Product Consistency Testing (PCT) of Thermochemically Converted Asbestos
Dale M. Timmons, R.G.
ARI Technologies, Inc.

August 15, 2001

INTRODUCTION

One of the goals of vitrifying hazardous and/or radioactive waste is to achieve a high level of chemical durability in the vitrified product. This results in a vitrified product that has the greatest chance of maintaining its integrity for geologic time periods. This minimizes the release of hazardous and/or radioactive materials to the environment. Vitrification of waste materials typically results in products exhibiting superior chemical durability. Thermochemical conversion does not result in complete melting of the waste so superior chemical durability cannot be assumed. The chemical durability of converted products should be tested.

One of the methods used to evaluate chemical durability is the Product Consistency Test (PCT). The PCT test methods are described by the American Society for Testing and Materials (ASTM) Designation C 1285-97. The PCT test is essentially an extraction that is performed on a crushed sample of the vitrified product. The extraction procedure emulates long-term exposure to worst-case environmental conditions by presenting a large surface area of the treated product to a relatively corrosive extraction medium. The extraction medium is then analyzed to determine the rate of elemental release from the product to the medium. The measured elemental release rates can then be compared to those from other treated waste products and a determination of relative product quality can be made.

The ASTM methods for PCT testing include methods A and B. Method A is performed on a crushed sample of vitrified product. The crushed sample is exposed to ASTM-Type I water (deionized water) at 90°C for a period of 7 days at static conditions (no agitation or circulation). Test method B is also performed on a crushed sample of the vitrified product under static conditions. However, method B allows for variation in the test duration, leachant volume, test temperature and leachant types.

TEST PROCEDURES

Two samples of converted products were selected for PCT testing. The two samples were collected at random and were arbitrarily named Samples A and B. They were prepared by statistically splitting the material. Splits for each of the samples were analyzed for bulk chemistry using X-ray fluorescence and other splits were prepared for PCT testing by crushing and rinsing. The crushed samples were then subjected to particle size analysis. PCT testing was performed using dionized water as an extraction fluid in non-reactive vessels at a temperature of 90º C. The pH of the extraction fluid was 7.0 at the initiation of the tests. The samples were exposed to the extraction fluid for a period of 7 days after which the fluid was subjected to elemental analysis.

TEST RESULTS

Whole rock analyses were performed on the vitrified product samples using X-ray florescence (XRF) for 13 metal oxides. Inductively coupled plasma spectroscopy (ICP) analysis was also performed on the leachant from the PCT tests for 27 elements. The concentrations of the elements in the leachant were then normalized relative to composition and surface area of the product. Normalizing the results facilitates a comparison between elements. The normalized value is obtained by dividing the product of the blank-corrected concentration of an element in the leachant and the volume of the leachant by the product of the mass fraction of the element in the sample and the surface area per unit mass of the crushed sample particles in the test.

The normalizing formula (Spalding) is:

Where:

NR_e = Normalized release of element e, from sample s
C_es = concentration of element e in leachate from sample s
B_e = concentration of element e in leachate from blank (detection limit for all analyses)
V_s = initial volume of leachant in test vessel containing sample s
f_e = average mass fraction of element e in the sample = (mass fraction oxide in material) x (atomic weight of metal in oxide)/(atomic weight of oxide),
m_s = original mass of sample s
SA_s = surface area per unit mass of sample s

The surface area per unit mass is found by the following equation:

This calculation assumes a mean diameter of spherical particles. A grain density of 2.5 g/cm³ was used for the calculation based upon measurements of similar products. The mean diameter of the vitrified product particles used in the test was the material passing the #100 sieve but caught by the #200 sieve (0.075mm to 0.15mm). The mean particle size for the two samples was 0.113mm.

The final normalized release values are expressed in g/m² (grams of element released per square meter of sample surface area).

Eight elements are represented. Only the elements for which detectable analytical results from the treated product and from the leachant are presented. Normalized release rate calculations cannot be performed without both analyses and the concentration in the vitrified product needs to be above detection limits.

The acceptance criteria for waste glass has been defined in the United States on the basis of releases measured with the PCT test (McGlinn, et.al.). The releases of Na and Si are typically considered the best indicators of glass quality. For PCT testing in deionized water at 90°C for 7 days, the releases for Na and Si should be less than 6.67 and 1.96 g/m² respectively.

The results of these tests can be compared to PCT tests previously performed on other vitrified samples. The PCT Tests previously performed included 7-day and 28-day tests at 90°C and 26°C. Similar tests using pH 10 buffered water were also conducted. Table 1 compares the results of the earlier PCT tests performed on vitrified products with the PCT tests performed on the converted product.

Table 1 shows that the PCT results derived from the ARI samples are one to two orders of magnitude better than the U.S. established criteria of 6.67 and 1.96 g/m² for Na and Si respectively. Furthermore, the Na and Si results are comparable to the PCT results that were obtained from PCT tests performed on vitrified products from a radioactive waste site in Australia. Figure 1 compares the data presented in Table 1 in graphic form. The graphical results show that, in general, the ARI product exhibits better chemical durability than the vitrified products.

References
McGlinn, P.J., Hart, K.P., Day, R.A., Harries, J.R., Weir, J., Thompson, L.E., Scientific Studies on the Immobilization of Pu by ISV in Field Trials at Maralinga, Materials Research Society Symposium Proceedings, Vol. 506, 1998.

Spalding, B.P., et. al., Tracer-Level Radioactive Pilot-Scale Test of In Situ Vitrification for the Stabilization of Contaminated Soil Sites at ORNL, Oak Ridge National Laboratory, Environmental Services Division, Publication No. 3962, 1991.