Investigation and prediction of lime agglomeration, sticking tendency, and blocks formation in vertical double shaft regenerative kilns for the production of high-calcium or magnesium-rich quicklime

This contribution reports preliminary results on the research activity performed on 25 carbonate rocks, including mud-supported, and grain-supported limestones, diagenetic dolomites, and granoblastic marbles, to evaluate their sticking tendency, and occasional melting at 1300°C, which could occur during the calcinations process in vertical double shaft regenerative kilns for the industrial production of high calcium or magnesium-rich quicklime. Samples, made available to Cimprogetti by different worldwide lime producers, have been characterized by means of a complete set of physical, chemical, mineralogical, petrographic, and thermal analyses performed either on powdered rocks, with the only exception of petrographic and porosimetric analyses, or on their insoluble rests. A particular attention was paid to the extraction of the insoluble rest by means of a weak acid attack at high temperature, followed by a centrifugation in deionized water using a spin-dryer device for purifying the sample. This procedure, specifically designed for clay minerals and organic matter analysis, permitted to concentrate some grams of insoluble rest from very pure carbonate samples. Moreover the so-called “overburning test” at 1300°C, according to the internal test method invented by Cimprogetti, has been performed to simulate and predict the lime agglomeration at the highest temperature of the kiln (Vola & Sarandrea, 2014). The quantitative phase analysis (XRD-QPA) according to the Rietveld method on lime blocks obtained from the overburning test, pointed out the formation of predominant lime (CaO), associated with significant content (3-6%) of tricalcium silicate (Ca3SiO5 or alite), and dicalcium silicate (Ca2SiO4 or belite). More rarely, tricalcium magnesium silicate (Ca3Mg(SiO4)2 or merwinite) has been observed too. The presence of such accessorial calcium silicates indicates that chemical reaction between the lime and clay minerals took place, pointing out an early stage of incomplete clinkerization process. Anyway, some blocks of burnt lime with a high sticking tendency (>40%) don’t present accessorial silicates, suggesting the lime agglomeration must be also related to other apparently not so obvious chemical or physical factors. Ongoing analyses, including the screening of the organic carbon (Cwt%, 13C‰), ultralight elements (Lithium, Boron), and halogens (Fluorine, Chlorine) by means of isotope ratio mass spectrometry (IRMS), inductively coupled plasma mass spectrometry (ICP-MS), and wet chemistry titration, will help to further clarify the key factor for understanding and predicting the lime agglomeration and blocks formation at the highest temperature of the kiln, as suggested by previous authors (Elsen et al. 2011). References: VOLA G. & SARANDREA L. (2014) – Investigating and predicting blockages. World Cement, October 2014, 85-92. ELSEN J., MERTENS G. & SNELLINGS R. (2011) – Portland cement and other calcareous hydraulic binders: history, production and mineralogy, EMU Notes in Mineralogy, Volume 9, Advances in the Characterization of Industrial Minerals, 452-457.