Smelting and processing of mineral commodities requires refining furnaces and vessels to undergo extreme temperatures.Molten metal is contained by layers of refractory lining,which undertake thermal stresses and chemical changes.The reactions within the furnace are abrasive and can cause substantial wear to the refractory.Non- destructive condition monitoring of refractory lining is essential for the continuous operation of processing vessels and ensure safety.Improved maintenance strategies are achievable if the remaining refractory thickness is known and the worn region is identified in advance.Traditionally,refractory thickness is estimated by temperature measurements and the use of mathematical modeling.This method suffers from several limitations.Firstly,the method provides an indirect indication of the relative refractory thickness/condition based on heat flux and refractory thermal conductivity.Secondly,the coverage of the thermocouples is limited by their layout;thus,an overview condition of the entire furnace is not available unless a substantial number of thermocouples are installed.To overcome some of these limitations,a patented technique called the Acousto Ultrasonic-Echo ( AU-E) was developed in the late 1990s.This technique complements the thermal flux technique and can provide an accurate direct measurement of remaining refractory thickness.It uses both the time and frequency domains data analysis to determine material thicknesses and quality.It has been applied to evaluate the quality of the refractory of blast furnaces,non-ferrous furnaces and vessels under thermal stress.Furthermore,Furnace Integrity Monitoring System ( FIMS) was developed by Hatch for long- term continuous monitoring of the structural integrity of furnace crucibles.In this paper,case studies are used to illustrate the application of the AU-E to determine the remaining refractory thicknesses of sidewall and hearth of furnaces,either during operation or furnace shutdown.Thermal factor was considered in the AU-E analysis to account for the change in velocity of the stress wave in materials due to thermal conditions.The retrospective acoustic emission data analysis of the long-term monitored furnace crucible showed that waming period and problematic regions of the furnace lower hearth could be identified by the FIMS,which could greatly improve the furnace operation safety.In conclusion,with the aid of non-destructive testing techniques,effective maintenance measures and timely repairs can be scheduled to prolong the furnace campaign life.