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Mining and metals refining
Feb 8, 2017

Refractory lining materials testing

Measuring chemical durability of refractory lining materials at our Research Center in Pori, Finland.
Figure 1: Refractory lining in operation in a pyrometallurgical furnace
Figure 1: Refractory lining in operation in a pyrometallurgical furnace

In the selection of proper refractory materials, Outotec material testing services are here to assist. Testing of the durability of refractory materials in a specific chemical environment can be done at laboratory scale. This makes it possible to follow the melt-refractory reactions in small volume instead of at full scale. With well-chosen, valid refractory grades longer campaigns can be expected in operation. Technical risks for refractory failure can be reduced and health and safety risks subsequently minimized.

Smelter personnel are often driven into negotiations with refractory suppliers, whether they are old suppliers offering new grades, or new suppliers entering the market. The measurement of refractory chemical durability helps in quality assurance of their products. With old suppliers the properties of familiar grades may change due to a different raw material base and with new suppliers the quality is an unknown quantity. Outotec Research Center’s service is independent from any refractory supplier and can provide you the expertise required to make the right selection for your smelter.

The measurement

Our Research Center has performed refractory durability tests for over 35 years. Target groups have been flash smelting and converting units as well as copper melting and casting units. Ferroalloy smelting refractories have also been tested along with refractories suitable for some new process concepts.
Tested refractory materials include magnesia chrome, magnesia, alumina-silica and carbon bricks or masses. The process melt has been typically slag, but matte and metal melts have been investigated as well. Best results can be expected if authentic slag or matte from a customer can be used. If not available, a reference melt can be utilized.

There are two options to measure the chemical durability of a refractory material: a finger test or a crucible test. The finger test is best suited for refractory brick materials and is the one most used, because it is dynamic and most accurately resembles the real operating conditions. The crucible test is best for refractory masses and for cases where the reactions with the refractories are unknown, i.e. new process concepts. It is static and does not mimic the vigorous melt movements as well as the finger method. Which method to use must be considered case by case.

The finger and the crucible methods differ in the way the investigated refractory is introduced. In the finger test the refractory is the finger and the melt is held typically in an Al2O3 crucible. In the crucible method there is no finger and the melt is held in a crucible made out of the investigated refractory material. 

Testing starts with the preparation of the raw materials. Refractory brick fingers are drilled out of larger blocks and crucibles are prepared by drilling a cavity in the middle of a cubic or cylindrical sample block. The process material to form the melt is ground to a preferential particle size of < 2 mm, in order to allow filling the crucibles as full as possible. In some cases premelting of the material is required to prevent foaming in the actual test.

In the finger test, the crucible is then heated in an induction furnace where the melt is vigorously agitated by induction current, making the conditions dynamic. In the crucible test, crucibles are heated in a chamber furnace, rendering the conditions static. In both methods typically a temperature of 1450 oC, N2 or Ar atmosphere and duration of 6 hours has been applied. Other conditions are possible but then comparison to previous samples becomes cumbersome. After the test the fingers are lifted from the melt and the induction furnace current is turned off. In crucible tests the samples are cooled down with the furnace.

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After the test the refractory fingers or crucibles are cut in half and the surfaces and inner structures are investigated. Typically the structures are photographed and special features like dissolving of the refractory and melt infiltration are scrutinized. In Figure 5. Examples of their structures are seen in Figure 6. If needed, the microstructures can be investigated further with optical microscopy, Figure 7 and with scanning electron microscopy. Figure 8 shows examples of scanning electron microscopy images taken for evaluation of the brick grades.

The results of the investigation of the refractory samples are reported as a short laboratory report or as a longer technical report if microscopy investigation is included. The reports will rank the studied refractory grades according to Outotec’s grading criteria to classes which, for example on magnesia chrome bricks are: good quality – satisfactory – tolerable – low quality – very low quality bricks. The grading criteria has been developed over the 35 years the tests have been done at Outotec, Pori.

The cost of testing will depend on the amount of samples as the crucible in the finger test holds 3 samples. In crucible test only one refractory grade can be measured in one test. Also factors that affect pricing are the extent of microscopy investigation and the extent of reporting.

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