Acid-Insoluble Matter: Definition, Hazards, and Determination Method
Acid-insoluble matter generally refers to substances that are insoluble in dilute hydrochloric acid (10%–20% HCl). For example, impurities such as Al₂O₃, Fe₂O₃, Mn₃O₄, and SiO₂ present in carbonate rocks—including dolomite and limestone used as refractory materials and chemical raw materials—are classified as acid-insoluble matter. These components are considered deleterious constituents in the aforementioned industrial raw materials. Therefore, the acid-insoluble content serves as an important technical indicator for evaluating the quality of such mineral resources.
Acid-insoluble residue refers to the material remaining after treatment of the sample with hydrochloric acid, followed by further treatment of the residue with sodium hydroxide solution. After neutralization with hydrochloric acid and filtration, the remaining residue is subjected to high-temperature ignition, and the final remaining substance is defined as the acid-insoluble matter.
Hazards of Hydrochloric Acid-Insoluble Matter
Impact on PVC Thermal Stability
A high content of silicon compounds in acid-insoluble matter may induce slight crosslinking or thermal degradation in polyvinyl chloride (PVC), thereby reducing its thermal stability. Conversely, when the calcium carbonate (CaCO₃) content exceeds 95% and the acid-insoluble content is low, the thermal stability of PVC is not adversely affected.
Effect on Filler Performance in Polypropylene Processing
The presence of silicon compounds in ground calcium carbonate (GCC) increases particle hardness. Calcite powders with high silicon content, when used in the production of polypropylene (PP) raffia or flat yarn masterbatch, can accelerate the wear of slitting blades due to increased abrasiveness.
Influence on AKD Sizing Efficiency
Su Yanqun et al. investigated the effect of acid-insoluble matter in lime mud calcium carbonate derived from straw pulp on AKD (alkyl ketene dimer) sizing performance. The study demonstrated that the total acid-insoluble content in lime mud calcium carbonate is a key factor leading to reduced AKD sizing efficiency.
Determination Method for Hydrochloric Acid-Insoluble Matter
The content of hydrochloric acid-insoluble matter is determined by dissolving the sample in hydrochloric acid, filtering the insoluble residue, igniting the residue at high temperature, and weighing the remaining mass. The required apparatus, reagents, and detailed experimental procedures are specified in Section 3.15 of GB/T 19281—2014, Methods for Chemical Analysis of Calcium Carbonate.
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