GB1589994A

GB1589994A - Method for dry spraying calcium sulphate hemi-hydrate

Info

Publication number: GB1589994A
Application number: GB30944/77A
Authority: GB
Original assignee: United States Gypsum Co
Current assignee: United States Gypsum Co
Priority date: 1976-07-29
Filing date: 1977-07-22
Publication date: 1981-05-28
Also published as: JPS5317501A; BR7704888A; AR214744A1; ZA773879B; IN146244B; AU2643077A; FR2359649A1; IT1079792B; DE2733540A1; CA1067301A

Description

(54) A METHOD FOR DRY SPRAYING CALCIUM SULFATE HEMI-HYDRATE (71) We, UNITED STATES GYPSUM COMPANY, a corporation of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for dry spraying calcium sulfate hemi-hydrate.

A severe problem encountered in underground mining is commonly referred to as air decrepitation of the mine roof and walls. Air penetration of these surfaces causes them to spall and crack creating safety hazards and unfit working conditions.

Several products and systems have been developed to solve this problem. These systems may be classified as either structural or nonstructural. The non-structural system may comprise mixes of cement and sand which are sprayed on the surface in thin layers as disclosed in U.S. Patent No. 3 224 203. These cement (concrete) coatings have high strength and are effective, but they are very expensive and costly to apply, they are relatively slow in setting, and they require special pumping equipment in the mine. For example, U.S. Patent No.

2 255 189 discloses a special system for applying cement coatings wherein the cement particles and water are admixed after discharge from the spraying apparatus and as or just prior to being deposited on the surface. U.S. Patent No. 3 645 762 discloses a hydraulic cement mix for spray application which is reported to be rapid setting, but it requires on the order of 6 hours to set and it includes several special ingredients which add to the cost and complexity of the system.

The other non-structural systems may employ premixes of cement and major portions of fillers such as vermiculite. U.S. Patent No.

3 055 434 illustrates a non-structural system comprising coating the mine roof and walls with asphalt and/or asbestos fibers. This is a low strength system intended to keep the air off the surfaces, and it is more economical than the hydraulic cement systems. However, it does not have the strength to hold in place loosened roof and wall material, and the asbestos fibres present a health hazard.

A recently developed method disclosed in U.S. Patent No. 3 892 442 employs a plastics foam or cement foam to coat the mine roof and walls, but this system is expensive and requires special equipment in the mine U.S.

Patent No. 3 900 333 discloses a gypsum anhydrite (CaSO4) particulate composition including an accelerator which has been developed for application to mine roof and wall surfaces. Technical literature published by Karl Brieden & Co. discloses that the gypsum anhydrite can be conveyed by air through a transport line and mixed with an aqueous stimulator (accelerator) solution in the nozzle mixing chamber and thereafter sprayed onto the roof or wall surface. However, this system is a slow setting system and the coating must be applied in thick sections.

The present invention provides a method of dry spraying calcium sulfate hemihydrate which comprises blowing an air stream of dry calcium sulfate hemihydrate (CaSO4 .1AH2 O) particles through a hose to a spray nozzle having a mixing chamber therein, introducing water into the stream of dry hemihydrate particles in an amount of from 20 to 100 parts by weight per 100 parts by weight of the hemihydrate, intermixing the hemihydrate particles and water in the nozzle mixing chamber, and spraying the wetted hemihydrate through the spray nozzle.

The method of this invention is suitable for use in underground mines and can use conventional rock dusting equipment which is standard mining equipment. In accordance with this method thin sealant coatings can be applied to the mine roof and walls providing a relatively cheap and effective system. The resultant gypsum coating provides a desirable high reflectivity, and the dry spray application requires less energy consumption than slurry systems and also less cleanup. It has been determined that good physical bonding to the surface of the mine can be achieved using commercially available alpha or beta calcium sulfate hemihydrate particulate material, including mixtures thereof, without incorporating special stimulators or accelerators.

Whenever the term "calcium sulfate hemihydrate" is used in the specification or the claims which follow, it includes alpha and beta calcium sulfate hemi-hydrate and mixtures thereof.

The amount of water added to the stream of dry calcium sulfate hemi-hydrate particles is a vital factor in achieving a satisfactory composition. First, there should be sufficient water admixed with the hemi-hydrate particles to assure adequate wetting for complete hydration upon discharge from the spray nozzle. In order to reduce dusting, it may be desirable, as a modified procedure, to add some of the water at the nozzle orifice.

Secondly, the water content must be selected so that the clacium sulfate is firmly adhered to and tightly seals the surface to which it is applied. According to the invention the said amount of water ranges from about 20 to about 100 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate, with a particularly preferred range being 50 to 55 parts water per 100 parts hemi-hydrate. The optimum ratio of water to calcium sulfate hemi-hydrate will depend on the type of hemihydrate and the particle size distribution.

It has been found that dust particles may be present in the periphery of the spray; to deal with this some of the water may be injected into the spray at the nozzle orifice by one or more water jets located on the end of the nozzle, the total amount of the water so injected and added to the dry hemi-hydrate remaining at about 20 to about 100 parts by weight per 100 parts by weight of hemihydrate.

One of the advantages of this method of dry spraying calcium sulfate hemi-hydrate is that the wetted hemi-hydrate particles set fast and develop high early strength. It has been found that about one hour after application, the composition may attain about 40% of its final dry strength (compressive). This permits fast application and also thin coatings which conserves energy and reduces expense.

Several problems may be encountered in performing the method of this invention.

Perhaps the most serious is dusting at the spray nozzle orifice. The mine atmosphere is normally dusty and additional substantial dusting is very undesirable. Dusting may be affected by several factors. In addition to the amount of water admixed with the hemihydrate particles, the particle size distribution is important. A moulding type hemihydrate (finer particles) generally requires more water than a gauging type (coarse particles). It is generally preferred that the particles range in size (strokes particle size) from about 0.2 microns (moulding type) to about 725 microns (gauging type).

Another factor affecting dusting is the flow rate used to transport the hemi-hydrate particles. Preferably, the air stream flow rate should range from about 50 cfm to about 140 cfm, and the line pressure should range from about 8 psi to about 22 psi.

In carrying out the method of this invention, a nozzle of the type used to spray gunnite (cement and sand) may be used, but it may require modification for optimum results.

Conveniently the calcium sulfate hemi-hydrate is fluidized with air and conveyed through a hose to a spray nozzle, and as it enters the nozzle, water is injected into the hemi-hydrate particle stream through an annular water ring surrounding the nozzle. Forward from the annular water ring, the hemi-hydrate particles and water are admixed in the mixing chamber of the spray nozzle. The turbulent stream of air, hemi-hydrate particles and water is activated by energy input resulting from the turbulence. After thorough mixing in the nozzle, the wetted calcium sulfate hemi-hydrate is discharaged from the nozzle and applied to the surface to be sealed. The hemi-hydrate sets rapidly (generally 15 to 20 minutes are required to achieve substantial strength) and it can therefore be applied in thin coats and still effectively seal the surface. The sealant coatings may range from about 1/8 inch to about 5 inches, with thicker coatings being present at surface indentations. One of the advantages of this method is that the resulting gypsum (CaSO4 .2H2 0) coating does not shrink when it sets, and in fact there is a slight expansion which provides adherence and sealing properties.

Thicknesses greater than about 5 inches involve unnecessary expense and so they are not preferred.

In many underground extraction processes or other mining activities, it may be necessary or desirable to incorporate a reinforcing material such as cut glass fibers in the sealant coating which perform as an aid in securing loosened material present on the surface. This may be accomplished in either of two ways.

The cut glass fibers may be dry mixed with the calcium sulfate hemi-hydrate particles and this admixture transported by air to the spray nozzle where it is mixed with water in the same manner as when the glass fibers are not present.

In this method, it has been found to be difficult to maintain a uniformly wet mixture possibly resulting in an unsatisfactory coasting. This difficulty may be due to plugging of the valves or nozzle parts, and the tests indicated that fiber glass adversely affects the wetting rate of the calcium sulfate hemi-hydrate. This may be alleviated by using various coatings on the glass fibers. A more preferred method for incorporat ing the cut glass fibers comprises introducing them into the stream of hemi-hydrate particles after it has been wetted by the water. This is accomplished by installing a glass fiber cutter or chopper near the spray nozzle discharge orifice whereby the glass fibers are incorporated in the wetted hemi-hydrate particles just after their discharge from the spray nozzle.

For example, a glass fiber rope is fed to the cutter or chopper and cut into short lengths, generally from about 1/4 inch to about 4 inches.

In general, the proportion of glass fibers ranges from about 0.5% by weight to about 6% by weight of the calcium sulfate hemi-hydrate. An air hose is connected to the cutter or chopper and pressurized air is fed thereto to blow the cut glass fibers into the wetted clacium sulfate hemi-hydrate particles. It is preferred that the air pressure fed to the cutter or chopper range from about 16 psi to about 100 psi, but this will vary according to the type of cutter or chopper.

The effectiveness of the method of this invention is surprising and unexpected because it was generally believed that an intensive energy input is required in order to set calcium sulfate hemi-hydrate. It has been found that sufficient and rapid strength can be developed without the substantial energy input previously thought necessary. Furthermore, establishment of an effective gypsum coating in an underground mine was considered to be unfeasible because of hydrostatic pressure causing water seepage in mines. It has been found that the method of this invention can be used in most mines which do not have a hydrostatic pressure problem.

The following Examples illustrate the invention, the word "gallons" herein means U.S. gallons.

EXAMPLE 1 At a coal mine in which a fire in the mine had made it necessary to seal off a portion of the mine from the other working portions of the mine, bulkhead seals had been constructed with solid concrete blocks. The surface of the seal on the return air side was coated using a trowel grade compound and inspection of the mine revealed that there were gas leaks in the concrete block seals. Prior tests using calcium sulfate hemi-hydrate as a sealant coating applied to the roof and wall areas in the mine to prevent decrepitation had proved the feasibility of using the hemi-hydrate as a sealant coating. In this particular job, the calcium sulfate hemi-hydrate was to be applied over the bulkhead seal surface at an adequate thickness to seal off any gas leaks in the bulkhead seal.

A total of 53 one hundred pound bags of calcium sulfate hemi-hydrate were loaded into a pressure tank rock dust distributor chamber (single tank-approximate 3 ton capacity).

Compressed air was then fed into the chamber causing the calcium sulphate hemi-hydrate to behave like a liquid. The hemi-hydrate was conveyed (air blown) through approximately 650 feet of 2 inch I.D. hose to the discharge end having a special spray nozzle containing an annular water ring. The tank pressure was about 22 psi and the line pressure was about 16 psi.

The calcium sulfate hemi-hydrate was wetted and then sprayed onto the roof, walls and bulkhead seal surfaces. The water was supplied to the spray nozzle (at a rate between 20 and 100 parts by weight water per 100 parts by weight hemi-hydrate) via a 1 inch hose from a 6 inch fire hose water line. The volume and pressure of the water was more than adequate.

The calcium sulfate hemi-hydrate performed very well. Although at times there was some surging of the dry hemi-hydrate particles, the hemi-hydrate particles were wetted at the spray nozzle reasonably well. The surging variation resulted in some dusting, but this was not detrimental in any was to the application of the calcium sufate hemi-hydrate.

The calcium sulfate hemi-hydrate was also sprayed onto the mine roof and wall areas adjacent to the bulkhead seal, extending for about six to seven feet into the mine entry. The calcium sulfate hemi-hydrate coating was applied in thicknesses ranging from 1/4 inch to a few inches at surface indentations with no dislodgement of the coating. During this application to the mine roof and wall areas, the air flow rate used to transport the hemi-hydrate particles was held at about 140 cfm (tank pressure was about 22 psi) and the line pressure was held at about 16 psi.

The approximate size of the bulkhead seal was 20 feet in width and 7-1/2 feet in height.

It required approximately four to five minutes to spray each bulkhead seal. It required at least 500 pounds of calcium sulfate hemi-hydrate material to coat one bulkhead seal consisting of approximately 600 square feet of wall, roof and and seal area.

Inspection of the sprayed bulkhead seals showed that the calcium sulfate hemi-hydrate coating was effective in sealing the bulkheads.

EXAMPLE 2 An additional spray test was conducted to evaluate the method employing the addition of cut glass fibers in the calcium sulfate hemihydrate coating. A glass cutter was mounted at the discharge end of the spray nozzle, and air was supplied from an air compressor. The air was supplied to the cutter at 60 psi. The glass fibers were blown into the stream of wetted hemi-hydrate particles just after their discharge from the nozzle.

The procedure for handling and air blowing the calcium sulfate hemi-hydrate was the same as that employed in Example 1. The dry spraying of the hemi-hydrate particles remained the same, with the only change comprising cutting and blowing one inch lengths of glass fiber into the wetted calcium sulfate hemi-hydrate. The wetted hemi-hydrate containing the cut glass fibers was sprayed onto the roof and wall areas in the mine entry. The application of the hemihydrate composition containing the one inch glass fibers was feasible. In this application, the glass fiber content in the sealant coating was approximately 4% by weight. The sealant coating containing the cut glass fibers had a dry density of 75.2 lbs/ft.3and a dry compressive strength of 2289 psi. It has improved resistance to cracking when exposed to physical abuse (bumping) from mine equipment.

EXAMPLE 3 A vertical pressure tank rock dust distributor was used to spray dry particles of calcium sulfate hemi-hydrate. The tank was charged with 1,000 pounds of calcium sulfate hemihydrate containing 1/4 inch lengths of chopped glass fiber at a ratio of 10 pounds of fiber per ton of hemi-hydrate, equivalent to 0.5% by weight. After charging, the tank was pressurized and the calcium sulfate hemi-hydrate was air blown through 150 feet of 2 inch I.D. heavy duty transfer hose to a special spray nozzle where it was wetted and sprayed. The line pressure was held steady at 11 psi, and the hemi-hydrate particles containing the cut fiberglass were conveyed at a flow rate or approximately 80 pounds per minute. There appeared to be good, uniform conveyance of the dry material.

However, there did appear to be excessive dusting at the spray nozzle discharge orifice.

Periodically, there was poor wetting of the hemihydrate particles which resulted in inconsistent application of the coating.

Water rate during this application was limited to approximately three gallons (about 25 pounds) per minute. It was decided to perform additional tests using increased water flow.

EXAMPLE 4 The vertical pressure tank rock dust distributor was again charged with 1,000 pounds of calcium sulfate hemi-hydrate containing 1/4 inch lengths of chopped glass fibers. The tank was pressurized and the line pressure was held at 11 psi. The dry hemi-hydrate/glass fiber material flow rate was approximately 80 pounds per minute. There was good, even conveyance of the dry hemi-hydrate particles.

In this example, water was fed to the spray nozzle at a rate of five gallons per minute. At the spray nozzle orifice there was periodic dusting. A uniform wet spray could be maintained at times, but this was not consistent.

When the hemi-hydrate particles were ad equately wetted, they sprayed and applied to the surface very well. The 1/4 inch pieces of glass fiber appeared to disperse in the material quite well.

EXAMPLE 5 The next test was run using the pressure tank rock dust distributor in a horizontal pos ition instead of a vertical position. This tank and set up are identical to that used in the mines for rock dusting. The tank was charged with 1,000 pounds of calcium suflate hemihydrate containing 1/4 inch lengths of glass fiber. Pressurizing and air conveying of the hemi-hydrate particles was identical to that used in Example 4. It was determined that the rate of water flow was erratic, varying from 31h to 5 gallons per minute. The dry hemihydrate/glass fiber material was air blown at 80 pounds per minute. It was calculated that the water available for wetting the hemi-hydrate particles ranged from 29 to 42 pounds per minute for 80 pounds of hemi-hydrate. The standard consistency for this particular hemihydrate mtaerial would be 52 pounds of water for 80 pounds of hemi-hydrate. It appeared that the water was sufficient for wetting the hemi-hydrate even though its rate varied. Again, there was poor wetting of the hemi-hydrate and there was dusting at the spray nozzle orifice. It was concluded that the fiberglass was impeding the wetting of the hemi-hydrate.

EXAMPLE 6 The same pressure tank rock dust distributor, in the horizontal position, was charged with 1,000 pounds of calcium sulfate hemi-hydrate containing 1/2 inch lengths of glass fiber. The glass fiber content was approximately 10 pounds per ton of hemi-hydrate. A change was made in the water line wherein a 2 inch I.D.

hose was installed which was reduced to a 1/2 inch entry at the spray nozzle. It was felt that this would supply a more uniform rate of water to the nozzle. The water rate after attaching the nozzle was determined to be 5 gallons (about 42 pounds) per minute.

The tank was pressurized and the line pressure for the dry material was held at 11 psi, yielding a dry material flow rate of 80 pounds per minute. Again, there was good, even conveyance of the dry material.

Spraying performance was again inconsistent.

When the calcium sulfate hemi-hydrate was adequately wetted, it sprayed and applied well.

The 1/2 inch lengths of glass fiber appeared to disburse well in the wetted hemi-hydrate.

The molds which were sprayed in Examples 3-6 were inspected after the coating material had set. The molds were peeled and the appearance of the coating was quite acceptable.

The results obtained indicated that this type of dry calcium sulfate hemi-hydrate spraying is a feasible method for applying a sealant coating.

EXAMPLE 7 The pressure tank rock dust distributor was charged with 1,000 pounds of calcium sulfate hemi-hydrate. The tank was pressurized with the line pressure held at 11 psi and the dry material was sprayed at the rate of 80 pounds per minute with water supply as in Example 6.

In this test, a glass cutter was held above the spray nozzle and chopped glass fiber was fed into the wetted hemi-hydrate while spraying the molds. The chopped glass fiber varying in length from 1/2 to 1 inch appeared to lay down well in a mat form in the molds.

EXAMPLE 8 A test was carried out to evaluate air convey ing and spraying calcium sulfate hemi-hydrate particles using a Bantam rockduster pump.

During the tests, 400 Ibs. of No. 1 Moulding Plaster hemi-hydrate, 300 lbs of HYDROCAL White hemi-hydrate, and 100 Ibs of TUF-ART Plaster hemi-hydrate were sprayed. The pump provided an air flow rate of 140 cfm and a line (2 inch hose) pressure ranging from 8 to 20 psi. The hemi-hydrate material was dumped into a one bag hopper, a feed screw transferred the material into the air valve box, and the dry hemi-hydrate particles were moved with air through the hose to the spray nozzle. In the spray nozzle, water was injected into the hemihydrate particles through a specailly designed water ring collar. The wetted hemi-hydrate was sprayed through the nozzle orifice by air pressure without undue pulsation.

The densities and compressive strengths of the various mixes was determined. The compressive strengths of the dry gypsum (CaSO4 .2H2 0) was measured using a molded cylinder having a 2 inch diameter and a 4 inch height.

No. 1 No. 1 Hydrocal TUF-ART Molding Molding White Plaster Plaster Plaster Water/Hemihydrate (pts. by 50.6 50.2 29.2 61.2 wt./100 pts. by wt.) 1. Hr. wet density (Ibs./ft.3) 102.6 102.4 120.3 92.4 Dry density (lbs./ft.3) 83.6 84.0 109.4 74.4 Dry compressive strength (psi) 2530 2069 4456 557 At the time of measuring the compressive strength, the TUF-ART Plaster material was still wet when broken which explains the poor compressive strength. The compressive strengths of the other materials illustrates the feasibility of the dry spray method of this invention.

WHAT WE CLAIM IS: 1. A method of dry spraying calcium sulfate hemi-hydrate which comprises blowing an air stream of dry calcium sulfate hemi-hydrate particles through a hose to a spray nozzle having a mixing chamber therein, introducing water into the stream of dry hemihydrate particles in an amount of from 20 to 100 parts by weight per 100 parts by weight of the hemihydrate, intermixing the hemi-hydrate particles and water in the nozzle mixing chamber, and spraying the wetted hemi-hydrate through the spray nozzle.

2. A method according to Claim 1 in which the wetted calcium sulfate hemi-hydrate is sprayed onto a surface to be coated to a thickness from 1/8 inch to 5 inches.

3. A method according to Claim 1 or 2 in which the said amount of water is from 50 to 55 parts by weight per 100 parts by weight of hemi-hydrate.

4. A method according to any of claims 1 to 3 in which the dry hemi-hydrate particles are of from 0.2 microns to 725 microns particle size.

4. A method according to any of Claims 1 to 4 in which the air flow rate used to transport the dry hemi-hydrate particles to the spray nozzle is from 50 cfm to 140 cfm and the line pressure is from 8 psi to 22 psi.

6. A method according to any preceding claims wherein the dry hemi-hydrate particles are blown to the spray nozzle m admixture with reinforcing filler.

7. A method according to any of Claims 1 to 6 which includes introducing reinforcing filler into the stream of wetted hemi-hydrate particles issuing from the spray nozzle.

8. A method according to Claim 7 in which the reinforcing filler comprises glass fibers which are cut by a glass cutter mounted near the discharge orifice of the nozzle, an air stream being passed through the cutter to blow the cut glass fibers into the wetted hemi-hydrate spray under pressure.

9. A method according to Claim 6 or 7 or 8 in which the reinforcing filler comprises cut glass fibers from 1/4 inch to 4 inches long.

10. A method according to Claim 9 in which the amount of cut glass fibers if from 0.5% to 6% by weight of the dry hemi-hydrate.

11. A modification of a method according to any of Claims 1 to 10 in which only some of the said amount of water is introduced into the stream of dry hemihydrate particles, the remainder being added to the wetted hemihydrate spray issuing from the spray nozzle.

12. A method of dry spraying clacium sulfate hemi-hydrate, the method being substantially as hereinbefore described with reference to any one of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

EXAMPLE 8 A test was carried out to evaluate air convey ing and spraying calcium sulfate hemi-hydrate particles using a Bantam rockduster pump.

During the tests, 400 Ibs. of No. 1 Moulding Plaster hemi-hydrate, 300 lbs of HYDROCAL White hemi-hydrate, and 100 Ibs of TUF-ART Plaster hemi-hydrate were sprayed. The pump provided an air flow rate of 140 cfm and a line (2 inch hose) pressure ranging from 8 to 20 psi. The hemi-hydrate material was dumped into a one bag hopper, a feed screw transferred the material into the air valve box, and the dry hemi-hydrate particles were moved with air through the hose to the spray nozzle. In the spray nozzle, water was injected into the hemihydrate particles through a specailly designed water ring collar. The wetted hemi-hydrate was sprayed through the nozzle orifice by air pressure without undue pulsation.

The densities and compressive strengths of the various mixes was determined. The compressive strengths of the dry gypsum (CaSO4 .2H2 0) was measured using a molded cylinder having a 2 inch diameter and a 4 inch height.

No. 1 No. 1 Hydrocal TUF-ART Molding Molding White Plaster Plaster Plaster Water/Hemihydrate (pts. by 50.6 50.2 29.2 61.2 wt./100 pts. by wt.) 1. Hr. wet density (Ibs./ft.3) 102.6 102.4 120.3 92.4 Dry density (lbs./ft.3) 83.6 84.0 109.4 74.4 Dry compressive strength (psi) 2530 2069 4456 557 At the time of measuring the compressive strength, the TUF-ART Plaster material was still wet when broken which explains the poor compressive strength. The compressive strengths of the other materials illustrates the feasibility of the dry spray method of this invention.

WHAT WE CLAIM IS:

1. A method of dry spraying calcium sulfate hemi-hydrate which comprises blowing an air stream of dry calcium sulfate hemi-hydrate particles through a hose to a spray nozzle having a mixing chamber therein, introducing water into the stream of dry hemihydrate particles in an amount of from 20 to 100 parts by weight per 100 parts by weight of the hemihydrate, intermixing the hemi-hydrate particles and water in the nozzle mixing chamber, and spraying the wetted hemi-hydrate through the spray nozzle.

2. A method according to Claim 1 in which the wetted calcium sulfate hemi-hydrate is sprayed onto a surface to be coated to a thickness from 1/8 inch to 5 inches.

3. A method according to Claim 1 or 2 in which the said amount of water is from 50 to 55 parts by weight per 100 parts by weight of hemi-hydrate.

4. A method according to any of claims 1 to 3 in which the dry hemi-hydrate particles are of from 0.2 microns to 725 microns particle size.

4. A method according to any of Claims 1 to 4 in which the air flow rate used to transport the dry hemi-hydrate particles to the spray nozzle is from 50 cfm to 140 cfm and the line pressure is from 8 psi to 22 psi.

6. A method according to any preceding claims wherein the dry hemi-hydrate particles are blown to the spray nozzle m admixture with reinforcing filler.

7. A method according to any of Claims 1 to 6 which includes introducing reinforcing filler into the stream of wetted hemi-hydrate particles issuing from the spray nozzle.

8. A method according to Claim 7 in which the reinforcing filler comprises glass fibers which are cut by a glass cutter mounted near the discharge orifice of the nozzle, an air stream being passed through the cutter to blow the cut glass fibers into the wetted hemi-hydrate spray under pressure.

9. A method according to Claim 6 or 7 or 8 in which the reinforcing filler comprises cut glass fibers from 1/4 inch to 4 inches long.

10. A method according to Claim 9 in which the amount of cut glass fibers if from 0.5% to 6% by weight of the dry hemi-hydrate.

11. A modification of a method according to any of Claims 1 to 10 in which only some of the said amount of water is introduced into the stream of dry hemihydrate particles, the remainder being added to the wetted hemihydrate spray issuing from the spray nozzle.

12. A method of dry spraying clacium sulfate hemi-hydrate, the method being substantially as hereinbefore described with reference to any one of the Examples.