SCALING RATE MEASUREMENT USING FTIR WITH ATR ACCESSORY

Scale formation in various parts of a Bayer refinery can, through reduced tank and pumped availability, lower the production capacity. Further, removal of scale by chemical, mechanical or other means increase the alumina production. Scale rate formation depends on the type of scale, liquor composition and temperature and range from micrometer per hour to micrometer per week.
Measuring scaling rate is cumbersome task and is, for practical reason (simplicity), often based on equipment descaling frequency. For scaling studies coupons are for extended time placed in a tank or pipe with manual measuring of scale thickness. The scaling rate is then related against average liquor composition and temperature. Liquor conditions and temperature that often can’t be controlled, and hence less well suited to establish a robust relationship.
A Fourier Transform Infrared equipped with an attenuated total reflection (FTIR-ATR) accessory is sensitive to scale formation causing a change in the absorption spectrum. Scale of several angstroms can already be detected. The feature that a FTIR-ATR is sensitive to scale formation has the potential that a FTIR-ATR can be a tool to study scaling rate. This potential is explored by studying silica scale rate in dilute Bayer process liquor.
Information that can be obtained by measuring continuously the Infrared absorption spectrum between 650 and 4000 cm-1 is the type of scale from the absorption peak, the induction time and crystallisation rate of the scale formation. The silica scaling of diluted Bayer process liquor was measured using a KEIT IRmadillo connected to a 1 litre stirred reactor with temperature control. Duration of a test is typically 1 to 2 days. The silica scaling rate was also estimated from the difference in silica concentration between start and end of the test.
The scaling rate measurement of diluted Bayer process liquor using FTIR with an ATR probe showed induction time varied between 10 and 20 hours and two different type of sodium alumina silicates are formed. The silica scaling rate is: 𝑆𝑖𝑙𝑖𝑐𝑎 𝑠𝑐𝑎𝑙𝑖𝑛𝑔 𝑟𝑎𝑡𝑒 [μ𝑚𝑦𝑒𝑎𝑟⁄]=2.88 𝑥 𝑆1 [𝑔𝐿⁄]−60.6
The estimated silica deposition using the above equation is in good agreement with the scale deposition found in refinery equipment. The FTIR-ATR shows to be a useful tool to establish scaling rate of well fined conditions in a short period of time and can be used to evaluate the effectiveness of descaling additives.