Vanadium staining in fired clay bricks

Abstract This paper reports on a series of efflorescence tests and instrumental techniques that have been used to determine the nature and mechanism of vanadium staining, and the effects of additions of magnesite, calcite, and fluorspar to the brick mix to eliminate the problem. Scanning electron microscopy, electron probe microanalysis, inductively coupled mass spectrometry (with laser ablation attachment), and X-ray photon spectrometry have been used to more fully understand the elemental composition, oxidation states, and formation of vanadium stains in fired clay bricks. It was found that the stains consist of V, K, Na, Mo, S, and O. Vanadium +4 compounds form initially within the brick and are slowly oxidised to vanadium +5 compounds as the water transports the potassium and sodium salts to the brick surface. X-ray diffraction led to a reasonably successful search match for a KVO(SO4)2 phase for the stains. Evolved gas analysis showed loss of vanadium compounds between 700 and 900° C which are most likely VF3 and VCl4. Inductively coupled plasma spectrometry laser ablation studies on thin sections of brick showed areas of glassy phases corresponding to the agglomeration of Mg/V/Al/Si and Mg/Al/Si. The increased alkalinity of magnesia and the comparable sizes of V4+ and Mgions are driving forces for locking vanadium into what is otherwise an acidic glassy phase. The addition of 1 wt-% magnesite to the brick mix led to fired products on a commercial scale that are free from vanadium staining.