Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF PLASTER OF PARIS BY BATCH CALCINATIONS"

Abstract

An Improved Process For The Preparation Of Plaster of Paris by Batch Calcinations A process has been developed for the preparation of plaster of Paris which comprises calcining gypsum in a reactor such as herein described, passing high pressure steam through the pipe of the reactor in the range of 9 - 15 kg/cm2 to heat the obtained gypsum powder , at the temperature in the range of 175-200°C for a period ranging 1-2 h to get the desired plaster of paris. The invention is based on the finding that when the gypsum is heated using high pressure steam the formation of CaSO4 is minimum due to the fact, the temperature does not go beyond 190°C.

Full Text

This invention relates to an improved process for the preparation of plaster of Paris by batch calcination. Gypsum is calcium sulphate dihydrate and has chemical formula CaS04,2H20. The dehydration of gypsum is known as calcination. When gypsum is heated, the water as steam is liberated till 155 C when calcium sulphate hemihydrate, CaS04,2H20 is formed. This hemihydrate is plaster of Paris. If the gypsum is further heated to 190-210 C, anhydrous calcium sulphate, CaSO4. (with no H2O molecule) is formed. The palster of Paris has quick setting properties with water and its presence in good plaster of Paris is undesirable. So during calcination of gypsum, the maximum temperature of calcination should be 155 C far getting plaster of Paris of ceramic grade. Kettle calcination using indirect hot gas such as combustion flue gas heating is most commonly used in batch mode for production of plaster (Encyclopedia of Chemical Technology, 3rd Ed., Vol.4 (1978), p-437). Continuous calcination in kettles using indirect hot gas heating was reported (Belgium Pat 624, 555, Feb 28, 1963, R.C. Blair, to British Plaster Board Ltd.). Moving bed calcinator with direct heating of gypsum particles with hot gas such as combustion flue gas when the particles are moving in single or multiple layers was reported (British Patent, 886, 602, Jan 10, 1962, A.N. Kanauf to Gebr. Kanauf Saar Gipswerke). Flash calcination of gypsum powder with direct hot gas such as combustion flue gas heating has been reported (Canadian Patent 984, 145, March 23, 1976, J.A. Keller and R.T.Spitz, to National Gypsum Company). Batch kettle calcinator shown in (Fig-1) of the drawing accompanying this specification is a metallic cylindrical vessel (1) having multiple horizontal flue pipes (2) at different heights through the side wall of the vessel and an aqitator with arms (3)in between flue pipes for powder agitation. There are two ports at the top of calcinator, one (4) for feeding gypsum powder and other (5) for exit of water vapour generated during calcination. There is a quick opening door (6) at the kettle bottom for hot plaster exit. The calcinator is enclosed in a heat insulated casing (7) with an uniform gap on all sides for movement of hot gas around the calcinator. There is a combustion chamber (8) and burner (9) below the kettle. The liquid or gaseous fuel is burnt in the combustion chamber and the hot combustion flue gas generated moves through the horizontal flue pipes and around walls to heat up the gypsum powder inside the kettle. The cooled flue gas goes out through stack (10). Usually the hot combustion gas temperature varies from 500 to 900 C. The kettle is first filled slowly with gypsum powder through port (4) while the agitator and burner is on. The gypsum powder gets heated up and release water from gypsum as vapour to exit through port(5). When the powder temperature reaches 155 C, the hemihydrate conversion is complete and the hot plaster is discharged through the quick opening gate (6) located at the bottom of the kettle. Fuel firing rate and the operation of gate (6) are to be monitored through elaborate control systems. Drawbacks of calcination in present batch kettle calcinator : (1) As the hot combustion gas temperature is between 500°C to 900 C, the powder layer near the inside wall of kettle body and flue lines is overheated to above 190°C to form soluble anhydrous CaS04 as contaminants to deteriorate the physical properties of plaster. (2) Rigorous control system of fuel firing for better temperature control and of quick opening of discharge valve to avoid the overheating of the product above 190~C resulting in the formation of contaminant anhydrous CaS04 as waste product. (3) The corrosion rate of the kettle body in contact with high temperature flue gas at 500 C to 900 C is high. The main object of the present invention is to provide an improved batch calcination process for the preparation of plaster of Paris. Another object of the present invention is to provide an improved process for the preparation of plaster of Paris of good quality ceramic grade with minimum or nil anhydrous CaS04 as contaminant. The invention has been developed as a finding that if the calcination of gypsum is affected by employing high pressure steam it is not required to employ a temperature beyond 160 C thereby avoiding the formation of calcium sulphate as a contaminat and facilitate getting a constant good ceramic grade plaster of Paris. The invention is based on the finding that when the gypsum is heated using high pressure steam the formation of CaSO4 is minimum due to the fact, the temperature does not go beyond 190°C. Accordingly the present invention provides an improved process for the preparation of plaster of Paris by batch calcinations which comprises calcining gypsum in a reactor such as herein described, passing high pressure steam through the pipe of the reactor in the range of 9 - 15 kg/cm2 to heat the obtained gypsum powder , at the temperature in the range of 175-200°C for a period ranging 1-2 h to get the desired plaster of paris. The process can be carried out in the calcinatory usually employed for the preparation. By way of illustration the process is described with reference to the calcinator shown in Fig 1 of the drawing accompanying the provisional specification. Calcinator shown in Fig 2 has a cylindrical metallic vessel (1) having multiple horizontal steam pipes (2) at different heights through the side wall of the vessel and an agitator with arms (3) in between the pipes. There are ports for gypsum powder feed (4) for exit of water vapour (5) generated during calcinations. There is a door (6) at the bottom of the kettle for powder discharge. The calcinatory is covered with heat insulation material (7) at the walls. The high pressure steam from a boiler, whose pressure is set 9-15 kg/cm2 (175-200°C) is used as heating fluid. A manually opening valve for steam at the inlet of calcinatory, passes through the pipes to heat the powder in the temperature range 150-165°C. the discharge of plaster after calcination is done with an ordinary door (6). Advantages of the present invention (1) In the present invention, pressure steam is used as heating fluid instead of hot combustion flue gas. (2) No control of heating rate is necessary because change in duration of heating will not affect the quality of the plaster of Paris produced. (3) The duration of draining has no effect on the plaster of Paris produced. So no control of draining valve is necessary. (4) The temperature of steam pipe wall is below 190 C, with maximum at 170°C. So the product will have almost no anhydrous CaS04 as contaminant and the plaster will tae very good quality. (5) As the maximum wall temperature of steam is 170 C or below, the chance of corrosion of the equipment is low. Because the temperature do not go beyond 190°C so the life of calcinator will be longer in comparison to the usual calcinator. The details of the invention are provided in the examples given below which are given to illustrate the invention and therefore should not be construed to limit the scope of the invention. Example — 1 : Steam at 9-10 Kg/cm2 was used in a calcinator of 90 cm diameter and 150 cm height with 5 cm dia steam pipes in 3 rows and having an agitator with 4 arms for stirring the powder content of the calcinator, 500 Kgm selenite gypsum was fed to the calcinator over a period of 1 hr and the steam at the above pressure was passed through the pipes. The calcination is continued for one and half hrs. till the temperature reached 150 -165 C and the powder was drained for 15 minutes. The yield of powder plaster of Paris was 410 Kgm (82%). Example - 2 : 550 Kgm selenite gypsum was fed to the calcinator over a period of 1 hr 15 min. when the steam at a pressure of 15 kg/cm2 was passed. The calcination was continued far 2 hrs till the temperature of powder reached 15O0 -1650C and the powder (plaster of Paris) was drained for 15 minutes. The yield of powder was 450 Kgm (81.8%.). We Claim: 1. An improved process for the preparation of plaster of Paris by batch calcinations which comprises calcining gypsum in a reactor such as herein described, passing high pressure steam through the pipe of the reactor in the range of 9 - 15 kg/cm2 to heat the obtained gypsum powder , at the temperature in the range of 175-200°C for a period ranging 1-2 h to get the desired plaster of paris. 2. An improved process as claimed in claim 1 wherein the preferable pressure of steam ranges from 9-10 kg/cm2 (i.e. 175 - 180°C). 3. An improved process for the preparation of plaster of paris by batch calcinations substantially as herein described with reference to the examples and with reference to figure of the drawings accompanying the provisional specifications.

Documents:

639-del-1996-abstract.pdf

639-del-1996-claims.pdf

639-del-1996-correspondence-others.pdf

639-del-1996-correspondence-po.pdf

639-del-1996-description (complete).pdf

639-del-1996-form-1.pdf

639-del-1996-form-2.pdf

639-del-1996-form-4.pdf

639-del-1996-form-6.pdf