If it involves lime slaking, storage of dry materials, pneumatic transferring, reagent preparation systems or any applications that go along with them, STT has seen it. Below are answers to some of the most commonly asked questions we receive. Cannot find an answer to your question here? Contact one of our experts directly. Just click on the form below.
Can you design for a hazardous location system?
Yes, our Engineers are able to meet the design standards required by any industry, or jurisdiction, globally.
STT has extensive experience designing for various different classifications of hazardous locations, and are familiar with North, Central & South American standards, as well as standards from many other countries we’ve served (Australia, Africa, Middle East, Asia/Mongolia). STT can provide a design conformant to the site requirements for various hazards including explosive and corrosive environments and have operating plants around the world in various industries where these hazardous locations frequently exist (including mining, power generation, petrochemical, and chemical processing industries).
Do you provide full-automated control/system?
Yes we do, the level of controls and automation provided is customized to meet the needs of each customer. We serve a wide range of customers, from oil & gas, power, mining applications, to municipal water and wastewater treatment plants.
If you already have a lime slaking system, consider updating it to current automation standards - it will be safer, easier to operate, and will save you money.
What communications protocols can your automation system support?
We have experience using a wide variety of communication protocols; Commonly using EtherNet/IP, Modbus TCP/RTU, & Profibus. Although these are the most common protocols used, we have the ability to work with other project specific communication protocols as well.
What kind of documentation do you provide? What documentation options do you have?
We can provide any level of documentation that your project requires.
The 3 common levels are classified as:
Within each of those three levels, documents can be issued for information or issued for review.
A base level of documentation will include: P&IDs, Foundation Loading & Anchoring Diagrams, Plot Plan & Elevation GA’s, Single Line Electrical Diagrams, and Instrument Block Diagrams. As well a base level will include Operation and Maintenance Manuals, Recommended Spare Parts Lists, and many other documents.
At the other end of the spectrum a Custom level of documentation will meet the stringent needs of a large EPCM Engineering firm’s international requirements.
What kind of utility do you need for your system?
A typical slaking system or dry chemical make down system requires only standard utilities:
- Electrical supply
- Compressed air supply
- Water supply (potable or service water or recycled water)
- Safety shower and eyewash station (depending on site requirements)
What industries does STT provide dry chemical systems for?
We have worked in just about every industry imaginable.The most common being Mining, Power, Oil & Gas, Municipal Water/Wastewater, Pulp & Paper, Food, and Steel.That said, any customer who requires bulk chemical handling would benefit from our expertise.
What types of products can STT’s systems store and handle?
We can store and handle just about any product you can imagine.In general, we say if you have a material delivered to your facility in a bulk truck or rail car, we can help.Some of the more common products being Lime Products (Quicklime, Hydrated Lime and Limestone), Magnesium Oxide, Soda Ash, Fly Ash, Powder Activated Carbon (PAC), Cement, Trona, Salt, Alum, Polymer, and Sodium Bicarbonate.
Where in the world are STT’s reagent preparation systems working?
We have systems located around the world -Canada, USA, Mexico, Central America, South America, Africa, Asia, Australia and the Middle East. Our design team will work with you to select and provide a system that is suitable for your location.
What is STT’s typical scope for a complete dry chemical handling system?
STT will not only supply the system but we can also help you with construction/installation of your bulk chemical handling system.
We will typically take on a scope that starts with unloading the material from the truck and ends with pumping or conveying the material being handled to a process.How that chemical is being used is your business, getting it there the most effective way possible, is ours.For simplicity, when considering a system, we generally divide it into seven categories:
- Material Receiving
- Bulk Dry Storage
- Flow Promotion
- Feeding & Conveying
- Wet Makedown or Slaking
- Slurry/Solution Storage
- Piping & Pumping
Should I use quicklime or hydrated lime for my application?
The choice to use quicklime or hydrated lime depends principally on two factors: availability and consumption rate.
Not all lime producers have hydrated lime available. When they do there is a difference in cost between the two materials. The cost per tonne of quicklime and hydrated lime is often very similar, but the big difference is that a tonne of quicklime when slaked yields 1.32 tons of hydrated lime (the difference in molecular weight between CaO and Ca(OH)2).
To use quicklime however, you will need to install a slaker. We generally find that it is more economical for facilities consuming more than 4 tonne/day of lime to use quicklime. If you will be using less than 4 tonne/day, then the extra cost of hydrated lime is offset by the additional cost and complexity of the slaker.
How does incoming water temperature affect the slaking process?
Incoming water temperature has a direct impact on the final slaking temperature. Lime slaked at the optimal slaking temperature will result in lime slurry of higher reactivity, reducing the amount of slurry needed in the process. If the incoming water temperature is low, optimal slaking temperature may not be achieved during the reaction, producing inferior quality lime slurry, increasing lime consumption and cost.
Final slaking temperature follows this formula:
Incoming water temperature + heat of reaction = final slaking temperature
The final slaking temperature has a direct impact on the hydrated lime particle size, which is linked directly to the reactivity of the lime slurry.
If the final slaking temperature is below 85°C (depending on elevation), it may be economical to add a water heater to ensure that optimal slaking temperature is achieved.
What temperature should my slaking water be?
Slaking water temperature should be sufficiently high so that the final slaking temperature is at least 85°C (185°F) - depending on site elevation.
Final slaking temperature will follow this formula:
Incoming water temperature + heat of reaction = final slaking temperature
For example, if you are slaking high calcium quicklime (93% available CaO) at a 4:1 water:lime ratio, the temperature rise from the heat of reaction can be up to 57°C (103F). In this case to achieve a slaking temperature of 85°C (185°F) incoming water temperature would need to be 28°C (82°F). Please note that every quicklime source will have its own temperature rise characteristics - this information can come from your quicklime supplier, and can be varified independently.
Why do solids build up in slurry systems?
Material builds up in slurry tanks and slurry piping for one of two reasons, settling or scaling. If you have build up in your slurry system it is likely from one or both of these reasons. It is important to determine which is the cause because the method(s) for addressing them are different.
Look into your tank and/or pipe and it should be relatively easy to determine which is causing your issues. If you have build up predominantly at the bottom of your pipe or tank you are likely looking at a settling problem. Settling is normally a result of material falling out of suspension due to low flow velocity or dead legs in your piping.
If the build up is on the walls of the tank, or around the full perimeter of the pipe you are dealing with scale. Scale is a result of a chemical reaction between the chemical being handled and the impurities in the water making up the slurry.
Do I need to heat my slaking water?
The more reactive your lime slurry is, the less of it you will use in your process. The final slaking temperature has a direct impact on the hydrated lime particle size, which is linked directly to the reactivity of the lime slurry.
Final slaking temperature will follow this formula:
Incoming water temperature + heat of reaction = final slaking temperature
If the heat of reaction is not sufficient to reach the optimal slaking temperature (higher is generally better), you should consider adding a water heater to pre-heat the slaking water.
How can I heat the water?
The two most common options for heating slaking water are:
- Inline electric water heater with Silicon-Controlled Rectifier (SCR) or thermostat controls
- Direct injection Steam heater
Alternatively, any source of waste heat available in your process could be used.
What is the recommended line velocity for lime slurry?
In the past general practice has been to recommend a minimum slurry line velocity of 4 ft/s (1.2 m/s) for lime slurry piping with a diameter of 3” or larger. This velocity was thought to be sufficient to maintain turbulent flow and prevent settling.
Current practice recommends a minimum slurry line velocity of 8 ft/s for lime slurry piping with a diameter of 3” or larger.
For 2” lime slurry piping we recommend a minimum of 8 - 10 ft/s (2.4 - 3.1 m/s).
Line velocity above 10 ft/s (above 3.1 m/s) is excessive and may lead to increased pipe wear and inefficient pumping.
What safety issues should I be looking for in my Lime Slaking System?
There are three main safety hazards on a lime slaker to be aware of:
- Moving Parts
- Chemical Lime
A slaker should be operated at as close to boiling as possible to get the best possible lime slurry quality. As such, there are numerous hot surfaces that could potentially cause burns. The slurry itself is also very hot so hatches should remain closed to prevent splashing.
The slaking system should also be designed to measure and address high temperature excursions, by stopping the feed of lime and cooling the reaction with water where necessary.
Regardless of the type of slaker being used there will be some form of rotating equipment used for mixing. These moving parts should be suitably guarded to prevent personal contact.
Lime, even when cool, can be irritating to the skin and damaging to the eyes. The MSDS (Material Safety Data Sheet) for the particular product being handled at your facility should be referred to for treatment and protection recommendations.
How do you handle grit produced during lime slaking?
‘Grit’ produced during slaking of quicklime can be handled in one of two ways:
- Separated at the discharge of the detention or paste slaker
- In a ball mill slaker the grit will be ground during the slaking process
Grit can be separated from the lime slurry using a vibrating screen, or with an inclined screw conveyor, with washdown. Grit separated this way will need to be collected and disposed of.
Alternatively, grit can be ground within a ball mill slaker to a particle size small enough carry on through the process without settling or causing abrasion/erosion.
What is product bridging and/or rat holing in a storage silo? What methods or technologies are available to overcome these conditions?
Product bridging describes the obstruction condition where a stable powder arch forms above the outlet of a silo or forms across opposite walls of a silo. In both of these circumstances normal product flow is interrupted. In the worst case scenario when a large product bridge collapses, serious, often catastrophic damage can result to the silo.
Rat holing is a condition where a narrow channel of product preferentially flows to the outlet of the silo, leaving a stable internal mass of product that does not flow at all.
Proper design and sufficient bulk product experience are required to prevent these flow obstructions from occurring. There are minimum discharge or outlet diameters, different for each bulk solid, that must be taken into consideration during the design phase of a properly engineered system.
What is "angle of repose" and why does this affect the sizing of a storage silo?
The angle of repose of a material is the steepest angle that can be formed between the sloped edge of a free standing pile of material and the horizontal axis. Each material will have its own angle of repose specific to that material. In general ‘sticky’ materials will have a steeper / larger angle of repose, while more free flowing materials will have a shallower / smaller angle of repose.
The angle of repose affects the silo’s sizing because the top of a pile of material will form a cone where the cone angle is related to the angle of repose for that material. In center fill silos, this cone will form within the silo, with the point of the cone at the center of the silo. When the silo is ‘full’ the tip of the cone will be at the top of the main cylindrical section of the silo. This will leave some free space around the silo periphery at the top of the silo. If this is not correctly accounted for, your silo will be undersized and will not hold the anticipated amount of chemical.
Do you treat each of the chemicals you store and feed differently?
Over the years we have developed best practices for each specific product that we handle.While there are unique properties to each chemical, there are also some commonalities that can be used to make system design more modular. For example, when we consider flow promotion in a silo we will group the products into three general categories:
- Granular materials – such as quicklime, raw sodium bicarbonate, and salt
- Powders – such as Hydrated Lime, Magnesium Oxide, Soda Ash, Trona, and Alum
- Fine Powders – such as Fly Ash, Cement and Powder Activated Carbon (PAC)
And, if we have not built a system for your specific product before, we can arrange to have it tested to determine how best to handle it.
What do I need to consider if I have more than one fill line?
There are a few things that should be considered if you will be filling a silo from multiple sources:
- Will both sources be used simultaneously? This will affect bin vent filter sizing.
- Will multiple sources provide an uneven silo fill? This will either affect silo structural design or fill line/target box location.
- Will there be backflow along an unused fill line? Some measure will need to be taken to prevent product from exiting via an unused fill line. This can be as simple as a cap on the line or as complicated as an automatic valve.
How is a slaker controlled?
Each type of slaker has its own method of control. In general there are three means to control a slaker, temperature, torque, and input measurement. While each slaker is controlled using one of these methods, they may also monitor some or all of the others.
Temperature is the most reliable means of control when the desired outcome is consistent highly reactive lime slurry. Slaking temperature is directly related to reactivity. Detention Slakers and Vertical Ball Mill slakers measure temperature directly in the product zone and are therefore the easiest to control. Horizontal Ball Mill slakers measure temperature at the discharge so control has some lag. Paste slakers and Batch slakers usually monitor temperature but it is not the control variable.
Paste slakers use torque either directly (via a clutch) or indirectly (via amp draw measurement) to control the consistency of the paste in the slaker. Ball mill slakers also monitor amp draw but usually with an eye to maintaining sufficient ball charge rather than controlling the reaction.
Batch slakers make no attempt to control the reaction once it has begun; instead they measure the amount of lime and water introduced into a given batch. Assuming that the lime and water properties remain stable, this will result in a consistent output. Almost all other slakers will use input measurement in some form, and may use this as the control mechanism until the unit has reached steady state.
Do I need an internal coating on my lime slurry tank?
In general an internal lining is not required. Lime slurry is not corrosive and will in fact inhibit rust formation. However, if you intend to shut the system down frequently and flush with raw water, or perform a regular acid flush to remove scale from the tank you may want to consider a protective coating compatible with the water/or flushing fluid. Some customers also find that selecting a coating with a smooth finish can help limit build-up on the tank walls.
Why do I need to pre-wet some dry products?
Because of the effects of surface tension some powdered materials such as fly ash, PAC, and Bentonite, will not immediately sink below the surface of the fluid when added to a tank. If feeding too fast, material can build up on the surface forming clumps that are difficult to mix, and eventually completely blocking material from being entrained.
(An everyday example of this effect can be found in coffee, if you add sugar to your coffee it immediately drops below the surface (easy to wet) however, if you are adding powdered cream this will tend to float on the surface for a period before submerging (difficult to wet). There are several different strategies for pre-wetting.
The solution we recommend will depend on the specific material the feed rate and desired concentration.
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.
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):
- detention slaker
- paste slaker
- 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.
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 and off during the feeding period to achieve consistent feed rates - the timing of the on / off periods will depend on your material.
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.
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.
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%.
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.
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.
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.
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.
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)
- Radar through a horn
- Load cells
- Strain gauges
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.
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).
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:
- Material Receiving – How do we get the material from the truck to storage?
- Bulk Dry Storage – Once material is on site how is it stored for later use?
- Flow Promotion – How do we get the material from storage to process?
- Feeding & Conveying – How do we control how much material is sent to process?
- Wet Makedown or Slaking – How is the chemical combined with water prior to being sent to process?
- Slurry/Solution Storage
- Piping & Pumping – Once made down or slaked, how is it sent to process?
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.
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.
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.
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.
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…”
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.
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. There are some key distinctions between the different slaker types. STT can help recommended the best choice for your application, get in touch.
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.
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.
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).
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 vapor 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 (returning 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.
Operations and Maintenance
Why is the wet scrubber/ducting plugged?
The wet scrubber is comprised of two important operational elements. Water flow and air flow, in balance. If either of these is not set up correctly, a plug may develop.
The water flow to the spray nozzles is counter-current to the air flow and needs to be adjusted to wash down any lime or powder laden vapours.
- Too much water flow and unacceptable process dilution might result.
- Too little water flow will result in the venturi blower pulling in airborne powder that has not been washed down by the nozzles. The powder will then accumulate over time and eventually cause plugging.
Airflow needs to be adjusted using the blower discharge damper or the balancing tee on the scrubber. The airflow should be set to pull airborne dust and vapour into the wet scrubber.
- Too little airflow and dust and vapour will not be exhausted from the mixing vessel.
- Too much airflow will result in the water spray, along with airborne powder, to be pulled into the ducting. This will cause plugging in very short period of time.
Why is the silo level reading not proportional to the actual silo storage capacity?
Most dry chemical storage silos utilize the guided wave radar method of level measurement. This means that a radar probe is suspended by a rope from the transmitter on the roof of the silo, to the point where the main silo section meets the cone. The radar probe does not extend all the way into the cone and usually only measures product level in the cylindrical portion of the silo. Depending on the product being stored and the size of the silo, the ‘unmeasured’ cone section of the silo may account for a significant amount of the storage capacity.
This issue can be addressed by having an appropriate offset in the calibration of the instrument.
How often should I change my dust collector filters?
Filter cartridges need to be changed before they become plugged.
Many operating plants schedule a change out of their dust collector filters on an annual basis. Some sites, with difficult to handle materials schedule these changes twice per year.
How often you need to change your filter cartridges will depend on the number of product deliveries that your plant receives. A site that receives six deliveries per day will have to change out their dust collector filters more often than a site that receives one delivery every two weeks.
These filters should also be changed out when the normal built in cleaning cycle, whether reverse pulse jet or shaker, does not adequately clean these filters.
Operating Tip: check to be sure that your compressed air source is clean, dry air - and that the minimum recommended operating pressure is maintained during the entire silo fill cycle.
Should I be slaking with Pebble or Powdered Quicklime?
Effective lime slaking can be accomplished with both Pebbled or Powdered Quicklime.
What’s best for you will depend on your slaking system and your ability to deal with the potential extra dust created by using only powdered quicklime. It is generally not necessary to use this more expensive powdered quicklime to create high quality / highly reactive milk of lime in a lime slaker.
High calcium pebbled quicklime is perfectly suitable for most lime slakers. It is a common misconception that powdered quicklime contains less of a grit component.
How do I keep personnel safe while working on and around the slaker?
As a minimum, follow all standard on-site policies and procedures for working around operating equipment.
Lock out / Tag out procedures are critical.
In addition, slaked lime can be hot and caustic, so be sure to wear appropriate personal protective equipment (PPE).
Typical appropriate PPE will include eye protection, face shield, gloves and coveralls. In dusty conditions a dust mask, or a half mask respirator, may be required. Make sure to dilute and cool down a slaker before working on any wetted components.
How often should I recharge my VERTIMILL grinding media?
Your VERTIMILL grinding media should be recharged whenever your VERTIMILL drive motor power draw drops below the minimum set point. More frequent charging, adding smaller amounts, will result in steadier power draw and steadier performance.
Modern VERTIMILLS used in lime slaking duty are designed to operate best when this power draw is between 70% and 80% of of full load amperage (FLA).
Never exceed 80% of FLA on the Vertimill drive motor.
Power draw dropping can also be an indicator of screw liner wear.
How do I check the level of grinding media in my VERTIMILL lime slaker?
Safety First! The level of your grinding media in the Vertimill can only be checked when the Vertimill is properly Locked Out / Tagged Out!
This is called the ‘Static Ball Charge Measurement’ in the Vertimill manual. Once the mill is stopped, a calibrated rope with a non-magnetic weight securely fastened at one end can be dropped in the ball charge chute or down the main Vertimill body inspection hatch. (Often the ball charge chute is angled and convoluted so the inspection hatch is more practical.) This ball charge depth is compared to the depth measured when the Vertimill was first commissioned with a new and full ball charge.
Call us for help determining the correct level.
One important side note - if the static ball charge rises by one foot (0.30 m) as compared to when it was first commissioned, in order to maintain the same 80% FLA maximum, your screw liners are worn and require replacement.
What should I check on the Slaker on my shift rounds?
Some things to look for are:
- Verify that the slaker temperature sensor is reading correctly. (We suggest you measure slurry temperature with a hand held Infrared (IR) Temperature Thermometer.
- Look for product leaks from gaskets or rubber connections connecting different parts of the feed system.
- Ensure correct flow through water lines to the slaker seals, wet scrubber and vibrating grit screen.
- Look for quicklime buildup in/around quicklime inlet.
- Slaked lime buildup at outlet or inside the vibrating grit screen.
- Sufficient exhaust suction from the wet scrubber.
- Check the HMI/DCS alarm history.
When should I change the packing in the Slaker seals?
A properly adjusted and lubricated seal assembly should only require weekly adjustments and perhaps monthly packing additions.
The most common type of slaker seal in horizontal agitator shaft detention slakers is a square cross-section ‘donut’ packing. Depending on the model and size of the slaker, there might be 3, 4 or 5 packing rings in a seal assembly. The first step is always to adjust the packing by tightening the Seal Packing Retainer (also known as a follower gland). Tighten just as much as it takes to stop a leak. If the Seal Packing Retainer is bottomed out, packing donuts must be added or replaced.
Always verify that adequate packing seal flush water is flowing. Clean seal flush water is critical. Replacing the flush water with air or grease will not work.
If it seems that packing has to be replaced too often, look for worn out agitator shafts or worn out stub shafts as a possible cause. Inspect and replace these as necessary.
The silo Pressure/Vacuum Relief Valve (PVRV) opened during fill, why and what should I do?
The PVRV is a key safety component for the silo. Whenever a PVRV activates (either under pressure or vacuum) - you must investigate!
A functioning PVRV can prevent a catastrophic structural silo failure. If it activates, the silo has either been over pressurized, or was under excessive vacuum.
Find out why:
- Over-pressurized? A common cause is plugged or blinded-off filter elements in the Bin Vent Filter (dust filter) on top of the silo. Inspect, clean and/or replace filter elements as necessary.
- Excessive Vacuum? Either your Bin Vent Filter cartridges are plugged and not allowing air flow into the silo, or you’ve experienced bridging in the silo and sudden movement of product (which is also dangerous).
- Perhaps your silo was over-filled? One possible cause is that the silo has actually been over filled. The cause might be a faulty silo level indicator or might be non-functioning high level alarms.
Under no circumstances should you ever bypass or prevent the normal operation of a PVRV!
Should I completely empty the silo or can I leave chemical in the silo for a long term shutdown?
Some bulk chemicals may degrade over time or absorb moisture from the air; some bulk chemicals may even solidify over time so it may be best to empty the storage silo in advance of a long term shut down. This will depend on your specific circumstances.
In addition, this may be an opportune time to inspect the silo’s internal structure and any instrumentation inside the silo.
What is 'feeder calibration' and how does this information affect my slaking system?
A feeder calibration is a method of collecting a sample or product from a feed to correlate the RPM (speed) of the feeder to the amount (weight) of product being delivered.
The slaking control scheme requires accurate information about the quantity of quicklime being discharged into the slaker in order to properly calculate the correct flow of water to best react with the lime. A minor discrepancy in quicklime addition is compensated for in the controller. If the expected quicklime feed rate vs actual quicklime feed rate is significantly different, the slaker may overheat or not generate enough temperature resulting in a poorer quality of lime slurry.
Why is my slaker overheating or setting off a high temperature alarm?
The most common reason a slaker will be alarming with a High-High temperature alarm is if there is too much quicklime being fed into the slaker relative to the amount of water or if the incoming slaking water is bringing heat with it. Check the actual output of the feed device (see “What is ‘feeder calibration’?).
Other possible reasons include:
- Quicklime is flooding though the feed device
- Water supply is not supplying enough water (restrictions)
- Look for possible low flow alarm
- Incoming slaker feed water temperature is too high
- The temperature sensing instrument is failing
- Check the temperature of slaking water coming in before it enters the lime slaking reaction zone.
What are the advantages of automating the slaking process?
Automatic slaker controls initially adjust the water flow to match the quicklime feed rate (often to a set ratio of 4:1 water to lime by weight). Once a stable operating temperature is reached, automatic controls fine tune the water flow to maintain a final slaking temperature set point.
Automating the process ensures that the slaker is running at the optimal temperature for lime slaking - producing the most effective and most efficient lime slurry for your process. Having the most effective and efficient lime slurry will save on quicklime costs and will indirectly reduce the carbon footprint of lime use by reducing lime consumption.