Operating Challenges in a Stone Crushing Plant

This post explores the various operating challenges that arise in a stone crushing plant. It is advisable to read my previous post ‘Anatomy of a stone crushing plant’ before proceeding, as it explains the different material flow, the machines involved and the working principles of these machines. I used the Vrish Industries plant at Biwan, Haryana, India as a case study to illustrate these concepts. In this post, I will share some of the operating challenges that we encountered while running the plant, and suggest some possible solutions to overcome them.

Power Consumption

One of the operating challenges in a stone-crushing plant is the high power consumption of the machines. The machines require very high-power motors that have very high power draw. As a result, power cost accounts for a large portion of the production cost.

Quality Estimate

Another operating challenge is the difficulty of estimating the quality of stone aggregate in real time. The quality of stone aggregate is measured in cubicity (sphericity), which is a measure of the percentage of the stone aggregate samples having a cubic or sphere shape. Having a high cubicity rating has many advantages in usage, which were discussed earlier. However, it is extremely hard to measure the real-time cubicity of the product, which is generally done by sending a sample to a testing lab and waiting for about 48 hours for the results.

Oversize boulders in Jaw Crusher

A common operating challenge is the occurrence of boulders that are larger than the opening of the jaw crusher. As the raw material is dumped into the grizzly feeder using trucks, some of the raw boulders may be too large for the jaw crusher to crush. In this situation, the jaw crusher is unable to crush the boulder and gets stuck at the top of the crushing chamber. This also prevents the rest of the material from being fed into the jaw crusher. In this case, the operator has no other choice but to halt production and either drill the large boulder into smaller boulders or make a small controlled dynamite blast that breaks the boulder. This also means that an operator needs to monitor the jaw crusher at all times.

Wear Parts

All the machines in the plant have a few wear parts that need to be replaced regularly, which are listed below:

1. Cone Crusher:

2. Jaw Crusher and Granulators:

3. Vibrating Screens:

4. Conveyor Belts:

If the parts are not replaced within the correct time frame, we will experience losses in other processes downstream. For example, if the cone mantle and liner are not changed at the correct time, we will experience a close side setting that is too large even when the main shaft is at the topmost position. This is because the manganese alloy on the mantle has worn so much that even at the highest point of the main shaft, the mantle will not be close enough to the liner. This will cause the output feed to be larger than desirable and more material will recirculate, causing the machines to use more power than needed.

Bushing and Bearing

The cone crusher bushing and the bearing in the jaw crusher and granulator should last a long period, but in practice, they need to be replaced regularly because we are unable to constantly monitor the expected life of these parts and what actions to take to prolong the life of these parts. These parts are also extremely expensive and cause production delays. A major problem that plant managers face is the inability to predict the expected life of a part and thus only get to know when the part has already failed.

Manual Controls for Feeders

The cone crusher and the jaw crusher both have an optimum amount of material in the crushing chamber that aids the best throughput and cubicity. The amount of material is regulated by the grizzly feeder for the jaw crusher and the vibro-feeder for the cone crusher. This is done by an employee who constantly monitors the crusher and makes adjustments accordingly. For example, if the cone crusher does not have enough material in it to run ‘choke fed’, then the employee would increase the speed of the vibro feeder, which will feed more material into the crushing chamber of the cone crusher. It is incredibly important for the cone crusher to run ‘choke fed’ since that encourages rock-on-rock crushing, and as explained earlier, that improves the cubicity of the aggregate.

Throughput Estimate (tons per hour)

The throughput estimate of the plant is an extremely important measurement to ascertain the efficiency of the plant. The technology to measure live throughput is prohibitively expensive, so throughput estimate is done through eye measure.

Motor Wear

Another operating challenge is the high wear and tear of the electric motors that power the machines in the plant. The motors have to handle varying loads depending on the feed, which causes more stress and friction on the motors, especially in the crushers. The motors have to be checked and serviced regularly to avoid breakdowns or malfunctions.

Gear Wear

All the machines have complex assemblies of gears that transmit the power and motion of the motors to the machines. The gears are also subject to wear and tear due to the high torque and speed of the machines. The gears have to be repaired or replaced regularly as and when they break down or the wear affects the performance of the machine.

Excessive Dust during Production

During production, when all the machines are running simultaneously, a large amount of dust is released into the air. This does not affect the quality of the product, but it creates a challenging and unhealthy working environment. The dust can cause respiratory and eye problems for the workers and also pollute the surrounding area. The plant has to install dust suppression systems that spray water or chemicals to reduce dust emissions. The dust suppression systems also require a constant supply of water and regular maintenance.

Motivation for Smarter Systems

The process of stone aggregate manufacturing is a highly manual process that requires constant monitoring and human intervention. The machines have to be adjusted and controlled by skilled and experienced operators who have intrinsic domain knowledge. There are also situations where oversized boulders in the jaw crusher and material overflow in the cone crusher need immediate attention from the operators.

Possible Solutions

There are existing solutions for these problems in other industries, but not in the stone aggregate industry. This may be because of the low inherent value of the stone aggregate itself. However, finding AI solutions for these problems will not only benefit the crushing industry but also other industries where heavy machinery is used.

Some of the challenges may not need deep learning or machine learning methods to find a solution. These are listed below:

Cone Crusher Choke Feed System

As discussed earlier, the cone crusher needs to run choke-fed to encourage rock-on-rock crushing for optimal quality. The feed into the crushing chamber is regulated by the vibro feeder from the stockpile. If the material in the crushing chamber is insufficient for the cone crusher, the vibro feeder’s motor is turned up in speed. Similarly, if the material is overflowing, then the motor is turned down. This is done manually by an operator and needs constant monitoring.

A possible solution for this problem is the use of an automated control system for the speed of the variable frequency drive (VFD) of the vibro feeder. This would require a sensor that measures the level of the rock in the crushing cavity of the cone crusher. There would be a fixed threshold that will represent the point at which the cone crusher is considered to be choke-fed. This would ideally be somewhere at the midpoint of the feed hopper. At the start of production, the VFD will be at full speed until the level measured by the sensor reaches the threshold point. Once the threshold is reached, the speed of the VFD will be adjusted in such a way that the feed level stays at or just below the threshold level.

Here is an example of choke feeding in cone crushers:

In this video, you can see that the feed manages to ‘choke’ the cone crusher, and the feed stays at a constant level. This needs to be constantly monitored by an operator. Our solution will have a sensor that will measure the level of the feed in the crushing chamber and using a PID controller we will control the speed of the vibro feeder in such a way that the cone crusher runs choke fed. The sensor will need to be a non-contact laser sensor that will need to be installed at the top of the cone crusher.

Machine Learning and Deep Learning Applications

The stone aggregate crushing industry has a huge potential for applying machine learning and deep learning techniques to automate and optimize the processes in industrial plants. However, a common challenge in implementing machine learning and deep learning in real life is that real life is much more complex and unpredictable than the curated datasets that are publicly available. This is especially true for the application of these techniques in manufacturing plants, where the working conditions vary constantly. This, along with the lack of publicly available datasets for this specific use case, means that a large effort in data collection is required. We plan to install several sensors to obtain a data stream of the plant during production.

Conclusion

In this post, we reviewed the various operating challenges that we face during the operation of a stone-crushing plant. We discussed some possible solutions to some of the challenges and the machine learning and deep learning techniques that will be used here. In the next post, we will shed light on the sensors needed to be installed for collecting the data to train these models and also to execute autonomous control using pre-trained models.

Anatomy of a Stone-Crushing Plant

In this post, we’ll be exploring the inner workings of a stone aggregate crushing plant, delving into the production process and the various kinds of machinery involved. Our focus will be on the Vrish Industries plant situated in Biwan, Haryana, India, where I have the honor of leading operations. This is an opportunity to gain a deeper understanding of the complexities of stone aggregate production.

Introduction

Stone aggregate, a versatile material used in everything from concrete and asphalt to railway ballast and construction fill, originates from large rock boulders that have been crushed by various types of stone crushers. These crushers break down the rock into a range of sizes, which are then sorted using vibrating screens.

The process of reducing the size of the stone boulders is typically carried out in multiple stages to achieve maximum cubicity. Cubicity is a measure of how much of the material can be considered cubic in shape. This characteristic is highly sought after as cubic particles contribute to better compaction, resistance to deformation, and workability.

In this post, we’ll delve into the fascinating process of stone aggregate production, shedding light on the complex mechanics behind it.

How does a stone aggregate crushing plant work anyway?

A crushing plant is a facility that transforms large rocks into smaller pieces for construction purposes. It consists of several stages, each with a specific function and equipment. The equipment has a reduction ratio, which measures how much the rock size is reduced after crushing. The optimal reduction ratio depends on the final product size and quality.

The stages of a crushing plant are:

• Primary crushing: The biggest rocks are fed into a jaw crusher or a gyratory crusher, which can crush them to about 15 cm across.

• Secondary crushing: The primary crushed rocks are further broken down by a cone crusher or an impact crusher, which can make them about 4 cm across. The machine used in this step is usually a cone crusher.

• Tertiary crushing: The secondary crushed rocks are processed by a vertical shaft impactor, sand-making machine, or another cone crusher with different settings, which can create fine sand or gravel of about 1 cm or less across.

• Segregation: The crushed rocks are sorted by size using a vibrating screen, which has different mesh sizes. The screen separates the rocks into different grades of stone aggregate, such as coarse, medium, or fine, that are fit for different uses.

• Transportation: The stone aggregate is moved from one step to another using conveyor belts, which are strong and can carry heavy loads. Sometimes, the plant also needs to store the rocks in stockpiles, for example, if there are maintenance issues or weather conditions that affect the working of some machines.

• Loading: The stone aggregate is loaded onto trucks using loading conveyor belts or wheeled loaders. The trucks then transport the material to the construction site or to a storage facility.

Plant at Vrish Industries

This illustration provides a step-by-step overview of a crushing plant:

1. Ramp: The starting point for the feed.

2. Grizzly Feeder: The feed is screened and regulated.

3. Jaw Crusher: The feed is crushed, reducing its size.

4. Vibrating Screen 1: The crushed material is segregated.

5. GSB out: The output at this stage is GSB (Granular Sub Base).

6. Twin Granulators: The feed is processed for further size reduction.

7. Stockpile: The material is stored temporarily.

8. Cone Crusher: The stockpiled material is crushed again.

9. Vibrating Screen 2: The crushed material is segregated for the final time.

10. Output 10-20 mm: The first output size category.

11. Output 2-10 mm: The second output size category.

12. Output 0-2 mm: The third output size category.

13. Loading Bay: The final destination for the different sizes of stone aggregate, ready for use in construction.

Crushing machines

Primary stage- Jaw crusher

Jaw crushers are usually used as the primary crushing machine in stone aggregate production plants. They have 2 jaws, one which is fixed and one movable. The feed is crushed between the 2 jaws when the movable jaw moves back and forth. When the movable jaw retreats from the closed position, the crushed material is discharged and the output feed size is determined by the difference between the open side setting and the closed side setting of the jaw crusher.

Secondary stage: Twin Granulators

Granulators are devices that reduce the size of stone materials by compressing them between two rotating cylinders. They are similar to Jaw Crushers, but they have a smaller maximum feed size and a lower reduction ratio.

To optimize the performance of the granulators, we only feed them with stone materials that are larger than 60mm in diameter. The smaller materials are bypassed and sent directly to the final stage. This way, we avoid overloading the granulators with unnecessary feed and we maintain a consistent output size. This also improves the throughput of the plant, as we only process the materials that need further reduction.

Tertiary stage - Cone Crusher

Our plant relies on the cone crusher as the final machine in the crushing sequence, which determines the shape, quality, and size of the stone aggregate. Some advanced plants use a Vertical Shaft Impactor as the tertiary stage, which produces higher-quality stone aggregate because it has a different crushing mechanism. However, these machines are also much more costly. We managed to achieve similar quality with just a cone crusher as the tertiary stage, thanks to our smart design. We will explain how we did it later.

The cone crusher receives material no larger than 80mm through the opening. The material drops and gets crushed between the mantle and the liner. These are the bright red parts of the machine in the illustration. A powerful electric motor drives the crown gear, which moves the eccentric bushing. The main shaft connects to the eccentric bushing and acts as a bearing. It also supports the gyratory movement of the main shaft and distributes the crushing force evenly across the crushing chamber.

We can adjust the CSS and OSS by changing the height of the cone mantle, which affects the distance between the cone mantle and the cone liner and thus the size of the output feed.

The illustration shows how the eccentric motion of the eccentric bush causes a gyratory motion of the main shaft and consequently the cone mantle. This squeezes the rock between the cone mantle and the cone liner.

Optimal Operating Conditions for a Cone Crusher

Operating a cone crusher at its optimal conditions is crucial for efficient functioning. Here are some key factors to consider:

• Choke Feeding: The crusher should be fully covered with feed material. This prevents erratic power and pressure profiles, excessive wear, and high peak loads. A higher feed rate results in a smaller product size and higher throughput.

• Rock-on-Rock Crushing: To achieve less flaky output, the crushing chamber should be filled with material. This promotes rock-on-rock crushing over rock and liner/mantel contact.

• Oil Temperature: The oil temperature should be within the operating range, typically between 45-55 degrees. This can vary due to weather conditions.

• CSS Setting: The CSS (Closed Side Setting) determines the output size. Material larger than 20 mm(in our case) is recirculated back inside the cone crusher. While recirculation results in more cubic material, it also increases power consumption. Therefore, an optimal CSS needs to be determined.

• Wear and Tear: The wear rate of the grade 7 manganese cone mantel and cone liner varies with different rock types. As these parts wear, the CSS needs to be adjusted to compensate for the wear.

• Power Consumption: Power consumption is a good indicator of the health of the cone crusher. Excessive power draw for the amount of material in the crushing chamber may indicate a fault in the internal parts of the crusher.

By keeping these factors in mind, one can ensure the efficient operation of a cone crusher. Remember, optimal conditions lead to optimal output!

Vibrating screens

Screening is the process of separating a mixture of particles into two or more fractions based on their size. One of the most common methods of screening is using vibrating screens, which are devices that consist of a screen mesh or a sieve surface, a vibrator, and a vibration-damping device. The material to be screened is fed onto the screen, and the vibrating motion of the screen causes the material to separate into different sizes. The smaller particles fall through the openings in the screen mesh and are discharged, while the larger particles remain on the screen and are discharged from the edge of the screen.

The size of the openings in the screen mesh is carefully selected according to the desired size of the products.

For example, if we want to produce aggregate sizes of 20-10 mm, 10-2 mm, and 2-0 mm, we need to use screen meshes with openings of 10 mm, 2 mm, and 0 mm, respectively. Anything above the size of 20 mm is recirculated back to the crusher for further crushing. The screen mesh with openings of 10 mm will receive the material that is below 20 mm, and will separate it into two fractions: 20-10 mm and below 10 mm. The fraction below 10 mm will pass through the screen mesh and will be discharged, while the fraction between 20-10 mm will remain on the screen and will be discharged from the edge of the screen. A similar process will segregate the 10-2 mm aggregate using the screen mesh with openings of 2 mm.

In our case, we have two locations where we use vibrating screens, as shown in the figure. They are located at 4 and 9. The first screen is used to feed the twin granulators the correct feed size, so this screen has only two outputs: above 60 mm and below 60 mm. The material that is already below 60 mm does not need to be crushed in the secondary stage, so we pass it straight to the stockpile. The material above 60 mm is crushed again by the twin granulators with the closed side setting (CSS) set so that the output size is 60 mm.

Feeders

The feed into the jaw and the cone crusher needs to be regulated in a way so that the optimum amount of material is crushed at any given point during production. Below we describe the different types of feeders used in the plant.

Grizzly feeder

The grizzly feeder has two objectives:

• To regulate the feed of large boulders into the jaw crusher

• To segregate boulders and GSB(granular sub-base), and send the GSB to the appropriate output conveyor

The working principle is similar to a screen but without the multi-layer mesh design. For the segregation of GSB, ‘teeth’ with wide openings are present at the end of the feeder.

Vibro feeder

The vibro-feeder takes the material from the stockpile and feeds it to a conveyor going into the cone crusher. The working principle is similar to vibrating screens grizzly feeders but they don’t segregate at material.

Both of these feeders make use of a variable frequency drive to regulate the amount of material fed into the crushers.

Conveyors

Heavy-duty conveyor belts are used to transport material from one machine to the other. They have different size and motor ratings depending on the amount and type of feed that is being transported on the conveyor.

Conclusion

In this post, we introduced stone aggregate crushing plants, how they work, what are the different machines used, and the different design considerations. We had a look at the Vrish Industries plant at Biwan, Haryana as an example to understand these concepts.