Equipment Selection

What's the difference between a Detention Slaker and a Ball Mill Slaker?

A ball mill slaker is a subset of the class of slakers commonly called a detention (or ‘retention’) slaker. Detention slakers are slakers that operate on either a batch or continuous basis with a water to lime ratio such that the contents of the machine remain a liquid slurry (<28% concentration) rather than forming a paste. They are also referred to as “slurry slakers” for this reason.

A ball mill slaker, either horizontal or a VERTIMILL ®, is unique because instead of having to separate the grit from the lime slurry as it leaves the slaker, the mill grinds the grit as the lime is slaked allowing it to carry on through the process as part of the milk-of-lime slurry.

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What is a lime slaker, and do I need one?​

A lime slaker is a machine designed specifically to control the reaction of water with quicklime (called ‘lime slaking’) to produce Calcium Hydroxide (CaOH2) or Hydrated Lime slurry. If your process uses lime slurry and you will have quicklime (CaO) delivered, you will need a lime slaker to carefully control the slaking reaction and the resulting lime slurry reactivity.

We generally recommend one of three types of lime slakers (and they each have applications where they are better suited):

  1. detention slaker
  2. paste slaker
  3. ball mill slaker

If you will be using less than 4 tonnes/day of lime, then you may be able to use hydrated lime and will not need a lime slaker.

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What type of flow promotion should I use in my silo?

The choice of flow promotion equipment depends on the exact properties of the material being handled. In general we would recommend using aeration for products that are “powdered” and vibration for products that are pebbled or granular. If the product is a mix of powder and pebble/granules, a combination of aeration and vibration can also be used.

It is also important to understand that flow promotion equipment may need to be cycled on & off during the feeding period to achieve consistent feed rates - the timing of the on / off periods will depend on your material.

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When should I use a bolted silo?

We will generally recommend using a bolted silo whenever the size or shipping constraints require the silo be field erected. In North America the practical limit on a shipable silo diameter is 14’ so anything larger than that will need to be field erected. We may also recommend bolted silos for shipments that are going overseas, or where there is no permanent road access. For applications requiring larger than 14 foot diameter, bolted silos are a good alternative to field welded silos, both from a cost and schedule standpoint.
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What does the scrubber do? And why do I need it?

The primary function of the dust and vapour scrubber is to reduce and minimize the dry solids feed inlet chute plugging. This is equally true for both cases where quicklime is being slaked and for cases where any dry powder is being prepared in a mix tank.

The scrubber will draw air through it, keeping the slaker or mix tank under slight negative pressure. Typically the air make up is drawn from as near the feed inlet chute as possible.

The key to successful scrubber operations is to correctly set up and adjust the airflow and water sprays as per the manual. Inspection of the scrubber is necessary from time to time, with that inspection interval varying depending on operations.

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What type of Feeder should I use?

The type of dry chemical feeder that you should use will depend largely on the degree of accuracy and control that your particular process requires.

In general for the greatest feed rate accuracy a Loss-In-Weight feeder (LIW) is recommended. A LIW feeder incorporates a hopper that can be on load cells or level control, which is refilled by a rotary valve from above. The product is then fed to the process make up tank from the LIW hopper by a feed screw. This arrangement can have a feed accuracy of +/- 0.5% once calibrated.

For process requirements that are not as stringent, a simple volumetric screw feeder can be used which has a feed rate accuracy of approximately 2.5% to 5%.

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What is the recommended material for a Lime Slurry System?

Carbon steel is the most commonly used material for both the dry and wet sides of a lime slurry service. Systems have operated successfully with downstream materials including stainless steel, fiberglass reinforced plastic (FRP), PVC / CPVC, and sometimes exotic alloys (duplex stainless steels, etc.). Selection of materials of construction, and certain coatings, is driven by the water chemistry and not by the inherent nature of the lime slurry.

With ‘typical’ process water the high PH created in the lime slurry dramatically reduces corrosion and mild carbon steels are a good selection for this service (this is assuming there are no significant corrosive elements present in the process water – such as high chlorides).

One other point worth noting is that aluminum is not recommended in lime slurry service as there are known material incompatibilities, and significant corrosion can be expected with the use of aluminum in lime slurry service.

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What technology do you recommend for level measurement?

Reliable continuous level measurement of a bulk solid in a silo can be attained by using guided wave radar sensors. Low level and high level alarm sensors are often but not limited to capacitance type probes.

Level measurement of slurry levels in a tank can be reliably attained with the use of ultrasonic level sensors. If your process slurry is susceptible to foaming due to water quality or additives or physical arrangement, special consideration should be taken.

For slurry level measurement differential pressure and high frequency radar sensor technologies are also used.

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What technology do you recommend for slurry flow measurement?

For measuring the flow of slurries the two most common types of flow meters used are magnetic flow meters (magmeters) and ultrasonic flow meters. Flow measurement in slurries requires more attention to design than other applications do because of the abrasive quality of particles in suspension and the possibility for scale formation. It is possible to reliably measure the flow of slurries.
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What type of valve do you recommend for a Slurry System?

The primary consideration when selecting a slurry valve is to anticipate eventual build-up due to settling or scale. Pinch valves are by far our preference because the flex of the valve will allow scale to break away if they are cycled regularly but in reality just about any full bore valve will work.

When selecting a valve for slurry service it is also important to distinguish between valves used for isolation or dosing. Pinch valves can work well in dosing applications particularly if it is allowed to pulse between fully open and fully closed when feeding slurry to process. If the valve needs to be fully modulating a V-port ball valve can work well but exotic materials such as ceramic may be appropriate to prevent wear.

The final thing to consider when selecting a valve is the properties if the water being used for flush. If there is potential for excursions, where hydrocarbons may be present for instance, most elastomers are not appropriate.

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What technologies are available to measure product level in a silo? Which ones are preferred?

Guided wave radar is the preferable technology to measure continuous product level in a silo.

Other technologies that are available include:

  • Weight and cable (often used as a check or to calibrate)
  • Ultrasonic
  • Radar through a horn
  • Load cells
  • Strain gauges

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Are there differences in the dry chemical systems you provide for each industry?

Whether our system is being provided to a Mine, Power Plant, Municipal Water/Waste Water Facility, Oil & Gas Plant, or any other facility, STT provides the option of a custom designed or pre-engineered Standard System. Pre-engineered systems can cover a broad range of applications requirements and can save you significant cost and lead time.

There is a lot in common from one industry to the next and there are great opportunities to learn from experiences in one industry and apply it to another.For example, our mining customers have a great deal of experience with grinding mills like the Metso VERTIMILL®, and this machine makes an excellent slaker. We have taken this experience and integrated these mills into complete lime slaking systems for customers in the power and chemical industries.

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How does a site’s location effect the design of a dry chemical handling system?

Local conditions play an important role in the design of a dry chemical handling system.We have systems in the extreme cold of Canada’s arctic, and the extreme heat of the Middle Eastern desert.Our systems on the US coast must be designed to withstand hurricane force winds, while our facilities in Chile are faced with one of the highest seismic loads in the world.

In addition, the proximity to a major city center also influences the systems design. A remote mine in the US Rockies, or the Peruvian Andes’ must have sufficient storage to continue operation if there is a disruption to access.While systems built in major city centers on the other hand, need to be cognisant of noise and aesthetic appearance.And of course language must be considered for local operators, our signs and manuals have been translated to French (Quebec), Spanish (Mexico and South America), Cantonese (China), and Arabic (Jordan, Saudi Arabia).

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What are the design elements that make up a dry chemical handling system?

A well designed dry chemical handling system will look quite simple once complete.However, its simplicity is misleading as there are hundreds of possible variations that lead to the correct design for your specific product, process and location.As engineers we have a habit of categorizing things, and a chemical system is no different. There are seven elements that need to be considered to design a perfect “simple” solution those are:

  1. Material Receiving – How do we get the material from the truck to storage?
  2. Bulk Dry Storage – Once material is on site how is it stored for later use?
  3. Flow Promotion – How do we get the material from storage to process?
  4. Feeding & Conveying – How do we control how much material is sent to process?
  5. Wet Makedown or Slaking – How is the chemical combined with water prior to being sent to process?
  6. Slurry/Solution Storage
  7. Piping & Pumping – Once made down or slaked, how is it sent to process?

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Do I need a stationary truck unload blower?

Many pneumatic convey trucks have their own onboard blowers.If this is the case you most likely do not need to provide your own blower at the site.However, if the trucking company you are planning on using to deliver to your site does not have blowers on their trucks, it is a relatively simple piece of equipment to add to a system scope.We would need to understand the connection size, the truck’s pressure rating and the desired offload rate in order to select an appropriate blower for your application.

Stationary blowers may also be needed if the desired offload rate is greater than what would be possible using a standard onboard blower.This is most often necessary at facilities using large quantities of lime where multiple trucks will be delivering each day.

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Why do I need a truck bypass assembly on a truck unload blower?

The pneumatic convey rate is directly related to the air flow rate, so the more air that is used, the faster a truck can be unloaded.

However, there is a limit to the amount of air that can be passed through pneumatic convey trucks before over-pressuring them. A common strategy to increase the unload rate is to use a blower that moves more air than a truck is capable of handling, and bypass some of the air from the blower directly to the silo fill line. This effects the convey rate in two ways.First, it allows the operator (or an automatic valve) to maintain the pressure in the truck as high as possible, even as the back-pressure begins to drop as the hopper empties. Second, the air that is bypassed is, in effect, whisking the air away from the truck faster.If you were to dig into the physics behind this phenomenon it would reveal a hybrid dense phase/dilute phase pneumatic conveying.

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Why shouldn’t I put a limit switch on the truck fill line?  What alternatives are there?

As plants are becoming more and more automated there is a natural desire to ensure that the cleaning mechanism on the bin vent filter is automatically initiated when a truck is filling the silo. Putting a limit switch on the fill line is an obvious way to tackle this problem. The problem is that these devices are notoriously fragile, in a notoriously rough and tumble application. Nevertheless, there have been some creative solutions provided over the years (attaching the cap to a chain on a limit switch is a particular favourite).

A great alternative to a mechanical limit switch is to measure the difference in pressure between the silo and atmosphere. If there is a measurable pressure then the silo must be being filled. A differential pressure switch or transmitter on the bin vent filter (a commonly available option) measures this pressure, the transmitter option has the additional benefit of allowing operators to trend the silo filling pressure and predict maintenance intervals on the bin vent filter cartridges/bags.

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What is a target box?

When talking about silo filling a “target box” generally refers to a means by which material being conveyed into a silo is slowed and redirected to allow the material to free fall into the silo.A “target box” can take many forms, but the most common on our systems is a 20” diameter cylinder attached to the center top of the silo.The cylinder has a bolted top with a deflector plate opposite the incoming fill line.

We prefer to us target boxes when filling a silo as they do less damage to the material being conveyed than a sweep elbow, and generate less dust than conveying directly into the silo at speed.A target box is also a convenient method used to allow multiple fill lines to center fill a single silo.

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Do I need lightning protection on my silo?

As you might imagine, “convincing” a lightning strike to selectively travel down a marginally more conductive path provided by a rod and copper wire attached to a large steel silo is not terribly realistic.Provided the silo itself is grounded, there is no need for additional lightning protection.This is supported by NFPA 780 Standard for the Installation of Lightning Protection Systems:

“5.4 Metal Towers and Tanks. Metal towers and tanks constructed so as to receive a stroke of lightning without damage shall require only bonding to grounding electrodes as required in Chapter 4…”

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Do I need a fan on my bin vent filter?

The objective of a bin vent filter is to remove the dust from the air entering the silo, either by way of pneumatic convey, flow promotion, or displaced by material stored in the silo.

The need for a fan on a bin vent filter will depend where the air needs to go once it’s cleaned.Bin vent filters are frequently located on the roof of a silo, in this case the air needs to only get to the other side of the filter.In this scenario, there is no need to blow the air any further.However, if the bin vent filter is located in a penthouse or a building where the discharged air will need to travel some distance to atmosphere, a fan or blower is recommended.

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What type of slaker should I use?

When helping our customers select the proper slaker for their application we consider five types of slakers:

  • Detention Slakers
  • Paste Slakers
  • Horizontal Ball Mill Slakers
  • Vertical Ball Mill Slakers
  • Batch/Mixtank Slakers
In order to make the best choice for your application you need to consider, throughput, lime quality, grit quantity/disposal facilities, water chemistry, available space on site, among other variable. This page highlights some of the key distinctions between the different slaker types. STT can help recommended the best choice for your application, get in touch.
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What is the best geometry for my mixtank/slurry storage tank?

We will generally start with a tank geometry as close to a 1:1 diameter to height ratio as possible when designing a slurry tank. This is done primarily to minimize the mixing power (and therefore cost) required to maintain a homogeneous suspension. However with proper mixer selection, shorter/squatter tanks or taller/skinnier tanks can also work just as well when required due to space availability.

In round tanks with center mount mixers it is also important to incorporate baffles on the sidewalls to break the vortex that would otherwise form and maintain turbulent mixing.

For the most part we will use a flat bottom tank with the suction connection a reasonable distance above the bottom. While this reduces to “usable” volume in the tank it allows for some of the inevitable build up to collect at the bottom of the tank rather than being drawn into the pump suction.
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Can I use bolted tanks for slurry mixing/storage?

Absolutely! We have been using bolted tanks as part of slurry systems for years. Sometimes these are tanks that are integral to a silo skirt, in other cases they are standalone tanks that would otherwise have been too large to fabricate in a shop and ship to site. In addition to the advantages bolted tanks have from a cost and erecting time perspective, the factory applied coating can increase the life of the tank considerably.
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What type of level instrument works best for slurry tanks?

Because slurry tanks are by their nature turbulent environments, with an inevitable element of scale build up, non-contact measurement is the key. Ultrasonic transmitters are good solution provided there is not too much dust in the air space above the liquid level, such as in a slurry storage tank following a slaker. When there is potential for dust in the air and/or foam on the tank surface as would be the case in a Hydrated Lime makedown tank, we would recommend switching to a radar transmitter. Some customers (with strong preventative maintenance programs) have also had success with “bubbler” type level transmitters. For more information on how to reduce dust click here (q 7).
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Why do I need a scrubber on my slurry tank?

The primary purpose of a scrubber on a dry chemical handling system is to clean the air that is vented from the tank. This venting is the critical element to designing a system that will operate reliably. Dust from the dry feed into the tank combined with water vapour in the air leads to build up of solids in the space above the liquid level. This build up can be particularly problematic at the feed inlet as it will gradually restrict the flow into the tank. A well designed scrubber will actively draw air from the tank, remove the dust (retuning it to the tank), and limit build up at the feeder inlet. A slight negative pressure on the tank will also help prevent dust from escaping and building up in the area surrounding the feed system.
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