storage tank  104  to high pressure reactor  124 , and further allows the flow of liquid CO 2  to be safely shut off. In the current embodiment, gate valve  112  is used, however in alternate embodiments, a ball valve may be used for the same function. A person skilled in the art will recognize that different valve types may be used to control the flow of liquid CO 2  and for the safe shut off of the flow of liquid CO 2 . In-line pressurization system  116  maintains the pressure within CO 2  feed line  108  to ensure that the CO 2  remains in a liquid state. While not necessary in the current embodiment, as the liquid CO 2  is already in the liquid state to be introduced into high-pressure reactor  124 , in-line pressurization system  116  may also change the pressure, while maintaining lower temperature than the critical temperature, if storage tank  104  holds CO 2  in a different state, such as gaseous CO 2 , inducing a phase change from gaseous CO 2  to liquid CO 2 . It will occur to a person skilled in the art that gate valve  112  and in-line pressurization system  116  are optional, and that storage tank  104  may send liquid CO 2  to high-pressure reactor  124  without the need for gate valve  112  or in-line pressurization system  116 . In other embodiments, first production line  160  may include gate valve  112  and lack an in-line pressurization system  116  or alternatively, production line  160  may include in-line pressurization system  116  and lack gate valve  112 . 
     Ca(OH) 2  feed line  120  allows Ca(OH) 2  particles to be dispersed into high-pressure reactor  124 . High-pressure reactor  124  includes stirrer  128 , allowing the Ca(OH) 2  particles to be further mixed with the liquid CO 2  to ensure that the Ca(OH) 2  particles are thoroughly distributed and coated with the liquid CO 2 . 
     The resulting mixture can then be sent through throttle valve  132  into separator vessel  136 . The resulting mixture goes through a phase change while it passes through throttle valve  132 , resulting in dry ice particles containing calcium hydroxide particles. 
     The second production line  160 A is similar in layout to the first production line  160  but uses an inline mixer  148  in place a of high-pressure reactor  124 . The second production line  160 A includes storage tank  104  connected to inline mixer  148  via a CO 2  feed line  108 A. Gate valve  112 A and in-line pressurization system  116 A are positioned along CO 2  feed line  108 A. Inline mixer  148  receives liquid CO 2  from CO 2  feed line  108 A, and further receives Ca(OH) 2  from Ca(OH) 2  feed line  120 A. The liquid CO 2  and Ca(OH) 2  are mixed in inline mixer  148  and sent through throttle valve  132 A. Throttle valve  132 A flashes the liquid CO 2  surrounding the Ca(OH) 2  particles into dry ice, where it is received by separator vessel  136 . The dry ice sublimates, and the resulting products of gaseous CO 2  and calcium carbonate coated calcium hydroxide particles are discharged through their respective outlets, gaseous CO 2  outlet  140 , and calcium carbonate coated calcium hydroxide product particle outlet  144 . 
     CO 2  feed line  108 A leads from storage tank  104  to inline mixer  148 , allowing liquid CO 2  to be sent from storage tank  104  to inline mixer  148 . CO 2  feed line  108 A includes going through gate valve  112 A, and in-line pressurization system  116 A. Similar to gate valve  112  of first production line  160 , gate valve  112 A of second production line  160 A allows control of the flow of liquid CO 2  from the storage tank  104 , and further allows the flow of liquid CO 2  to be safely shut off. Similar to in-line pressurization system  116  of first production line  160 , in-line pressurization system  116 A of second production line  160 A maintains the pressure within CO 2  feed line  108 A to ensure the CO 2  remains in a liquid state. Similar to first production line  160 , gate valve  112 A and in-line pressurization system  116 A are optional. Liquid CO 2  is received into inline mixer  148  through a leak-proof port. 
     Ca(OH) 2  particles may be dispersed into inline mixer  148  through Ca(OH) 2  feed line  120 A, where the Ca(OH) 2  particles may be further mixed with liquid CO 2  to ensure that the Ca(OH) 2  particles are thoroughly dispersed into liquid CO 2 . Inline mixer  148  may also have a stirrer to further promote the dispersion of the Ca(OH) 2  particles. The resulting mixture can then be sent through throttle valve  132 A where dry ice is formed. The dry ice is then fed into separator vessel  136 . 
     Separator vessel  136  allows the dry ice to settle and allows gaseous CO 2  to leave without carrying particles with it. In both the first production line  160  and second production line  160 A, separator vessel  136  may include a filter to aid in separating gaseous CO 2  and calcium carbonate coated calcium hydroxide product particles. The filter (also known as a mist eliminator) captures dust or particles that are leaving with the CO 2  vapors. The mist eliminator is also generally used in evaporators and is known to persons skilled in the art. Furthermore, separator vessel  136  may include a pressure control module to change and maintain pressure within separator vessel  136 , and a heating element to accelerate dry ice sublimation if needed. 
     Once the dry ice has been sublimated, the resultant products of gaseous CO 2  and calcium carbonate-coated calcium hydroxide products may be collected through gaseous CO 2  outlet  140  and calcium carbonate-coated calcium hydroxide product particle outlet  144  respectively. 
     In alternate embodiments, separator vessel  136  may be substituted with cyclone  832 , where centrifugal forces and cyclonic separation allow the separation of the calcium carbonate coated calcium hydroxide product particles and the gaseous CO 2 . Cyclone  832  will be further discussed below. 
     In another embodiment, system  100 C depicts the same two production lines  160  and  160 A, however, gaseous CO 2  from gaseous CO 2  outlet  140  is returned to the storage tank  104  via return line  156 , after going through in-line pressurization system  152 . In-line pressurization system  152  raises the pressure within return line  156  to induce a phase change in the CO 2 , converting the gaseous CO 2  into liquid CO 2 . It will occur to a person skilled in the art that in-line pressurization system  152  may be any component that induces a phase change in the CO 2 . 
     Returning to  FIG.  1   , while system  100  depicts the two production lines  160  and  160 A for producing calcium carbonate-coated calcium hydroxide, it will occur to a person skilled in the art that the two production lines  160  and  160 A are not limited to running in parallel and may in fact run independently. 
       FIG.  2    depicts method  200  for producing calcium carbonate coated calcium hydroxide particles using first production line  160 . 
     In the first production line  160 , block  205  depicts introducing liquid CO 2  into high-pressure reactor  124  from storage tank  104  via CO 2  feed line  108 . Block  210  depicts feeding Ca(OH) 2  particles into high-pressure reactor  124  via Ca(OH) 2  feed line  120 . 
     In the current embodiment of method  200 , liquid CO 2  is received first by high-pressure reactor  124 , and then Ca(OH) 2  particles are then dispersed into high-pressure reactor  124 . In other embodiments, it is contemplated that Ca(OH) 2  particles could be fed into the high-pressure reactor before the liquid CO 2 . In another embodiment, it is contemplated that Ca(OH) 2  particles are placed into a small chamber or an auxiliary chamber, and then flushed with a small amount of liquid CO 2 , prior to being moved into high-pressure reactor  124  to be further mixed with additional liquid CO 2 . The Ca(OH) 2  particles can be flushed with a small amount of liquid CO 2  into a nozzle which directs the mixture into an opposing nozzle ejecting liquid CO 2 . The spray from either nozzle is designed to overlap with one another, to further promote dispersion. 
     Once both liquid CO 2  and Ca(OH) 2  particles are received, they may be mixed/dispersed in high-pressure reactor  124  using stirrer  128 . This is depicted in block  215 . Throughout the steps in blocks  205  to  215 , the liquid CO 2 (1) may be kept at a range of 0.518 MPa to 16 MPa and -56.56° C. to 30.98° C. In a preferred embodiment, the liquid CO 2  is kept at 8 MPa and - 25° C., as this is easily achieved, and is readily used in industry. The dispersion of the Ca(OH) 2  particles in liquid CO 2  is to ensure a thorough and uniform coating of liquid CO 2  surrounding the Ca(OH) 2  particles. 
     At block  220 , the liquid CO 2  and Ca(OH) 2  particle mixture is then sent through a throttle valve  132 . The liquid CO 2  undergoes a phase change from liquid into a solid, creating a dry ice shell surrounding the Ca(OH) 2  particles. 
     In the current embodiment, the liquid CO 2  and Ca(OH) 2  mixture is sent through throttle valve  132 , and the liquid CO 2  is flashed into dry ice at preferred normal atmospheric pressure of 0.1 MPa creating a dry ice shell surrounding the Ca(OH) 2  particle. Alternatively, the dry ice shell may also be formed through throttle valve  132  at different temperatures and pressures by undergoing a phase change.  FIG.  6    is a pressure-temperature phase diagram that depicts other conditions at which liquid CO 2  and dry ice can be obtained.  FIG.  7    is a pressure-enthalpy phase diagram depicting conditions at which dry ice can be formed from liquid CO 2  through an isenthalpic process.  FIG.  6    can be found at “CO 2  as a Refrigerant - Properties of R744” by Andre Patenaude published on May 14, 2015, located at the following URL: “https://emersonclimateconversations.com/2015/05/14/co2-as-a-refrigerant-properties-of-r744/”. The data for  FIG.  7    can be found at Plank, R., and Kuprianoff, J., Z. ges. Kalte-Ind., 1, 1 (1929); Z. tech. Physik, 10, 99 (1929). It will occur to a person skilled in the art that a change of state may be performed through a change in pressure or temperature, and as such, the liquid CO 2  and Ca(OH) 2  mixture is not limited to being sent through throttle valve  132 . Other apparatus or devices may be contemplated to aid in the change of state from the liquid CO 2  and Ca(OH) 2  mixture into dry ice. 
     The dry ice shell surrounding the Ca(OH) 2  particles are then fed into separator vessel  136 . Pressure is maintained within separator vessel  136 , allowing the Ca(OH) 2  particles to react with the dry ice shell, affecting the thickness of the CaCO 3  shell. The longer pressure is maintained, the thicker the CaCO 3  shell. The residence time of the Ca(OH) 2  particles and the dry ice within separator vessel  136  while pressure and temperature are maintained, correlates directly to the thickness of the CaCO 3  shell surrounding the Ca(OH) 2  particles. In a preferred embodiment, separator vessel  136  operates at 0.1 MPa, however separator vessel  136  may be maintained at a pressure range between 0.01 MPa to 0.518 MPa to thicken the CaCO 3  shell. In a preferred embodiment, separator vessel  136  may be maintained above —78.5° C. Once the desired thickness of the CaCO 3  shell is achieved, the dry ice may be sublimated, producing gaseous CO 2 . The dry ice may also be heated using a heating element, to further accelerate the sublimation process. This is depicted at block  225 . 
     At block  230 , the resulting calcium carbonate-coated calcium hydroxide product is collected. It will occur to the person skilled in the art that the size of the calcium carbonate-coated calcium hydroxide product may be controlled by the choice of the Ca(OH) 2  particle size that is fed into high-pressure reactor  124  through Ca(OH) 2  feed line  120 . 
       FIG.  3    depicts method  200 A for producing calcium carbonate-coated calcium hydroxide particles using second production line  160 A. As previously mentioned, second production line  160 A uses inline mixer  148  instead of high-pressure reactor  124 . Liquid CO 2  is introduced into inline mixer  148  at block  205 A and Ca(OH) 2  particles are fed into inline mixer  148  at block  210 A. The liquid CO 2  and Ca(OH) 2  is then mixed/dispersed in inline mixer  148  at block  215 A, before continuing through method  200 . It will occur to the person skilled in the art that method  200  may be used with different embodiments of systems used to produce calcium carbonate-coated calcium hydroxide products. 
       FIG.  5    depicts method  200 C for producing calcium carbonate coated calcium hydroxide particles using first production line  160  and further taking the gaseous CO 2  product and cycling it back to be reused in first production line  160 . Method  200 C has similar steps to method  200 , and blocks  205  to  230  follow the same process. Block  235  shows the collection of gaseous CO 2  from gaseous CO 2  port  140 . Block  240  depicts in-line pressurization system  152  inducing a phase change on the gaseous CO 2 , changing it to liquid CO 2 . It will occur to a person skilled in the art that similar to block  220 , the phase change may not be limited to be induced by a change in the pressure, but may also be a change in temperature, or a change in both temperature and pressure.  FIG.  6    depicts the temperatures and pressures that liquid CO 2  may be obtained at. 
     Returning to  FIG.  5   , once liquid CO 2  has been obtained, it is then reintroduced into first production line  160  at storage tank  104 , and is depicted by the line between block  240  and block  205 . It will occur to a person skilled in the art that the recirculation of CO 2  may also be applied to method  200 A in second production line  160 A. 
     The size of the resultant calcium carbonate-coated calcium hydroxide product particle collected may be determined based on the Ca(OH) 2  particles fed into either high-pressure reactor  124  or inline mixer  148 . The larger the Ca(OH) 2  particles fed into the system, the larger the resultant calcium carbonate coated calcium hydroxide product particles. Likewise, nanoparticle calcium carbonate-coated calcium hydroxide particles can be achieved by feeding nanosized Ca(OH) 2  reactant particles into the system. 
     A person skilled in the art will recognize that method  200  and method  200 A may be performed with particles other than Ca(OH) 2  particles. Particles may be fed into high-pressure reactor  124  through a feed line in method  200 , where the particles are mixed with liquid CO 2 . Alternatively, particles may be fed into inline mixer  148  through a feed line in method  200 A, where the particles are mixed with liquid CO 2 . 
     In other embodiments, different methods may be used to coat calcium hydroxide particles with calcium carbonate. For example, in an alternative embodiment, liquid CO 2  may be flashed within a reactor by suddenly dropping the pressure within the reactor to below 0.518 MPa. Dry ice forms on Ca(OH) 2  particles that were previously fed into the reactor, where the Ca(OH) 2  particles act as heterogeneous nucleation sites for the formation of said dry ice. The dry ice around the Ca(OH) 2  particles reacts with the outer shell of the Ca(OH) 2  particles producing a shell of CaCO 3 . 
     In a preferred embodiment, liquid CO 2  may be throttled to induce a phase change into an exit stream wherein it is mixed with Ca(OH) 2  particles and where, through heterogeneous nucleation, dry ice covered Ca(OH) 2  particles are created. These particles are then collected in a cyclone, where the remaining dry ice and the calcium carbonate (CaCO 3 )-coated calcium hydroxide Ca(OH) 2  particles are separated and collected. An advantage of this embodiment is that throttling to induce a phase change to the liquid CO 2  is simple to implement leading to a system with low maintenance and less failure points. Another advantage of this embodiment, is that similar to method  200 , the production time of this embodiment is significantly faster than that of existing production methods. In addition, similar to method  200 , the resultant calcium carbonate-coated calcium hydroxide particles from this embodiment also have a higher biocidal activity in comparison to those in the previously cited Vance et al. (2015) ‘Direct Carbonation of Ca(OH)( 2 ) Using Liquid and Supercritical CO 2 : Implications for Carbon-Neutral Cementation’, leading to an increased number of uses, especially in environments where a biocidal effect is advantageous. 
     An exemplary system for implementing this preferred method is depicted in  FIG.  8   , where a liquid CO 2  storage tank  104  may feed liquid CO 2  into thermally insulated hose  804  through gate valve  112 . Both gate valve  112  and flow meter  808  may control the flow rate of liquid CO 2  into thermally insulated hose  804 . The thermally insulated hose  804  is connected to expansion nozzle  816  (also referred to herein as a throttle  816  or a throttle valve  816 ), where liquid CO 2  may be throttled and so as to induce a phase change from liquid to a mix of solid and gaseous states. The resulting mixture of solid and gaseous CO 2  is propelled through exit stream  828 , where Ca(OH) 2  particles are added via screw feeder  824 . Through heterogeneous nucleation, dry ice-covered Ca(OH) 2  particles are created, which are then introduced into cyclone  832  where sublimation occurs. Any remaining/excess CO 2  and CO 2  from the sublimation may be collected though gaseous CO 2  outlet  140 , and returned to liquid CO 2  storage tank  104  after going through a phase change from gaseous state to liquid state through in-line pressurization system  152 . The produced calcium carbonate coated hydroxide particles are collected via the connected calcium carbonate-coated calcium hydroxide product particle outlet  144 . 
     As previously indicated flow meter  808  and gate valve  112  control the rate at which liquid CO 2  is introduced into thermally insulated hose  804 . The flow rate of the liquid CO 2  entering throttle  816  is proportional to the kinetic energy of exit stream  828 , where a high kinetic energy of exit stream  828  may be achieved due to the initial flow rate of the liquid CO 2  and the pressure differential between the entrance of throttle  816  and the exit of throttle  816  where exit stream  828  begins. A high kinetic energy of exit stream  828  allows for the suspension of solid dry ice and also Ca(OH) 2  particles. In a preferred embodiment, the flow rate of the liquid CO 2  in thermally insulated hose  804  and upon entering throttle  816  is approximately 173.5 kg/d. The speed of the mixture of solid and gaseous CO 2  propelled through exit stream  828  measured in proximity to the exit of throttle  816  may range between 6 m/s to 600 m/s. In a preferred embodiment, the speed of the mixture of solid and gaseous CO 2  in exit stream  828  measured in proximity to the exit of throttle  816  may be 60 m/s. 
     In the current embodiment, thermally insulated hose  804  allows for the flow of liquid CO 2  from liquid CO 2  storage tank  104  to throttle valve  816 . Thermally insulated hose  804  also ensures that the liquid CO 2  that is flowing through is kept at a pressure range of 0.518 MPa to 16 MPa and a temperature range of —56.56° C. to 30.98° C. at position  812  prior to liquid CO 2  entering throttle valve  816 . Furthermore, thermally insulated hose  804  may provide additional distance for liquid CO 2  to reach a specific flow rate. However, if the liquid CO 2  is kept at said pressure range and temperature range within liquid CO 2  storage tank  104 , and the liquid CO 2  may be discharged as a specific flow rate, insulated hose  804  may be optional. In alternate embodiments, liquid CO 2  from liquid CO 2  storage tank  104  may be introduced directly into throttle  816 , where flow meter  808  and gate valve  112  control the rate at which liquid CO 2  is introduced into throttle  816 . 
     Between position  812  and position  820 , liquid CO 2  is throttled through throttle valve  816  and undergoes a phase change from liquid to a mixture of gas and solid. More specifically, the liquid CO 2  is changed into a mixture of gaseous CO 2  and solid dry ice. Undergoing a phase change using throttle  816  is advantageous due its simplicity. The throttling occurs at approximately constant enthalpy, also known as an isenthalpic process, per energy balance on throttle valve  816 . The phase change is induced through a change of pressure or a change of temperature, which can be determined through  FIG.  7   . In the current embodiment, at position  820 , the pressure is below 0.518 MPa. A person skilled in the art will recognize the different configurations and variables, such as temperature and pressure, of inducing phase change from the liquid CO 2  to a mixture of gaseous CO 2  and solid dry ice. 
     As the mixture of gaseous CO 2  and solid dry ice leave throttle valve  816 , the mixture enters exit stream  828 . Exit stream  828  has high kinetic energy due to throttle valve  816  and also the initial kinetic energy from liquid CO 2  enter thermally insulated hose  804  from liquid CO 2  storage  104 . The high kinetic energy allows the particles of Ca(OH) 2  introduced from screw feeder  824  to be suspended as they flow along exit stream  828 . Exit stream  828  may be encompassed by an insulated hose, pipe or any form of physical structure that will not impede the high kinetic energy of exit stream  828 , while being able to maintain the temperature and pressure as required in exit stream  828 , and direct the flow of exit stream  828  towards cyclone  832 . Screw feeder  824  is used for the introduction of Ca(OH) 2  particles to ensure a steady and regular flow of Ca(OH) 2  particles into exit stream  828 . In a preferred embodiment, screw feeder  824  is in proximity to the exit of throttle valve  816  and the beginning of exit stream  828 , where kinetic energy is at its highest after exiting throttle valve  816 , and also allowing time within exit stream  828  for heterogeneous nucleation, which will be further discussed below. Other forms of feeder or introducing Ca(OH) 2  particles may be contemplated, as long as the introduction of the Ca(OH) 2  particles are done in a regular and controlled manner. 
     As the Ca(OH) 2  is introduced via screw feeder  824  into exit stream  828 , the Ca(OH) 2  particles act as heterogeneous nucleation sites for the dry ice. The dry ice forms around the Ca(OH) 2  particles and reacts with the outer shell of the Ca(OH) 2  particles producing a shell of CaCO 3 . Due to the high kinetic energy, the Ca(OH) 2  particles are suspended in the gas in exit stream  828 , allowing the Ca(OH) 2  particles to act as a core and exposing the entire surface of the Ca(OH) 2  particles, allowing for a uniform coating of dry ice. As the Ca(OH) 2  particles and the mixture of gaseous CO 2  and solid dry ice travel through exit stream  828 , and as heterogeneous nucleation occurs around the Ca(OH) 2  particles, sublimation may also occur, where any excess dry ice in the exit stream  828  that does not undergo heterogeneous nucleation around the Ca(OH) 2  particles, and any excess dry ice that has grown as a result of heterogeneous nucleation around the Ca(OH) 2  particles may undergo a phase change into gaseous CO 2 . Similarly, Ca(OH) 2  particles that undergo heterogeneous nucleation early on after entering exit stream  828  from screw feeder  824  may begin reacting and becoming dry ice coated core-shell calcium hydroxide-calcium carbonate (CSCC) particles. In addition, changes in temperature and pressure within exit stream  828  may cause gaseous CO 2  to become dry ice as it travels through exit stream  828 . Furthermore, while the high kinetic energy promotes heterogeneous nucleation of the Ca(OH) 2 , there may still be a minority of Ca(OH) 2  particles that remain uncoated. As such, the resulting mixture introduced into cyclone  832  may include gaseous CO 2 , solid dry ice, Ca(OH) 2  particles, dry ice covered Ca(OH) 2  particles, and dry ice covered CSCC particles. 
     As said mixture enters and spirals within cyclone  832 , sublimation continues to occur, where any excess dry ice, whether coated on the Ca(OH) 2  particles or excess dry ice from the phase change from throttle valve  816  that was introduced into cyclone  832  from exit stream  828  may undergo a phase change into gaseous CO 2 . In addition, uncoated Ca(OH) 2  particles may undergo heterogeneous nucleation within cyclone  832  if heterogeneous nucleation did not occur within exit stream  828 . Furthermore, the Ca(OH) 2  particles coated in dry ice continue to react to create dry ice covered CSCC particles. The mixture in cyclone  832  may also undergo an increase in temperature and a pressure drop while in cyclone  832 . The increase in temperature may be due to a lack of insulation surrounding cyclone  832 , or it may be due the addition of heating elements to increase the speed of sublimination. The pressure drop arises due to the shape and design of cyclone  832 . Similar to method  200 , the residence time of the dry ice coated CSCC particles while undergoing sublimation will affect the thickness of the calcium carbonate coating on the calcium hydroxide particles. The residence time of the dry ice coated CSCC particles in cyclone  832  may be affected by various factors, including the shape and design of cyclone  832 . 
     Cyclone  832  further separates gaseous CO 2  from the CaCOs-coated Ca(OH) 2  particles through cyclonic separation and/or centrifugal force, where due to the weight of the particles or as a result of the CaCO 3  particles losing momentum when colliding against the wall of cyclone  832 , the CaCO 3 -coated Ca(OH) 2  particles settle at the bottom of cyclone  832  due to gravity and are collected at the bottom of cyclone  832 , at calcium carbonate-coated calcium hydroxide product particle outlet  144 . In the current embodiment, collector  836  collects the CaCOs-coated Ca(OH) 2  particle product, however as will be evident, collector  836  is optional, and if present, may be of any shape or size for the collection of the CaCOs-coated Ca(OH) 2  particle product. 
     Gaseous CO 2  is collected at the top of cyclone  832  at gaseous CO 2  outlet  140 , due to the spinning effect of cyclone  832 . Similar to embodiment of system  100 C, the gaseous CO 2  that is collected at gaseous CO 2  outlet  140  may be returned to storage tank  104  via return line  156 , after going through in-line pressurization system  152 . As previously discussed, in-line pressurization system  152  raises the pressure within return line  156  to induce a phase change on the CO 2 , converting the gaseous CO 2  into liquid CO 2 . 
     In alternative embodiments, cyclone  832  may be replaced with electrostatic precipitators or separator vessel  136  to allow for sublimination of dry ice and the separation of dry ice covered CSCC particles from the remaining mixture. A person skilled in the art will recognize that different equipment may be used to allow for sublimation of dry ice and the separation of dry ice covered CSCC particles from gaseous CO 2  and other mixture components. 
     Referring to  FIG.  9   , method  900  is depicted for producing calcium carbonate coated calcium hydroxide particles using system  800 . At block  905 , liquid CO 2  is discharged from liquid CO 2  storage tank  104  through gate valve  112 , thermally insulated hose  804 , and throttle valve  816  into exit stream  828 , where the liquid CO 2  undergoes a phase change as it is throttled through throttle valve  816  from liquid CO 2  into a mixture of gaseous CO 2  and solid dry ice. 
     As the mixture of gaseous CO 2  and solid dry ice travels through exit stream  828 , screw feeder  824  introduces Ca(OH) 2  particles at a regular and controlled rate into exit stream  828 , where it joins the mixture of gaseous CO 2  and solid dry ice. This is depicted at block  910 . 
     At block  915 , as the Ca(OH) 2  particles travel through exit stream  828 , the Ca(OH) 2  particles act as heterogeneous nucleation sites for dry ice. As previously discussed, due to the high kinetic energy of exit stream  828 , the Ca(OH) 2  particles are suspended in the air, allowing the exposure of the surface of the Ca(OH) 2  particles for the build-up and formation of dry ice around the Ca(OH) 2  particles. Once covered with dry ice, particle agglomeration of the dry ice covered Ca(OH) 2  particles is limited. 
     At block  920 , the dry ice covered Ca(OH) 2  particles are introduced into cyclone  832  for cyclonic separation. While being exposed to the rotational effects within cyclone  832 , the dry ice covered Ca(OH) 2  particles and the excess solid dry ice from exit stream  828  that did not form around the Ca(OH) 2  particles are sublimated, changing the phase of the dry ice into gaseous CO 2 . This is depicted at block  925 . 
     Through cyclonic separation, the gaseous CO 2  and the calcium carbonate coated calcium hydroxide particles are separated, with the gaseous CO 2  discharged through the top of cyclone  832  through gaseous CO 2  outlet  140  (as depicted at block  935 ), and the calcium carbonate coated calcium hydroxide particles falling to the bottom of cyclone  832  and collected through calcium carbonate coated calcium hydroxide particle outlet  144  (as depicted at block  930 ). 
     In certain embodiments, the collected gaseous CO 2  may be optionally recycled by inducing a phase change from gaseous CO 2  to liquid CO 2  as depicted at block  940 , where the liquid CO 2  may be returned to liquid CO 2  storage tank  104  to be discharged again at block  905 . 
     Although the foregoing description and accompanying drawings relate to specific preferred embodiments of the present invention as presently contemplated by the inventor, it will be understood that various changes, modifications and adaptations, may be made without departing from the spirit of the invention.