GEOL-312 lab 10 ARD potential test

The objective of this lab is to understand some of the procedures involved in testing rock samples for their potential to generate acid rock drainage, and to understand why some rocks have the potential to produce acidity, while others have the potential to consume acidity.

Sample preparation

Five rock samples with varying amounts of sulphide minerals and carbonates have been crushed to minus-10 mesh (approx. 1 mm).  A representative 250 g portion of each has been split out, dried and pulverized to minus-200 mesh for assay and the titration test.

Assay

The pulverized samples have been assayed for sulphide sulfur. That means that each has been analyzed for total sulphur and also for sulphate sulphur.  The difference between the two is sulphide sulphur.  The acid production potential (APP) of the sample, expressed as kg of sulphuric acid** per tonne of sample, is calculated on the basis of the sulfur assay: 

Acid production (APP) = 31.25 * S(%)

The results are as follows:

Titration Test

The acid generation part of the experiment is already done, therefore, and we are going to be testing the samples for their acid consumption potential.  In other words, we are going to see how much acidity they might consume as a result of their carbonate mineral content.  The test is done by titrating with sulphuric acid until a stable pH of 3.5 is reached.  pH of 3.5 is chosen as above this value the acid-generating bacterium Thiobacillus ferrooxidans is not active.

Reagents

·         distilled water

·         1.0 N sulphuric acid

Equipment

·         balance

·         250 ml beakers

·         magnetic stirrers and magnets

·         burettes and stands

·         pH meter

A l0 g portion of the pulverized sample is added to 100 ml of distilled water and stirred for approximately 15 minutes. (For samples R-Yl-002 and R-BL-061 we will use 1 g only, as these are carbonate-rich.) The natural pH is recorded. The sample is then titrated to pH 3.5 with 1.0 N sulfuric acid. The test is continued until a stable pH of 3.5 is reached.  In fact for some of these samples this would take way more than the 3 hours that we have available, so we either have to be happy with an unstable final pH (in which case we’ll estimate the amount of acid that would have to be added), or someone has to be prepared to come in tomorrow to complete the titration process.

For a 10 g sample, the acid consumption is; given by:

Acid consumption = ml 1.0 N H₂SO₄ x 4.9 kg/tonne

Interpretation

If the acid consumption value (in kg of acid per tonne of sample) exceeds the acid-producing potential (kg per tonne), the sample will not be a source of acid mine drainage and no additional work is necessary. If the acid consumption is less than the acid production potential or the difference is marginal, the possibility of acid mine water production exists. In this case, further testing is needed to confirm the result.

This test procedure is adapted from the BC Research “initial test” method for acid rock drainage potential described at:

http://technology.infomine.com/enviromine/ard/Acid-Base%20Accounting/ABAdiscussion.htm)