Surface Analysis of Ground Calcium Carbonate Filler Treated with Dissolution Inhibitor Peter Pang, † Yves Deslandes, ‡ Stephane Raymond, ‡ Gerry Pleizier, ‡ and Peter Englezos* ,† Department of Chemical & Biological Engineering and Pulp and Paper Centre, University of British Columbia, 2216 Main Mall, Vancouver, BC, V6T 1Z4 Canada, and Institute for Chemical Process and Environmental Technology, National Research Council, Building M-12, Ottawa, ON, K1A 0R6 Canada This work demonstrates that the solubility of ground calcium carbonate (GCC) decreased when GCC was treated with phosphate-containing chemical inhibitors. The extent of inhibition of the dissolution process was found to increase with inhibitor dosage until a saturation point was reached, beyond which further addition of inhibitor did not have any further effect. The mechanism of the inhibition was investigated by conducting surface analysis of the treated GCC. X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and energy- dispersive X-ray analysis were employed to confirm the presence of phosphate on the surface of the treated GCC. Scanning electron microscopic pictures revealed that the treated GCC particles had a different surface morphological pattern than the untreated GCC particles. It is proposed that the inhibition was brought about by the precipitation of calcium phosphate phases such as hydroxyapatite on the GCC surface. 1. Introduction Calcium carbonate (CaCO 3 ) finds extensive use as a filler in papermaking. Apart from the general benefits of using a mineral filler such as improved brightness and opacity, paper filled with CaCO 3 has better aging resistance because of its buffering ability. In addition, there is an economic incentive to use CaCO 3 because of the ability to produce cheap precipitated calcium car- bonate on-site at the paper mill. 1-3 The application of CaCO 3 in acidic papermaking such as newsprint production has been limited by the exten- sive decomposition of the filler under acidic conditions. The pH strongly influences the solubility of CaCO 3 .A change in pH from alkaline to acidic conditions causes a dramatic increase in the filler solubility by several orders of magnitude. Not only does the dissolution increase the furnishes cost, but it also buffers the wet end pH to approximately 8.5, which can cause darkening of the mechanical pulp. In addition, dissolved calcium has been shown to have a detrimental effect on the efficiency of chemical additives such as retention aids. 4 Inhibition of the dissolution of CaCO 3 has been studied extensively in the field of geochemistry. Various inorganic and organic compounds, which are termed inhibitors, have been shown to have an inhibitory effect on the rate of CaCO 3 dissolution. It is generally believed that this inhibitory effect is due to adsorption of the inhibitor on the reactive regions of the CaCO 3 surface such as kinks, which retards the rate of dissolution and precipitation of CaCO 3 . As a result, most of the dissolu- tion inhibitors are also known to inhibit the growth of CaCO 3 crystals from a system that is oversaturated with respect to CaCO 3 . Inhibitors that have been reported in the literature include phosphate-containing chem- icals, 5-11 magnesium ions, 12-14 oxalate ions, 15 carboxylic acids, 16,17 and other organic compounds. 6,8,18,19 Various patents relating to the use of phosphate compounds for manufacturing acid-tolerant CaCO 3 filler have been discussed. 20-22 Ain and Laleg recently con- ducted several trials on a newsprint paper machine using a commercial acid-tolerant PCC. 23 The wet end pH was stabilized at neutral with minimal PCC dis- solution and pulp darkening. We have previously examined the inhibiting capabil- ity of phosphate, oxalate, magnesium, maleic acid, and succinic acid on the dissolution of papermaking-grade precipitated CaCO 3 (PCC) filler in water. 24 Phosphate was found to be the most effective inhibitor and was able to lower the solubility of PCC by 80%. The objectives of this work are, first, to examine the inhibiting capability of phosphate ions on the dissolution of ground CaCO 3 (GCC) filler by monitoring the con- centration of dissolved calcium, and second, to charac- terize the surface structure of GCC treated with the inhibitor in order to understand the inhibition mecha- nism. 2. Experimental Section 2.1. Dissolution Experiments. Ground calcium carbonate was obtained in the form of 70 wt % slurry from Columbia River Carbonates (Woodlands, WA). The average GCC particle size was measured to be 1.62 µm in diameter by the technique of light diffraction (Mal- vern Mastersizer 2000, Malvern Instruments, U.K.), and the specific surface area was calculated to be 4.44 m 2 / g. According to the supplier, the GCC contains dispers- ing agent and less than 1% of MgCO 3 and acid insolu- bles. A stock GCC suspension with a concentration of 100 g/L was first prepared by diluting 14.29 g of 700 g/L GCC thick stock suspension to 100 mL with deionized water (ELGASTAT UHQ II, Elga Ltd., Bucks, U.K.). * To whom correspondence should be addressed. Tele- phone: (604) 822-6184. Fax: (604) 822-6003. E-mail: englezos@interchange.ubc.ca. † University of British Columbia. ‡ National Research Council. 2445 Ind. Eng. Chem. Res. 2001, 40, 2445-2451 10.1021/ie000846b CCC: $20.00 © 2001 American Chemical Society Published on Web 04/27/2001