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Nov 18, 2021

Six Tips to Make Screening Aggregates More Efficient

Screening can be a bottleneck in aggregate production. Cases of quarries operating with high rates of return and contamination of materials in their screens are not uncommon, as are stoppages in times of higher humidity and rain. These challenges, can be met with relatively simple actions. We have prepared the guide below to help you through the process.
Metso Outotec screen
Many bet only on changing the screening media, when the bottleneck is the screen itself.

(TIP 1) First you need to measure

An ideal screen would theoretically work with an efficiency index in the 90% range. This means that the rate of return, that is, the amount of material that recirculates in the process, is only 10%. To measure efficiency, therefore, we need to assess the percentage of incorrectly sized particles: materials that are outside the particle size range established by managers for production. The measurement of screening efficiency is done by recording the amount of material that feeds the screens and the amount of materials retained in each deck. The procedure for analysis is simple: just collect 1m of material on the retention belt and weigh, in the laboratory, pass the material through a screen with the same opening as the screen on the deck, weigh the passing material and divide by the total weight of the sample, you will have the percentage of incorrectly sized particles present in the sample.

(TIP 2) Analyze the feed of the screens

When the percentage of incorrectly sized particles in the product is very high, it may be the case to look at the previous steps, including feeding. Changing the screening media of the first deck, for example, can relieve the second deck and improve screening performance. Direct feed to the screen is a common mistake in many quarries and efficient operations always have a feed box to even out the feed. Poor distribution also affects efficiency, as does a very fast feed flow, which does not allow stabilization of the material in the screening area. Amplitude is another point of attention: if the material is jumping a lot or being thrown out of the screen, for example, the amplitude will be high. A good feed allows for faster passage of fine materials right at the beginning of the deck, which should make the screens on the second deck, closer to the feed, receive more material and wear out first. If the wear on the second deck is concentrated more in the center of the screen, the feed process may be to blame.

Screening media image
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(TIP 3) Yes, it could be the equipment

Although most optimization challenges come from tweaking the screen media, it is possible that the improvement will only happen by replacing the screens. And there are many example of this, from homemade equipment to undersized screens. This is the case of the processing plant that has a modern high-capacity crusher, but that needs to operate below its possibilities because the screens cannot process the materials. Many managers bet on changing screening media to improve production, when the bottleneck is the equipment itself. Assess the total amount of material fed by the amount of through material present on the return belt: if the percentage of incorrectly sized particles is too high, contact a specialist to assess whether the screen size is adequate.

(TIP 4) Type of screens influences performance

There are several types of screening media on the market, from metallic wire to those made of flexible and highly resistant special rubber. Likewise, there are different systems for fastening. Current technologies include modular rubber media, which have a longer wear life, as well as easier change-out, which takes less than 30 minutes and can be performed by a single maintenance professional. Wire meshes – due to their weight – require more people, as well as equipment – ​​they are heavy – which also implies special attention to process safety. In general, rubber media tend to last at least 4 times longer than wire mesh. Therefore, consider 6 hours of time spent on changing the wire mesh, which means 24 hours of machine downtime, 72 hours of labor if 3 people are involved in the exchange, and 24 hours less in production. For a screen that produces 200t/h this means 4,800 tonnes less production.

(TIP 5) The influence of moisture can be minimized

Screening becomes critical in some humidity ranges and the wet climates intensify the problem. Conversely, the drier the material, the easier its screening. It is very common for fine material to stick to the larger particles, but there are technical solutions available that minimize the effects of moisture on the production of aggregates, including the use of screens with high resilience screening media and anti-blinding rods that help in the process. Anti-blinding rods should be installed close to screens to intensify mesh movement and prevent particles from clogging holes. Flexible and modular rubber screening media allow for less maintenance time, in addition to increasing production time, avoiding unnecessary downtime. By examining the material in the product stockpile, it is possible to check the amount of fines that remain stuck to the end product. For a more accurate analysis, sample collection should be done directly on the belt.

(TIP 6) Evaluate the payback of new solutions

Put your suppliers into action and use their experience to assess the adoption of new components such as screening media, fixtures and fastening systems. You can also calculate the cost of the screening media, for example, considering parameters such as: component durability (how many hours is their average life), number of changes required, time spent in operation and number of people involved in maintenance, production rate hours, number of days worked per year. With the data it is possible to determine how many hours per year you did not produce. You can then evaluate and see technical options that reduce this no production time and can deliver a short-term payback on any investment.

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