Method for bonding gypsum to paper

Claim 1

IN A METHOD FOR PREPARING GYPSUM BOARD COMPRISING CONTINUOUSLY ADVANCING A FIRST PAPER SHEET, DEPOSITING A SETTABLE SLURRY OF WATER AND CALCINATED BY-PRODUCT GYPSUM UPON SAID FIRST SHEET, FOLDING THE EDGES OF SAID FIRST SHEET TO PARTIALLY ENCLOSE THE SLURRY, CONTINUOUSLY ADVANCING A SECOND PAPER SHEET, APPLYING PASTE TO EDGE PORTIONS OF THE ADVANCING SECOND SHEET WHICH WILL CONTACT FOLDED EDGE PORTIONS OF SAID FIRST SHEET PRESSING THE SHEETS AND SLURRY THEREBETWEEN TO FORM A BOARD OF SUBSTANTIALLY UNIFORM THICKNESS, ALLOWING THE SLURRY OF THE FORMED BOARD TO SET, AND THEREAFTER DRYING THE BOARD THE IMPROVEMENT WHICH COMPRISES UTILIZING AS THE AFORESAID SETTABLE SLURRY A SLURRY OF WATER AND CALCINED BY-PRODUCT GYPSUM PREPARED BY SLURRYING BY-PRODUCT GYPSUM CONTAINING RESIDUAL FREE AND OCCLUDED PHOSPHORIC ACID AS AN IMPURITY WITH WATER, INCORPORATING AMMONIUM HYDROXIDE INTO THE RESULTING SLURRY IN AMOUNT SUFFICIENT TO BRING THE PH THEREOF TO BETWEEN 8 AND 10 AND TO NEUTRALIZE THE FREE PHOSPHORIC ACID TO FORM THE AMMONIA NEUTRALIZATION SALT OF THE PHOSPHORIC ACID IN THE SLURRY, DE-WATERING THE THUS-TREATED SLURRY TO FORM A DE-WATERED GYPSUM CAKE, CALCINING THE DE-WATERED CAKE WHEREBY CALCIUM SULFATE HEMIHYDRATE IS FORMED AND OCCLUDED PHOSPHORIC ACID IS LIBERATED, PULVERIZING THE CALCIUM SULFATE HEMIHYDRATE TO POWDER FORM, SLURRYING THE CALCIUM SULFATE HEMIHYDRATE POWDER WITH WATER, AND INCORPORATING AMMONIUM HYDROXIDE INTO THE SLURRY IN AMOUNT SUFFICIENT TO BRING THE PH THEREOF PRIOR TO SETTING TO BETWEEN 7 AND 8.5 AND TO NEUTRALIZE LIBERATED PHOSPHORIC ACID TO FORM THE AMMONIA NEUTRALIZATION SALT OF THE LIBERATED PHOSPHORIC ACID IN THE SLURRY.