Aspectos básicos del análisis granulométrico

In the Aggregate and Mining Industries, high volumes of rock and ore are produced and processed each day.  It is critical to know the exact sizes of the rock in each product or process stage for the following reasons:

• The salable products typically need to meet State or Federal Specification requirements, and
• Process specifications help optimize the efficiency of commodity recovery and reduce downtime in the plant. 

However, counting and measuring each stone or particle by hand is not a scalable solution for the size of these operations, so these industries depend on using Sieve, or Gradation, Analyses to determine the average material sizes of the overall product by periodically sampling the larger process stream.

Terms and Standards Used in Sieve Analysis

Before getting started, it is important to define some of the key terms and standards used when conducting a sieve analysis.  These terms are not used in common vernacular and are critical to the steps and processes that follow.

Key Terms

• Gradation: The particle size distribution of an aggregate.
• Sieve: A device used for separating the sample by particle size.
• Fraction: The portion of the sample retained on each sieve.
• Shaker: An instrument to mechanically separate particles by size.
• Percent Retained: The percentage of material retained above each sieve size.
• Percent Passing: Percentage of total material that passed through each sieve.

Identifying industry standards

ASTM International, previously American Society for Testing and Materials, is an international organization that produces technical standards.  Using Industry Standards ensures that the test will be conducted uniformly between plants, which yields a level baseline of products between each unique site conducting the test. Below are two standards that may be used as resources for sample reduction and conducting Sieve Analyses, which include guidelines and tips on each process.

• ASTM C136: Standard test method for Sieve Analysis of fine and coarse aggregates
• ASTM C702: Standard practice for reducing samples of aggregate to testing size

Preparing for the Test

Step 1:

The first step in conducting a successful Sieve Analysis is starting with a well collected, dry sample.

If the sample is wet, the material will not flow freely or separate properly. Thus, it is important to dry the sample in an oven with a temperature ≤110°C. Dry times vary by material type, sample size, moisture content, and oven efficiency, so check the sample intermittently until it is dry and free flowing.

Step 2:

After a sample has been dried, a preliminary weight is needed. With the pan in place, tare the scale; if the scale does not have a tare option, the pan will need to be weighed separately to be subtracted from the sample weight.

Using a splitter, separate the sample to the appropriate size.  If a splitter is not available, use the cone method to split the sample: Pour the sample into a conical shape and divide the sample into four quarters. Once sectioned, add the sample to the tarred pan to collect a preliminary total material weight.

After the preliminary weight is recorded, the entire sample will be transferred to the top tier of a sieve shaker.

Step 3:

Close the sieve stack and assemble the stack onto the shaker, if applicable, making sure that it is centered on the base and completely nested under the top cover before tightening. Lock/tighten the stack in place. Power on the shaker and set the time.

Conducting the Test: Cumulative Method

The most common method of conducting a sieve analysis is the cumulative method.

Step 1:

Start by removing the top tray and transferring the fraction into the tared pan on the scale. Record the weight of the fraction.

Leaving the material in the weigh pan, repeat the process by removing each subsequent tray and recording the total weight after adding each additional fraction to the weigh pan.

Once each fraction is added and the weights are recorded, compare the final total weight to the preliminary total weight; check for a difference less than or equal to 0.3%.

If the difference is greater than 0.3%, the test will need to be repeated with a new sample.

Step 2:

Calculate the cumulative percent retained for each line by dividing each cumulative gradational weight, as recorded from the test, by the total weight of the sample, and multiplying by 100%.

As an example, 12.6 grams of mass retained would be divided by the total weight of 309 grams to yield a calculation of 4.0% retained.

Step 3:

To calculate the cumulative percent passing, subtract the percent retained values for each line from 100%.

Continuing with the previous example, subtracting 4.0% retained from 100% yields a 96% passing product.

Conducting the Test: Fractional Method

Unlike the cumulative method, the fractional method for conducting a sieve analysis will measure and record each fraction individually. This occurs by emptying the tare pan between each weight. Other than this major difference, the fractional method is very similar to the cumulative method.

Step 1:

Weigh the material in each sieve individually and record the mass; tare the scale before each fraction weight.

Step 2:

Calculate the fractional percent retained for each line by dividing each fractional gradational weight, as recorded from the test, by the total weight of the sample, and multiplying by 100%.

Step 3:

Calculate the percent passing by subtracting the fractional percent retained, along with the fractional percentages retained for all sieves larger than the one under calculation, from 100%

As an example, with a 21.3% retained calculation, 21.3% needs to first be subtracted from 100%. Then, also subtract the fractional percentages that were retained above it. In the example above, that includes 27%, 25.8%, and 4%, which indicate a 21.9% passing.

Helpful Tips for Conducting a Sieve Analysis

When conducting a Sieve Analysis, unexpected events can occur during the process that may need to be addressed and/or accounted for. Download our list of recommendations and mitigation techniques which can aid in successful completion of the analysis.

This blog was adapted from a 2021 educational webinar with Polydeck experts, Craig Burke, director of Engineering, and Jamie Mills, New Product Development Application Engineer. To learn more about upcoming educational opportunities, subscribe to our newsletter.