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Agglomeration, a process integral to various industries, involves the consolidation of fine particles into larger, cohesive structures to enhance product performance and handling characteristics. Binders play a pivotal role in this process, specifically in compaction and wet granulation, by promoting adhesion and providing structural integrity to the agglomerates. Several binder types are commonly employed in agglomeration processes, each exhibiting unique properties that influence the final product's quality and functionality.
In this article, we will take a look at some of the most common binders used in agglomeration, from water and wax to clay and others. However, if you are completely new to binders and you’d like to learn more about what they are, why they are important, the different binder types and whether you need to choose one for your next project, check out our previous article: “An Introduction to Binders in Compaction & Wet Granulation”.
Common Binders
The three most common binder types that we interact with at Bepex are matrix-type, film-type (liquid bridge) and chemical-type binders. Below, we will take a look at specific binders within these three categories along with some of the pros & cons of each. Understanding the characteristics and applications of these common binders is crucial for optimizing agglomeration processes across diverse industrial sectors, ranging from pharmaceuticals and food production to mining and agriculture.
Water or Parent Material Solution (Film-type Binder)
When using water or a parent material solution as a film binder, it is going to act as both a binder and a lubricant between the individual particles and the rolls or the dye. Water in and of itself can react with some materials like soda ash to bind material together, and it will increase the plasticity of cloys by lubricating the boundaries of the particles, as well as contributing in other ways. In the case of organic or cellular products, water will promote bonding through Van der Waals' forces by increasing the true area of contact of the particles.
Below are some pros & cons of using water or a parent materials solution as a binder:
Pros:
- Acts as a binder and lubricant
- Can react with some materials (soda ash)
- Practically free, readily available
- Great for water soluble materials, forms salt bridges
- Drying/curing will increase the strength of the compacted material
Cons:
- Will typically increase the hardness while decreasing solubility of the final product
- Typically has a low green strength (the strength of the material right out of the compactor, before any curing is done to it)
As you can see, the biggest pros of this type of binder are that it is readily available and it's very cheap—practically free in many cases. It's a great binder to use when the parent material is soluble in water, because it's going to partially dissolve that material. And then when it's cured, it will form a salt bridge in between the particles since drying and/or curing materials after compaction will increase the strength of the material.
Potential cons of using this particular binder is that it will increase the hardness of the final product and it will also decrease its solubility somewhat. Also this binder typically has a low green strength, but it will harden when it's cured.
Application: This type of binder can be used either before or after compaction depending on the material and your desired end-product. To use before compaction, mix the binder with the material or raw feed to the level of moisture that is desired, followed by compaction and curing. If you will be applying this binder after compaction to add more coating, for example, spray on the binder in a tumbling drum, followed by curing if required. Bepex can provide a Turbulizer for this process if needed.
Steam Binder (Film-type Binder)
Similar to water, another low cost option is a steam binder. A steam binder is injected into the material before the compaction process. It has similar pros and cons to water:
Pros:
- Like water, but minimized addition
- Low cost
- Good for water soluble materials
- Drying/curing increases strength
- Heat aids in agglomerating
- Heat releases internal binder in some materials (for example: sawdust)
- Will increase hardness and decrease the solubility of the final product.
Cons:
- Will add moisture to the final product if not cured.
Like water, a steam binder will be low cost and readily available. One aspect of a steam binder that is slightly better than water, is that it will add some heat to the material, which typically will aid in the compaction process. With some materials, this added heat can also release internal binders.
For example, with sawdust or wood products, the increased heat will release the tannins and lignans that are naturally in the raw feed. These tannins and lignans act as additional binding agents without adding any costs to the product, making steam the only necessary material to be added to the mix before compaction. As we mentioned before, adding binder to a material will almost always increase cost, so steam can be an ideal option for materials that release natural binders at increased heat.
Of course, adding steam to a material means adding moisture to a product, so curing is necessary for reaching ideal hardness for the end-product. With curing, the end product should become quite hard, so if that is your goal, then steam is a great option. However, the product will become less soluble in return, so make sure to be mindful of this based on your product requirements.
Pellet mill, common in wood pelletizing
Application: Steam is added before compaction. Note that with the pellet mill above, the pre-conditioner on top of the mill injects steam into sawdust before the pellet mill creates pellets.
Coal Tar Pitch (Matrix-type Binder)
Another binder that we deal with frequently at Bepex is coal tar pitch, which is very common in the coal and graphite industry. One of the key components of this binder is that there are multiple melting and softening points available, which will ultimately dictate how they are used with each given process or product.
Pros and cons of coal tar pitch binders include:
Pros:
- Can be added as a liquid or a solid
- Can be heat treated for “smoklessness” or temperature resistance
- Allows for low pressure compaction
- Good green strength, hardens further on cooling
- Waterproof
Cons:
- Mid range price/limited availability
- Carcinogenic
- Will add carbon and sulfur to the product.
Because it is a matrix binder, generally it will lower the compaction pressure needed to get a good briquette. That means that the end-product is going to have a high green strength after briquetting—and it will harden even further after it cools. It is also a waterproof binder, so if your application involves a material that is going to be sitting outside in a pile for instance, you may want to use a coal tar pitch binder.
Coal tar pitch will typically have a middle-tier price point, but its availability will likely be limited depending on where your facility is located as it’s generally only available near places where the coal tar pitch is produced. Something to also consider is that this binder is going to add sulfur and carbon to the final product, so if you need a binder that will burn clean after compaction, coal tar would not be a good option.
Application: For low melt point pitch you will 1) heat the raw material, 2) melt the pitch, 3) mix liquid into the material, 4) allow to cool, and 5) compact the material. With a high melt point pitch, you want both the raw feed and the pitch to be a fine powder. First, you will dry mix the two powders together, and then compact the materials. This will work better in the compactor vs on the bench, because when you're compacting you're putting a lot more mechanical work into the material and warming up the pitch to help it bind.
Portland Cement (Matrix-type Binder)
Portland cement is a common binder in the mining reclamation sector, as well as with some minerals. The key pros and cons of portland cement to consider are:
Pros:
- Allows lower pressure compaction
- Waterproof after curing
- Relatively heat resistant after curing
- Low cost and readily available
Cons:
- Adding water or a wetter feed will be required
- Low green strength, but cures hard
- Extended curing time is required (days)
- Forced curing (drying) is detrimental
- Contaminants: Silica
With portland cement, you are either going to have to add water or your feed is going to have to be on the wetter side of the spectrum, since the cement has to be wetted to be activated. Due to having a wetter feed, this binder is going to have a fairly low green strength. However, it will be very hard once it’s cured. It will take at least a day, if not longer, for materials using this binder to cure, but it will be waterproof after curing. Be sure not to force cure the cement if you do use this binder, because if it dries-out it won’t be as hard as it could be.
Again, with matrix binders it will require less pressure in order to create the desired conglomerates. The cement is also going to be relatively heat resistant after it cures, so if you need to heat-treat a product after curing, this would be a good binder to consider. Portland cement is also fairly low cost and readily available everywhere.
Another point to consider when using portland cement is that it will add silica to the final product, so if you don’t want silica in the final product, this would not be a good option.
Application: When using cement as a binder, first dry-mix the cement powder with the raw feed, then add water if necessary. The material is then pressed and it must be allowed to cure afterwards at least overnight, but possibly for a few days to fully harden. This is simply done by letting the compacted material sit and cure on its own. The material will be soft, so careful handling is required before further processing.
Paraffin (Matrix-type Binder)
Paraffin wax is a less common binder, but we have used it in the past here at Bepex.
Some pros and cons of paraffin wax binder include:
Pros:
- Decent green strength/hardens on cooling
- Forced cooling accelerates hardening
- Allows low compaction pressure
- Waterproof
Cons:
- Feed needs to be dry
- No heat resistance
- High price and limited availability
- Contaminants: Carbon
To use a paraffin binder, the feed needs to be dry in order for it to work properly. Since paraffin doesn’t contain as much moisture as liquid binders, it will have a decent green strength and will harden further upon cooling—force cooling can also accelerate the hardening process. Paraffin also allows for lower pressure compaction and will be waterproof once the material is compacted, but it will not have any heat resistance.
Some downsides of this binder is that it is a fairly high-priced option that is limited in availability. It will also add carbon to the final product, so be mindful if this is acceptable or not before choosing it.
Application: Heat the feed and then add the melted wax before compaction. Mix everything thoroughly, allow it to cool slightly, then compact. Finish the process by allowing the material to fully cool. Alternatively, you can add shaved wax to the feed and heat and agitate it before compacting, then allow it to cool some, compact the mixture, and finally allow it to fully cool.
Bentonite Clay (matrix-type binder)
Bentonite clay is a common binder for minerals like iron ore, with the following pros and cons:
Pros:
- Feed can contain moisture
- Water addition will result in a film type bond
- Good green strength with no curing required
- Low price, high availability
Cons:
- Not waterproof, will disintegrate
- Good head resistance, but product may swell
- Contaminants: silica
When using bentonite clay, if the feed contains a fair bit of moisture, the addition of more moisture could result in a film-type bond instead of a matrix-type bond when compacting it dry. Thus, if you are using a wet feed or you’re adding water to the mixture, less bentonite clay will be required.
Bentonite clay will have good green strength and no curing will be required if you are only using the dry bentonite clay with limited moisture in the material. However, if you are going to be adding a significant amount of water, the materials may require some curing. Since materials with a bentonite clay binder will not be waterproof, do not use this binder if the end-product is going to be sitting outside in a pile or exposed to the elements in any way.
This binder will have a decent level of heat resistance, but if the product is going to be put into a furnace or something similar after compaction, the product might swell and crack. Bentonite clay will be a low price option and highly available, so if project economics are a big concern, you may want to consider this binder if it works with the material. Since this is a clay, it will add some silica to the final product, so be mindful of the end-products’ required specs.
Application: When conducting a dry mix method, simply dry mix the raw feed and bentonite clay before compacting the material. If adding water is necessary, also do this before compaction.
Starches (Film-type Binder)
Inline starch cooker
Starch binders are very common in food applications and can be used in a raw or pregelatinized form. If you're using pregelatinized starch, mix it into a slurry before adding it to the raw feed. When using raw starch, you can sometimes use an in-line thermal heater to cook it before adding it to the primary material. A starch binder can be added as a dry powder, but this will require adding some water before compacting since it ultimately is a film-type binder.
Some pros and cons of using starch as a binder:
Pros:
- High price, high availability
- Good green strength, but curing required
Cons:
- No heat resistance
- Possibility of mold issues if the product is not cured
Although starch carries a higher price tag, it is highly available, so project economics and alternative binder availability will need to be considered before choosing starch as the binder. A starch binder will have good green strength, but curing is required of the final product as mold issues can arise if it is not cured fully. Another factor to consider is that the end-product will not be heat resistant with a starch binder.
Belt dryer cooler
Application: Take a starch slurry and add it to the feed material before compacting, and finally curing it. Or, to mix the starch as a dry powder, simply add a little bit of water before compacting the material.
Lignosulphonates (Film-type Binder)
Lignosulfonate is a common binder in fertilizer applications. Lignosulfonate is a byproduct of the papermaking industry and there are two different forms you can find it in: either in powder form or in liquid form. Since this is a film-type bond, when using the powder form you will need to add liquid to the mixture before compaction in order for the binder to work.
Some pros and cons of lignosulfonates include:
Pros:
- Low price, limited availability
- Can be added as a powder or liquid
- Good green strength, curing required
Cons:
- Water addition required for powdered version
- No heat resistance
- Contamination: Sulphur
Lignosulfonate is lower on the price spectrum, but also has limited availability because it is a byproduct of the pulp and papermaking industry. This binder will have good green strength, but it will require curing. Lastly, it is important when considering it for your end-product that it will not be heat resistant and it's going to add sulfur to the final product.
Application: If the lignosulphonate is a powder, dry mix it with the feed then add water as required, then compact it before curing. If using the liquid version of the binder, add the liquid to the raw feed and compact it before curing.
Molasses (Film or Chemical-type Binder)
Molasses can be used as either a film-type or a chemical-type binder as it is added as a liquid and can result in both types of bonds. If you're using it as a chemical binder, you will want to also add some lime to the raw feed beforehand in order for it to self-cure. This is going to be a common binder in mining reclamation projects.
Some of the pros and cons molasses are:
Pros:
- Low price, limited availability
- Good green strength, curing required
- Reacts with lime to self cure
- Decent weather and heat resistance
Cons:
- Contamination: Carbon
Similar to starch, molasses will typically be a low price option, but is also going to have limited availability. It will have good green strength, but will require curing afterwards in order to reach this if you don’t use lime beforehand. Although molasses binder will have decent weather and heat resistance, it will not be completely weatherproof. Lastly, it will add some carbon to the final product, so be sure that the end-product is allowed to contain carbon.
Application: Dry mix the molasses powder with lime (CaOH) if self-curing, then mix in the molasses before compaction, and then move to storage to allow it to cure for up to 72 hours. If adding the molasses as a liquid to the raw feed, compact and then cure if required. Note that the color of the molasses can affect the color of the product.
Sodium Silicate (Film-type Binder)
Sodium silicate, as referred to as “water glass”, is a film-type bond that is very common in high temperature applications in the foundry industry for casting sand or in use cases such as agglomerating before a blast furnace.
Pros and cons of sodium silicate binder include:
Pros:
- Medium price, limited availability
- Reacts with several chemicals to self-cure
- Good weather resistance
- High heat resistance
Cons:
- Poor green strength, curing required
- Contaminant: silica
Sodium silicate falls into the medium price-range for binders, but is also going to be limited in availability. It isn’t going to have a good green strength before it's cured. During the curing process, however, it will react with a couple of different chemicals to self-cure and it will have strong weather resistance. Also, be mindful that sodium silicate will add silica to the final product.
Application: If adding a curing-aid to the mixture, add it to the raw feed before adding the silica and compacting. If you're not adding a curing-aid, add the sodium silicate before you compact it, and then cure the material afterwards. If you do use a curing aid, sometimes those materials can take up to 72 hours to fully cure after compaction, force-cured or not.
Which binder is right for your product?
Now that you are more familiar with some of the most common binders, choosing the right one for your next agglomeration project should be much clearer. The best binder for any given product ultimately comes down to the materials it will be mixed with and the end-product requirements, which should leave you with fewer options to select from. Familiarizing yourself with each of these binders and their pros and cons will allow you to provide options with much more efficiency, with project economics likely being the final determiner of which binder is the most cost-effective, while also meeting the product requirements.
Feel free to use this guide as a resource, but if you ever have additional questions about binders or need our help with a custom process, please don’t hesitate to reach out:
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