CA2016974A1 - Coating method for encapsulation of particulate matter - Google Patents

Abstract

COATING METHOD FOR ENCAPSULATION
OF PARTICULATE MATTER

ABSTRACT OF THE DISCLOSURE

Particulate and fibrous materials are encapsul-ated with a monolithic coating of controlled thickness through a multiple step simple rapidly bonding process by first treating the particulate material with an acid phosphate and particularly with sequestered ammonium polyphosphate or superphosphoric acid and a powdered mineral solid of the magnesium alkali earth metal oxide carbonate or silicate type capable of reacting with the said acid phosphate inagranulator drum or spraying pro-cess. Deposition of an excess amount of phosphate solu-tion on the particulate surfaces leads to well bonded encapsulated products. The process can be accelerated by heating either of the reactive components to 100°
Centigrade and by the use of magnesium oxide heavy powder, calcined dolomite, magnesite, olivin, serpentine or marble powder in leu of dead burned magnesium oxide in combination with powdered raw dolomite, magnesite, olivin, serpentine or marble. The process can be repeated until the desired coating thickness is achieved.

Description

2~ 69~7a - This invention relates to the deposition of a cementitious monolythic coating of particulate matter of diverse origin.
Several particulate material types could substan-tially benefit from surface protection as provided by encapsulation. Some particulates are porous (such as vermiculite or expanded perlite), hygroscopic (such as potash, ammonium nitrate and urea among others), flammable (polystyrene beads and wood fibers) or fragile (mineral wooIs, glass fiber or asbestos) and require some means of surface stabilization to impart properties such as a sealed surface, moisture resistance, some degree of fire retar-dancy and improved mechanical strength, respectively.
Pelletized or granulated nuclear waste materials could be safley transported by remote~pneumatic means if they could be solidly encapsulated to avoid radioactive contamination of pipes and pumping equipment used in the transfer pro-cess.
In a well designed encapsulation method, the size of the particulates to be protected can be immaterial and the mean particle diameter may range from 5 microns to as large as several centimeters.
In the past, encapsulation has been readily obtained by slurrying the particulate material in a suitable coating material such as varnish, paint, rubber, various plastics, molten sulphur and even molten glass and metals. However, such coatings are not universally usable, neither are they always waterproof or rigid enough nor do they have the required weathering and fire resistance as required in diverse applications.
It has been observed by Limes et al. (Canadian Patent No. 765 874, 1967) that when dead burned magnesite (MgO) was brought into contact with liquid ammonium polyphos-phate (10:34:00 or 11:37:00), a rapid exothermic reaction took place which resulted in the formation of a mono-lithic solid and liberation of ammonia. When the slurry was placed in a mould or pressed, a cementitious solid was obtained which was hard, heavy, insoluble in water, of high dimensional stability, fire resistant and quite strong.
In an invention to Paszner (Candian Patent No.
1 081 718, 1980) wood particulate material was mixed with ammonium polyphosphate and dead burned magnesium oxide in ce~tain proportions and when the cementitious mixture was allowed to cure, solid products were obtained that had high compressive strength, excellent dimensional stability and fire resistance inspite of the high wood content. The reaction was found to be fast enough so that the mixtures could also be used in spray equipment to produce fire retardant protective spray coatings over large surfaces.
During the course of the present investigations it was discovered that, depending on the reactivity of the dead burned magnesite and the level of ambient temperature during the magnesium oxyphosphate reaction, when granulated material were treated with a limited quantity of liquid ammonium polyphosphate or superphosphoric acid (syrupy) and subsequently mixed with a large excess of high reacti-vity dead burned magnesite or various forms of calcined dolomite and similar magnesium containing mineral com-pounds, the phosphate wetted particulate material became coated with a well adhered magnesium oxyphosphate (the ~ t7 reaction product of ammonium polyphosphate and dead burned MgO) to form rounded partlcles which have sealed surfaces without being glued together. If agitation during the reaction is vigorous enough and the MgO powder is available in large enough excess, the particles do not stick together and can be readily separated from the excess MgO powder by seiving through appropriately grated screens. This procedure could be repeated as many times as desired or required to build up a coating of the desired thickness around the particles.
It was further found that the process of particulate coating can be made continuous if a) the ammonium poly-phosphate-wet material is continuously fed into a drum, placed on a slight incline, containing the MgO powder mixture, b) rapidly mixing the ammonium polyphosphate-wet particluates by rolling into the MgO solids until all the free liquid is consumed on the surfaces and the particles become well coated, c) allowing the coated particles to rest so that the cementitious coating can sufficiently harden, d) removing the excess MgO powder by sieving or air separation, and e) allowing the resulting coating to fully cure and lose its excess moisture. In such con-tinuous coating processes it may be desirable to speed up the reaction to increase the throughput. This can be - achieved by 1) repalcing part of the MgO with calcined dolomite, magnesite, circonia,spent flue gas scrubbing dolomitic lime stone, calcined olivine, serpentine or marble powders, 2~ by chemically pure magnesium oxide heavy powder (Merck Chemicals) or 3~ by use of syrupy superphosphoric acid or 4) by use of warm (60-80C) ammonium polyphosphate or a combination of these options.

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Heating the MgO solids in the drum bed is a relatively simple way to achieve such elevation in temperature in the reaction mixture as the cement solids are capable of absorbing substantial amounts of heat. Heating the MgO
poweder and pre-heating the ammonium polyphosphate may be advantageous in some instances where reactivity of a cheaper source of MgO requires additional energy to initiate the exotherm reaction to obtain fast coating speeds. In most cases however, a combination of MgO sources is suf-ficient in modifying the reaction speed.
In the case when too much ammonium polyphosphate is applied to the particulate material, "cacking" and bond-ing between the particles may arise following such inital coating.
Alternately, coatings can also be applied to partic-ulates by a combination of dusting and spraying in whichcase air is used to transport (fluidize)the particles and to keep them detached from each other. ',~etting is obtained by spraying the particluate surfaces with finely atomized ammonium polyphosphate. In this case sufficiently high air-to-solids ratios have to be maintained to keep the particles well separated to avoid coalescense of the liquid droplets and to avoid sticking. Once the coating is adhered, the wet particulates are allowed to drop through space and partly supported by countercurrently flowing hot air to speed the curing rate.
When such coated particles contain sufficient excess ammonium polyphosphate and are impacted or deposited on solid surfaces before the reaction (hardening) of the coating is complete, solid continuous coatings of well en-capsulated particles will result whereby the processes of -5~

particulate encapsulation and bonding are sequential steps and combined in one process.
Thus the characterising features of the process of invention are as follows:
a) Combining said particulate materials with liquid phosphate solution in an amount just sufficient for forming a thin film over the said particulate material, b~ rapidly mixing such particulate material with an excess of reactive MgO containing cementitious . powder mixture of sufficiently small size to form a continuous coating on the particulate surfaces either in a drum or in form combining said wet particulates with an air stream to fluidize the said particulates and combining them with similarly fluidized MgO containing cementitious mixture in a suitable spraying ap-paratus in proportions defined by the wetting ability of the free phosphate solution film situated on the particulate material, c) transporting and agitating said coated particuiate material in an excess of MgO contalning cementi-tious solids or floating them in an air stream until the particulate coating has essentially and substantially hardened and lost its surface tackiness, d) separating such coated particulate material by known processes of segregation, sieving or such as may be advantageous from the excess mineral solids, and -6- ~ ~ ~

e) if the particulate material is sensitive to the residual moisture in the coating, it may then be dried by known means as in a drum drier or hot air stream until most of the moisture is driven off.
If the steps a) to b) are carried out in such a way that the MgO containing cementitious solids and ammonium poly-phosphate are proportioned in a ratio that a slight excess of liquid is available on the particulate surfaces then on deposition of such encapsulated particles on a solid sur-face hardening ~curing) will take palce and a layer of such coated particles is formed on that solid surface in manner that the said coated particles are also bonded to each other to form a continuous matrix of particles and air pockets, or a continuous matrix without air pockets if the encapsulated particulates are impacted on the sur-face. The density of such continuous matrices is readily controlled by the speed at which the impact of the coated particles on the surfaces occrres.
The temperature of the reaction medium or the coating components can be controlled in Step a) as by controlling the temperature of the phosphate solution, in Step b) by controlling the temperature of the MgO-containing cementitious mixture and in Step c) by ccnt-rolling the temperature of the fluidizing medium, e.g., the excess MgO cementitious solids or fluidizing air.
Such controlled temperatures may be readily generated by known means of heating and will result in substantially increased reaction rates to solidify the coatings and spead up the loss of surplus moisture (as in Step e)) and release of ammonia by-product gas.
A partial solubility of the particulate substrate in the liquid phosphate solution during the coating pro-cess does not seem to affect the the MgO-phosphate reaction rate or the hardness of the coating.
The MgO cementitious matrix may be comprised entirely of dead burned or calcined magnesite from mineral or seawater sources, or may be of magnesium oxide-rich dolomite,up to 99.5~ raw ground magnesite, dolomite, ziconia, olivine, serpentine or marble powder or other additives which would at least partially precipitate phosphate from ammonium polyphosphate or superphosphoric acid solutions by reacting with them under certain conditions as determined largely by the stoichiometry of the components and/or the reaction environment (temperature). The magnesium oxide may also be chemically pure magnesium oxide heavy powder.
Additives other than those described above may be added to the MgO cementitious mixture as required or sug-gested by the particular application for which the coating is designed without adversely affecting thecuring speed of the coating. Such additives may be for example magnesium nitrate, potassium chloride or sulphate, lignosulphonic acid, urea, ammonium nitrate, boric acid, iron oxide, fer~
rous sulphate, ferric chloride or virtually any inorganic solid from a few percent and up to 50 per cent of the cemen-titious solids. Such additives would be well blended with the MgO containing solids and usually ground to the same extent as the MgO.

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EXAMPLE I

About 50 grams of expanded perlite of 1.0 to 5 mm diameter was treated with 75 g of; ammonium polyphosphate (10:34:00) of commercial grade (Simplot Chemical Co.
California) in a suitable plastic bag. The liquid phos-phate was well distributed over the perlite by shaking.
To the wet perlite mass about 500 g of dead burned magnesite powder was added in a large plastic bag. The MgO had a particle size passing -200 mesh sieve (National Refractory Chemicals, Moss Landing, Ca.). The perlite-MgO mixture was vigorously tumbled until the particles became well separated and coated with magnesium oxyphosphate (MOP) (about 2 min). The mixture was allowed to stand for an -additional 5 min before shaking the bag again and separat-ing the unreacted MgO by using an 80 mesh sieve. Tumbling the coated particles for an additional 3 min resulted in "polished" pellets. The lose small powder was removed by sieving again on an 80 mesh sieve and added to the rest for reuse with the next batch.
The total weight increase on the perlite was 112 g of which 18 g was reacted ammonium polyphosphate solids, 77 g was MgO and 18 g was water. About 35 g of the ammonium polyphosphate remained unreacted and absorbed into the perlite pores.
On drying in a forced air oven at 50C with gentle tumbling in a drum for three days, the coated material lost a total of 36 g of moisture and ammonia which was liberated by the MgO-ammonium polyphosphate reaction.
The procedure was repeated up to 5 times, except that starting with the second coating mixture the amount -9- ~

of ammonium polyphosphate was reduced to 25 g for all subsequent applications. The average particle diameter after the 5th encapsulation ranged between 3 to 7 mm.

EXAMPLE II

The same mixing sequence was followed with the same proportions using perlite as the substr~te, as described in Example I with the exception, that the MgO powder was preheated to 100C in an oven before combining the phosphate wet perlite with the cement solids.
In this case the reaction was instantaneous as in-dicated by the almost immediate release of ammonia. The coated material weight was 118 g and the MgO uptake was 83 g following the second fines removal by sieving.

EXAMPLE III

To 50 grams of expanded perlite 150 g of ammonium polyphosphate was added in a plastic bag to which immediately 150 g of cementitious powder mix was added. The powder consisted of 30% (50 g) -200 mesh dead burned MgO (made from sea water by Kaiser Refractories, Oakland, Ca.) and 70% of -200 mesh ground garden dolomite. The cement powder was rapidly shaken with the wet perlite particles and the resulting mixture was poured into a dish and allowed to stand for 10 min by which time it hardened into a solid slab of low density. The encapsulated perlite particles were bonded together at their contact points. The slab was dried to 15 ~ moisture in the air in four days giving off only faint ammonia odor at that stage. The ammonia odor disappeared within a week or eight days.

EXAMPLE IV

A series of samples was prepared with the same mixing ratios as in Example I except that the MgO source was replacedby -200 mesh clacined dolomite. The calcined dolomite was prepared by heating garden dolomite (45~ MgO) content) at 850C in a muffle furnace for 8 h and on cool-ing the material was reground to pass 200 mesh.
The calcined dolomite to raw dolomite ore ratio was varied between 100% calcined dolomite to 95% raw dolomite (the rest calcined dolomite). The reaction rates with all the mixtures below 70~ raw dolomite content were very rapid (almost instantaneous) whereas those of the mixtures having more than 70~ raw dolomite were comparable in rate to those had with the dead burned MgO mixtures.
Equally rapid curing (hardening) rates were obtained when a maximum of 15 % chemically pure magnesium oxide heavy powder was added to the raw dolomite powder. Heavy powder MgO addition to raw dolomite as low as 0.5% was effective in intiation of the reaction and hardening of the coatings albeit at somewhat slower rate. Mixtures con-taining lass than 5% MgO heavy powder in raw dolomite cured at the same rate as observed with dead burned MgO
mixtures containing less than 50~ raw dolomite.

Ol~

EXAMPLE V

One hundred grams of prilled urea (ave. diameter 2.5 mm) ammonium nitrate prills or processed potash (KCl) were mixed individually with 35 mL of clear ammonium polyphosphate solution of 11:37:00 (TVA, Muscle Sholes, Ala.~composition in plastic bags. The wet particulates were treated with 500 g of -200 mesh dead burned magnesite -dolomite mixture (30% MgO and 70% dolomite) in plastic bags and rapidly tumbled around to prevent the prills from being glued together by the MOP matrix. After mixing for 2.5 min the prills were well separated and encapsulated with MOP coating. The coating thickness was about 0.4 to 0.7 mm with average prill diameter of about 3.5 mm for the urea prills.
Trials with 30% replacement of the dolomite in the cementitious mixture with finely ground magnesium nitrate or potash (they had to be dried before grinding) had no noticeableeffect on the curing rate and the coating thickneqs but noticeablyreduced the amount of ammonia released during the encapsulation process.
It was further observed that such coated prills were no longer sensitive to atmospheric moisture even at high (80-90%) relative humidity.

-12- ~ 7~-EXAMPLE VI

Twenty grams of polystyrene beads of about 5 mm diameter were treated with 15 mL of ammonium polyphosphate solution in a palstic bag and tumbled to distribute the liquid evenly over the beads. The wet mixture was then treated with 500 g of MgO containing cementitious mixture consisting of 150 g of dead burned magnesite and 350 g of finely ground olivine powder in a plastic bag. After shaking the bag contents for 3 min they were allowed to sit for 7 min and then separated into unreacted cement and the encapsulated polystyrene beads. The beads were free flowing and the coating thickness was measured as 0.5 mm.
When the same amount of beads was mixed with 30 mL
of ammonium polyphosphate and 55 g of dead burned MgO
and the mixture poured into a dish after a brief mixing, the mass solidified in 4.5 min giving a rather lightslab in which the encapsulated particles were bonded only at their contact points.
A similar product could also be made with the dead burned MgO/olivine cementitious powder mixture when the liquid ammonium polyphosphate cement powder ratio was 0.6 or less. It was noted that olivine could be replaced with wet-ground serpentine or dry marble powder without appreciable change in the reaction rates and hardness of the coating.

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EXAMPLE VII

Olivine, serpentine and marble chips (3 mm 0) were heated in a muffle furnace at 1100C overnight and S ground to -200 mesh in a ball mill (the serpentine was wet milled). These calcined products were mixed with various portions of raw magnesite ore also ground to -200 mesh to produce MgO sources for the various coat-ing applications.
Monolithic coatings of 0.2 to 0.6 mm thickness were produced on perlite, polystyrene beads as well as wood particles (sawdust) by the procedures described in Examples I to VI. The coatings had only slight color variations depe~ding on the source of the calcined com-ponents of the coatings.
Enhanced fire resistance was observed especially on the encapsulated perlite and wood particles.

EXAMPLE VIII

Trials with asbestos fibre encapsulation by the process described in Example I was only partly success-ful in that it was difficult to keep individual fibers and fiber segments detached from the neighbouring fibers.
Adhesion of the coatings however were excellent.
To 30 g of fluffy, apparently well separated filter aid grade asbestos mineral fiber was mixed with 20 mL of ammonium polyphosphate liquid in a plastic bag until the liquid was well dispersed and adsorbed by the fibers.
After gently tumbling the fibre mass it was allowed to stand for 15 min at 21C.
The fibre material was then added to 250 g of -250 mesh MgO containing cementitious powder mix con-sisting of 80% dead burned magnesite (Kaiser refractories) and 20% calcined olivine weighed into a plastic bag. The mixture was gently tumbled for 2 min whereupon the fibre cement mixture was alloed to cure for 10 min. The hardened fibre-cement powder mixture was separated on an 80 mesh sieve. Apart from a few larger fibre lumps which consisted of well bonded asbestos fibres, most of the fibres were well separated. The apparent fibre diameter increased from 100 ~m to 500 to 700 ~m showing uniform surface coat-ing (encapsulation). The weight increase on the fibres due to the coating (including the lumps) was 38.3 g. The coating appeared to be well adhered to the fibres and the lumps were well bonded.

EXAMPLE IX

A similar attempt with Asplund mechanical fibres as described in Example VIII was much less sucessful in producing detached encapsulated fibres. Therefore it was decided that spray coating should be attempted. The fluf-fed fibre mass showed a very strong tendency to bunch up as soon as the ammonium polyphosphate solution was added and possibly due to the light weight of such fibers they tended to bunch up rather than separate on addition of the cement solids. The fibres consisted mainly of western Canadian species possibly Douglas-fir, western red cedar and true fir.

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A large lump of about 2.5 kg of dried Asplund pulp was loaded into the hopper of a two-stage fibre blowing appartus wherein the fibres were effectively mixed with air in the first stage. The fibre-air mix-ture was further fed into the second stage fluidizing chamber along with metered amounts of dead burned mag-nesite laced with about 2% MgO heavy powder to increase the reaction speed. The fibre-air-cement mixture was pumped through a 5 m hose which had small nozzles pro-jecting about 20 cm ahead of the hose and spraying well atomized ammonium polyphosphate into the fibre-air-cement mixture. The second air pressure was so adjusted that the exiting fibre-cement mass was allowed to expard (slow) rapidly and be wetted by the liquid. The coated fibres were allowed to fall some distance gently supported by an up-flow of warm air. On the ground a mat of loose fibres were collected which had considerable amounts of ununiform-ly attached MOP on their surface.
The fibres thus produced were rigid after drying and showed properties of mineralized fibres, and good fire resistence.
EXAMPLE X

In an experiment similar to that of Example VIII
mineral wood (KAFCOAT H) and glass fibres prepared from insulation matting (phenol formaldehyde coated) were mixed in a plastic bag with polyphosphoric acid (1:0.5 ratio) until well dispersed. To this wet material a large excess (500 g) of MgO containing 20% calcined magnesite and 80% olivine was added with tumbling until most of the -16- ~ P ~ ~ -fibres were coated. Again considerable lump formation was -noted,however, the lumps were well bonded by MOP while the fibres which remained separated were well coated. The dried fibres were quite stiff and could be handled better like particles than would be expected of fibres which form bundles of unregular form. Some chipping of the coating was noted on the glass flbres when such were tumbled in a bag to simulate mechanical handling.

Claims (17)

1. In a process for uniformly encapsulating and optionally bonding particulate and fibrous materials by initiating an exothermic reaction on the particulate surfaces on sequential ad-dition of a powdered cementitious mixture con-taining magnesium alkali earth metal oxides, carbonates and silicates with a liquid phosphate capable of reacting with the alkali earth metal in a batch or continuous manner the steps of which comprise, a. treating the particulate material with an equi-valent or up to five times the weight of the particulate weight with an aqueous solution of acid phosphate reacting liquid, and b. vigorously mixing the wet particulate material with an excess (five to ten times the weight) of magnesium alkali earth metal powder of at least -100 mesh particle size until all particles are well coated, and c. allowing the coating on the particulate material to cure with agitation, and d. separating the unreacted powder from the said coated particulate material, and e. drying the coated particulate material to remove the excess water remaining after the reaction, f. repeating steps a) through e) if so desired up to five times or more.

2. The process of claim 1 where the alkali earth metal material is selected from a group of dead burned magnesite magnesium oxide heavy powder calcined dolomite calcined magnesite calcined olivine calcined serpentine or calcined marble powder.

3. The process of claim 2 where the said alkali earth metal oxide is a mixture of dead burned magnesite and one or any of up to 90% of raw dolomite, magnesite, serpentine and olivine.

4. The process of claim 2 wherein the said alkali earth metal oxide is a mixture of dead burned magnesite and one or any of up to 30% calcined dolomite, calcined magnesite, calcined olivine or calcined serpentine.

5. The process of calims 2 and 3 wherein the alkali earth metal oxide is magnesium oxide heavy powder.

6. The process of claim 1 wherein the liquid phos-phate is sequestered ammonium polyphosphate of 10:34:00 or 11:37:00 N(ammonia):K(phosphate):K
(potassium) composition.

7. The process of claim 1 wherein the liquid phos-phate is superphosphoric acid.

8. The process of claim 1 wherein the alkali earth metal powder is heated to 100°C prior to addition to the particulates.

9. The process of claim 1 wherein the amount of powdered cementitious binder is not in excess and such that the encapsulated particles remain wet on their surface and such coating develops into a binder between the said particles.

10. In a process for production of encapsulated particulate and fibrous materials and optionally bonding same by initiating an exothermic reaction on the particulate surfaces on sequen-tial addition of a liquid phosphate capable to react with alkali earth metal mixtures as pro-cessed in a spray gun and the steps of which comprise:
a) dispersing the said particulate material in a large excess of fast moving air to fluidize the particles, b) mixing said fluidized particles with finely powdered cementitious magnesium alkali earth metal oxide, carbonate or silicate, c) pumping such fluidized particulate mixture at low solids to air ratio, d. mixing said dry mix with a liquid phosphate in the fast moving air stream by atomization, e) Rapidly expanding said fibre-air stream to decelerate the coated fibres, and f) allowing said encapsulating coating to cure in an upwardly flowing air stream.

11. The process of claim 10 wherein the fibrous material may be any of a mineral wool, fibres or particulates of regular or irregular shape.

12. The process of claim 10 and 11 wherein the liquid phosphate is sequestered ammonium poly-phosphate or superphosphoric acid.

13. The process of claim 10 and 11 wherein the alkali earth metal oxide is dead burned magnesite and calcined dolomite, magnesite, olivine, serpen-tine or marble.

14. The process of claim 13 wherein the added alkali earth metal is extended with raw powdered dolo-mite, magnesite, olivine, serpentine or marble.

15. The process of claims 10 and 13 wherein the cementitious powder is preheated to 100°Celsius.

16. The process of claims 10 and 11 wherein the amount of liquid phosphate is in excess of the cementitious powder and the encapsulated sprayed particulates settle into a bonded fibre cement coating.

17. The process of 1 and 10 wherein the encapsula-tion process is repeated several times.