Process and apparatus for calcination of gypsum

Abstract

The invention pertains to a process for modifying gypsum, wherein a continuous feed of raw gypsum is provided; the water content of the raw gypsum is determined in the continuous feed by near infrared spectroscopy (NIR) measurement; the raw gypsum is calcined in a calcination unit at a fire rate to remove water from the raw gypsum and to obtain a continuous feed of calcined gypsum having a water content within a selected range; a water content of the calcined gypsum is determined by near infrared spectroscopy, and the fire rate is adjusted based on the water content of the raw gypsum and of the calcined gypsum. Further, the invention pertains to an apparatus for performing said process.

Claims

1. Process for dehydrating gypsum, wherein a continuous feed of raw gypsum is provided; the raw gypsum is calcined in a calcination unit at a fire rate to remove water from the raw gypsum and to obtain a continuous feed of calcined gypsum having a water content within a selected range; a water content of the calcined gypsum is determined by near infrared spectroscopy, and the fire rate is adjusted based on the water content of the calcined gypsum.

2. Process according to claim 1, wherein a water content of the raw gypsum is determined in the continuous feed by near infrared spectroscopy (NIR) measurement and the fire rate is adjusted based on the water content of the raw gypsum and of the calcined gypsum.

3. Process according to claim 1, wherein after calcination the calcined gypsum is cooled and the water content of the calcined gypsum is determined by NIR spectroscopy in a continuous feed of cooled calcined gypsum.

4. Process according to claim 1, wherein a gypsum sample is taken from at least one of the group of continuous feed of raw gypsum, continuous feed of calcined gypsum, and continuous feed of cooled calcined gypsum for analysis by NIR spectroscopy.

5. Process according to claim 3, wherein at least one external measuring station is provided for NIR measurement and the gypsum sample is transported to the at least one external measuring station for determination of the water content of the gypsum sample.

6. Process according to claim 4, wherein the gypsum sample is provided at the measuring station in a standardized sample form.

7. Process according to claim 1, wherein the adjustment of the fire rate is performed by adjustment of the feed rate of the raw gypsum to the calcination unit.

8. Process according to claim 1, wherein a first gypsum sample is taken from the continuous feed of raw gypsum, the continuous feed of raw gypsum is calcined in the calcination unit and a second sample is taken from the continuous feed of calcined gypsum, wherein the second sample is taken from a spot within the continuous feed of calcined gypsum that corresponds to a spot of the continuous feed of raw gypsum, where the first gypsum sample has been taken.

9. Process according to claim 1, wherein the water content of the raw gypsum comprises a moisture content of physically bound water and a content of water of crystallization, and a content of water of crystallization C.sub.r is determined in the raw gypsum by NIR spectroscopy; an amount is defined, corresponding to an amount of water of crystallization to be removed from the raw gypsum; a set-point for a content of water of crystallization S comprised in the calcined gypsum is determined by subtracting from C.sub.r (S=C.sub.r); the raw gypsum is fired at a fire rate to obtain calcined gypsum; a content of water of crystallization C.sub.c of the calcined gypsum is determined by NIR spectroscopy; and the fire rate is adjusted such that SC.sub.c=0, wherein is a maximum deviation.

10. Process according to claim 1, wherein a process controller is provided and the adjustment of the fire rate is performed by the process controller.

11. Apparatus for dehydrating gypsum comprising: a feeder for feeding a raw gypsum to a calcination unit; a calcination unit for calcining the raw gypsum; a discharging unit for discharging calcined gypsum from the calcination unit, and a first NIR (Near Infrared) spectroscopy unit to determine the content of water or water of crystallization comprised in the calcined gypsum.

12. Apparatus according to claim 11, wherein a second NIR spectroscopy unit is provided to determine the content of water or water of crystallization in the raw gypsum.

13. Apparatus according to claim 11, characterized in that a cooling unit is provided for cooling the calcined gypsum and the first NIR spectroscopy unit is provided at a position to determine the content of water of crystallization after cooling of the calcined gypsum.

14. Apparatus according to claim 11, characterized in that a third NIR spectroscopy unit is provided to determine the content of water of crystallization in the calcined gypsum before cooling of the calcined gypsum.

15. Apparatus according to claim 11, characterized in that a process controller is provided for adjusting a fire rate of the calcination unit based on data determined by the first NIR spectroscopy unit and on data of at least one of the second and third NIR spectroscopy units.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The process according to the invention will be described in more detail with reference to the accompanying drawings. The figures of the drawings show:

(2) FIG. 1: a scheme of the components of an apparatus according to the invention;

(3) FIG. 2: a scheme showing an embodiment in which a sample is taken from the continuous feed of raw gypsum for NIR analysis.

(4) FIG. 1 schematically shows the components of an apparatus according to the invention. Raw gypsum is transported on a conveyor 1, e.g. a belt conveyor. The conveyor has a constant transportation rate for providing a continuous feed of raw gypsum. The raw gypsum can be adjusted in a mill (not shown) to desired particle size and can be adjusted to a desired water content in a dryer (not shown). A scale 2 is provided at the conveyor 1 for determination of the feed rate of the gypsum feed transported by conveyor 1. In scale 2 the amount of raw gypsum provided in the raw gypsum feed is continuously or discontinuously weighed. A NIR spectroscopy unit 3 is provided for determination of the content of water or water of crystallization in the raw gypsum. The raw gypsum is then transported to a calcination unit 4, e.g. a flash calciner. The calciner unit 4 is equipped with a heating unit 5 for providing heat for calcination. Into the heating unit 5 are introduced fuel and air to be burned and the hot gases are then introduced into the calcination unit 4. As fuel can be used every suitable fuel, e.g. natural gas, light petroleum gas, heavy oil, coal, etc. After calcination the calcined gypsum is removed from the calcination unit (4). In case of a flash calciner the mixture of hot gases and calcined gypsum is transferred into a separating unit 6 to separate gases which are discharged through a discharging line 7 from the calcined gypsum. The calcined gypsum which is still hot is discharged from separator 6 and is transported by a conveyor 8, e.g. a conveyor screw. The hot calcined gypsum is cooled in a cooling unit 9 to about room temperature. Downstream of the cooling unit is positioned a further NIR spectroscopy unit 10 for determination of the water content or content of water of crystallization in the calcined gypsum.

(5) FIG. 2 shows a scheme showing an embodiment in which a sample is taken form the continuous feed of raw gypsum for NIR analysis.

(6) A raw gypsum feed from a silo 20 is transported on a conveyor 1 to a belt scale 2 to weigh and adjust the raw gypsum feed. The particle size of the raw gypsum is adjusted to less than 60 mm. Part of the raw gypsum feed is separated into a separation line 11 whereas a main feed is transported further by conveyor 1 towards the calcination unit 4. The separated raw gypsum is transported in separation line towards a mill 12 for adjustment of particle size to less than 2 mm. Downstream of mill 12 the milled raw gypsum is transported on a belt conveyor 13 to pass an adjustment device 14 to adjust layer thickness of the raw gypsum layer transported on belt conveyor 13 and to smooth the upper surface of the raw gypsum layer. The raw gypsum layer then passes NIR spectroscopy unit 3 for determination of the water content or content of water of crystallization of the raw gypsum. After having passed NIR spectroscopy unit 3 the raw gypsum is transported by a bucket elevator 15 back to the main feed of the raw gypsum transported on conveyor 1.

REFERENCE LIST

(7) 1 conveyor 2 scale 3 NIR spectroscopy unit 4 calcination unit 5 heating unit 6 separator 7 discharging line 8 conveyor 9 cooling unit 10 NIR spectroscopy unit 11 separation line 12 mill 13 belt conveyor 14 adjustment device 15 bucket elevator 20 silo