Solving Material Segregation in Batch Mixing Processes

material segregation in batch mixing processes

When solid materials mix, a degree of material segregation is inevitable. A variety of processes are used to achieve a predetermined level of uniformity in batch mixing processes, however other processes downstream can cause the materials to separate again. With proper design considerations and awareness of the material segregation physics at work, processes and products can be tested for separation, and product defects can be avoided.

Material Segregation Problems In Batch Mixing Processes

Sifting

How It Works

Scientifically known as granular convection and known in practice as the Brazil nut problem, sifting is one of the most common material segregation problems in batch mixing processes. Sifting occurs mainly through the relationship between the particles’ size and mass; particles significantly larger and more massive (like Brazil nuts) rise to the top of the mixture, and smaller, less massive particles (like cashews) fall between the spaces towards the bottom. It can occur without movement, but movement worsens the problem, since vibration or shaking causes smaller particles to relocate faster into empty spaces. The degree with which this material segregation problem occurs depends on the variation between the particles’ size and mass, and the amount of each.

What You’ll See

Sifting is a material segregation problem impacting many processes. A vibrating conveyor belt can cause sifting, as well as some stirring processes. Even shaking a completed package (like a jar of mixed nuts) can cause this separation.

Angle of Repose

How It Works

The angle of repose material segregation problem in batch mixing processes works similarly to sifting, but operates on a different principle. Instead of sinking to the bottom, smaller, finer particles form a hill when they are poured or dispensed. When thicker, coarser particles reach the hill, they tumble down towards the edges. The materials’ differing angle of repose—the angle at which it will be stable and not tumble down—causes this separation.

What You’ll See

This material segregation problem in batch mixing can be especially difficult. In silos and hoppers it’s often the cause of flow problems like ratholing and bridging. If the coarser particles stick to the sides of the hopper they can get rancid and contaminate the next batch. Since air flows through the coarse and fine materials at different levels when they are dispensed, it can create a pressure differential that can damage a holding unit, such as a silo. Most commonly in batch mixing, the material segregation causes different concentrations of ingredients when the mixture is dispensed from an improperly designed hopper.

Fluidization

How It Works

Fluidization occurs when a mixture is suspended in a gas or liquid. In batch mixing processes, this material segregation problem most commonly occurs in plain old air. When a mixture is aerated (which may occur simply through free falling), the finer, less dense particles retain air and move towards the top of the mixture, while the larger, denser particles which didn’t absorb air sink.

What You’ll See

This material segregation problem in batch mixing commonly occurs in powder ingredients. If the powder does not bind sufficiently to another material, it will separate through fluidization if aerated or allowed to free-fall. With all the powder at the top, the uniformity and product quality can be compromised. Fluidization, combined with the previous two material segregation problems, also poses workplace safety risks from powder explosions and respiratory hazards as large amounts of powders separate into the air.

Trajectory Segregation

How It Works

Unlike the previous three material segregation problems, trajectory segregation occurs through horizontal movement. This occurs by two different principles. In a fluid mixture, trajectory segregation occurs through a relationship between particle size, density, viscosity, and velocity. Particles with the same viscosity and velocity, but different size or density will travel at different rates. This causes the mixture to seperate.

In a solid mixture, trajectory segregation occurs through friction. Finer materials with more surface area and therefore more friction move slower and will deposit closer to the end of the horizontal path. Larger materials with less surface area and less friction move faster and deposit further.

What You’ll See

This material segregation problem in batch mixing processes commonly occurs in ribbon blenders and conveyors or chutes. In ribbon blenders, the particles in a fluid suspension separate in the blender due to their differing size or density. In chutes and conveyors, friction causes the materials to move at different rates, creating a pile of fine materials near the end of the chute and coarse materials further away.

Material Segregation Solutions In Batch Mixing Processes

Ingredient Testing

In order to solve material segregation problems in batch mixing processes, the materials and processes must be well understood. Knowing the particles’ density, size, mass and other properties can help you predict how the materials will segregate. While designing process automation equipment, ask your manufacturer if they will conduct ingredient testing for angle of repose, sifting capacity, and other issues. With this information, your manufacturer can design equipment to prevent material segregation problems in batch mixing processes.

Material Segregation Testing

When testing material segregation in existing batch mixing processes, make sure to test accurately. Remember that material segregation in the end product will also affect any tests on the end product. Use a sample thief or a riffler to get samples that accurately represent the whole, and see where and to what extent problems exist in the process chain. Remember to check the coefficient of variation at different points of the process, not just at the mixer, to see if your downstream process is causing segregation.

Feed Bin Design

Angle of repose problems are most commonly caused by an improperly designed feed bin and hopper. Using a mass flow hopper designed according to the materials’ angle of repose will prevent the mix from dispensing unevenly. In general, an angle of at least 70 degrees is recommended. Hopper inserts or low-traction coatings can also be used.

Mixing And Blending

Mixing and blending processes should be carefully selected and placed. The wrong mixer or blender can actually cause materials to separate through trajectory segregation. If the mixer is placed too early in the process, the materials may simply resegregate downstream. Placing a mixer immediately before dispensing can mitigate segregation effects.

Coating

Many material segregation problems only occur in free-flowing mixtures. Adding a binder can stop problems like sifting, though other problems like sticking and clogging should also be considered.

Agitation

Aeration or vibration can remix some materials, particularly if they are separated in a silo or hopper. With both of these methods, be careful not to introduce sifting or fluidization.

New Materials

More drastic differences between materials cause more drastic material segregation problems in batch mixing processes. If your materials are especially coarse, or a large variation between particle size and density exists, talk with your supplier about material quality. Or, consider additional processing to a diverse material; would adding another process for more material uniformity prevent material segregation problems? Would it be cost-effective?


Material segregation may occur in areas that operators never see. This means you might only see the results in low product quality or contamination. You might also experience repeated processing problems and machine errors due to clogging, flooding, low flow and other issues. If you’ve noticed these issues, assess your batch mixing processes and see where material segregation problems may occur. Once you identify the problem, a simple fix may increase product quality significantly.

5 Powder Flow Control Problems And Solutions

powder flow control problems and solutions

Powder flow problems cause frustration and hours of expensive downtime. They can also damage machines, create backups, and produce sub-par products. Some types of materials, machines, and working conditions make powder flow problems more likely. We’ve identified the most common powder flow control problems and flow control solutions to help you solve these troublesome inefficiencies.

5 Most Common Powder Flow Problems and Flow Control Solutions

1. Problem: No Flow

Under normal operating conditions, the material should flow through the system without interruption. If no-flow alerts are a regular occurrence, the system is not optimally designed for either the material or the environment. This may occur in environments with high humidity, materials with high moisture content, solid materials that are irregularly shaped, or materials with certain coatings.

Solution: Agitation

Depending on the cause of the no-flow problem, a few solutions are available.

  • A mechanical agitator before feeder entry
  • Vibrator added to hopper
  • Air pads to aerate product

Each of these are long-term solutions that will ultimately save your time and money by eliminating downtime. When making these upgrades, make sure to conduct proper testing. Consider carefully where and how to mount the devices, and how often they should operate to be most effective.

2. Problem: Low Flow

This powder flow problem may go unnoticed for long periods since it doesn’t directly cause downtime. However, insufficient flow can affect all downstream systems. Low flow maybe be caused by obstructions above the feeder, or misalignments. This may also occur if the materials are too thick or the feeder is too small.

Solution: Bigger or faster feeder

The ideal flow control solutions for this problem will either expand the feeder to increase volume at slower speeds, or speed up the feeder to push more material through faster.

  • Upgrade to larger feeder
  • Add variable frequency drive
  • Change reducer on drive

3. Problem: Decreasing Flow

Some powder flow problems do not cause a sudden stop, but rather a slow reduction in material flow. Unlike other powder flow problems which are caused by materials sticking together, this is generally caused by materials sticking to the feeder because of static build-up.

Solution: Eliminate static

This is particularly common problem in fast-moving, dry materials, but flow control solutions to this problem are generally easy to implement.

  • Ground the feeder frame to prevent static build-up
  • Use electro-polish on feeder
  • Add Teflon coating to feeder

4. Problem: Material flooding

If too much material is getting through or the material floods after shut-off, this can also cause production problems downstream, or result in inconsistent products. These flow control solutions create the opposite effect of the previous three, but they are implemented in similar ways.

Solution: Slower, interrupted feed

Upgrading the hopper or attached systems can stop flushing and flooding.

  • Vent hopper to reduce aeration
  • Install slide gate or butterfly valve at discharge point
  • Smaller feeder
  • Lower drive speed
  • Incline the feeder

5. Problem: Damage to feeder

If your system takes more damage and needs more repairs than comparable equipment, the materials or the system may be to blame. This may be a bulk solids or powder flow problem, and it can be caused by too much abrasion or improper system construction.

Solution: Slower speeds, stronger system

This flow control solution can be implemented by either slowing down the product or reinforcing the system.

  • Lower drive speed
  • Install larger feeder to slow materials
  • Add liner or coating to system

If you’ve inherited a system that constantly sees problems, or your materials have changed and it’s created new issues, consider these solutions. If you’re building a new system, take advantage of testing and proper construction beforehand and these powder flow problems will never occur. With the right material testing before installation, you can be sure that your system is made for your materials before it arrives.

4 New Food and Workplace Safety Regulations For Grain and Pet Food Manufacturers in 2018

workplace safety regulations for grain and pet food 2018Recent changes in food, environmental, and worker safety have caused many grain and pet food engineers to take another look at their manufacturing facilities. Some of these rules are newly changing, others are just recently subject to penalties for noncompliance and all deserve adequate attention. For greenfield sites or renovations, appropriate attention to food and workplace safety regulations from the outset will save thousands in fines down the road. No matter the situation for your grain or pet food facility, we’ve put together four key safety regulations for manufacturers to know in 2018.

4 New Food and Workplace Safety Regulations For Grain and Pet Food Manufacturing

1. Amended OSHA Slip and Fall Regulations

Violations exposing workers to fall or slip accidents occupied three of the top ten most common violations of workplace safety regulations in 2016, including non-compliant ladders, scaffolding, and fall protection equipment. Seeking to remedy these preventable accidents, OSHA amended Regulation 29 CFR 1910 Subpart D, which regulates safety requirements for walking and working surfaces, in 2016. Many of these provisions are already in effect, and several others are required as of November, 2018.

Grain and pet food manufacturers who are building up instead of out, as well as grain elevators and feed storage facilities should pay particular attention to these workplace safety regulations, some which are already enforced with fines.

  • Ensuring exposed workers are trained on fall hazards (required May 17, 2017)
  • Ensuring workers who use equipment are trained (required May 17, 2017)
  • Inspecting and certifying permanent anchorages for rope descent systems (required November 20, 2017)
  • Installing personal fall arrest or ladder safety systems on new fixed ladders over 24 feet and on replacement ladders/ladder sections (required November 19, 2018)
  • Ensuring existing fixed ladders over 24 feet … are equipped with a cage, well, personal fall arrest system, or ladder safety system (required November 19, 2018)

2. NFPA 652: New Standards on Combustible Dust

The dangers of combustible dust are no mystery to pet food and grain manufacturers. Between 2007 and 2016, 91 explosions occurred due to grain dust alone. In 2016 the National Fire Protection Association’s (NFPA) Standard 652 outlined best practices for evaluating risk and protecting against dust and powder fires. The new standard’s biggest departure from previous standards is the development of Dust Hazards Analysis on existing or future processes.

Though OSHA recently abandoned expanded regulations on combustible dust due to regulatory reform under the Trump administration, experts remind businesses that workplace safety regulations on dust hazards exist under a number of other OSHA standards. Project managers and engineers can manage their combustible dust risk in a number of ways;

  • Proper machine maintenance to eliminate dust leaks.
  • Separating feed mixing processes into different buildings to manage risk and loss.
  • Conducting thorough risk analysis to understand threats.
  • Utilizing temperature monitoring sensors to prevent sparks.
  • Installing dust monitoring and dust collection systems suitable for your facility and particulates.

3. OSHA New Respirable Dust and Crystalline Silica Standards

Combustible dust laws are not the only dust-related workplace safety regulation grain and pet food manufacturers should be aware of this year. OSHA regulations governing respirable dust particles require workers to use personal protection equipment (PPE) and requires facility managers to measure, manage and keep levels within Permissible Exposure Limits (PELs). In 2017, OSHA amended regulations on exposure to crystalline silica, one of the most common types of harmful respirable dust. When crystalline silica dust is inhaled it sticks to the lungs, causing scarring and irreparable damage.

Though these workplace safety regulations are most important in the construction industry where workers are regularly exposed to hazardous crystalline silica levels, some steps in raw material feed processing, particularly cleaning, can pose respirable dust hazards. To mitigate exposure, take the following precautions;

  • Know who is exposed, where, and what causes exposure.
  • Measure and monitor harmful or nuisance dust levels.
  • Make PPE available and cultivate a culture of safety compliance.
  • Utilize dust collection with the right air intake and appropriate filtering.

4. USDA and FDA: Food Safety and Modernization Act

The USDA and FDA jointly oversee provisions within the Food Safety and Modernization Act (FSMA), including those regulating food and pet food. Signed into law in 2011, many FSMA regulations are only now going into effect and under enforcement.

FSMA covers nearly all food and pet food facilities. The FDA’s current FSMA guidance document developed solely for pet food “covers facilities that manufacture, process, pack, or hold food intended for all animal species including food-producing animals (e.g., livestock, poultry, and aquaculture species), companion animals (e.g., dogs, cats, horses, and guinea pigs), laboratory animals, and animals maintained in zoological parks. “Animal food” means food for animals other than man and includes pet food, animal feed, and raw materials and ingredients (see 21 CFR 507.3).”

Pet food and grain processors undergoing process development should be aware of FSMA regulations which concern the following cases, among others;

  • Animal foods with high oil content which resist microbial heat treatments.
  • Mycotoxins (Aflatoxins, Fumonisins, Deoxynivalenol, Ochratoxin etc.) proliferating in grains.
  • Pesticides on grains.
  • Plant toxicants (lectin, protease inhibitors, cyanogenic glycosides etc.)
  • Animal-specific nutrient deficiencies and toxicity hazards.
  • Process or product cross-contamination.
  • Metal contamination from process equipment.

Evaluating and planning for food and workplace safety regulations during project design and installation will prevent future problems. Working with an experienced and reputable process equipment manufacturer specialized in your industry will help to anticipate safety concerns and hazards specific to your facility.