Process Design

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.

Related:
What is a slaker, and do I need one?
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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 oC (depending on elevation), it may be economic to add a water heater to ensure that optimal slaking temperature is achieved.

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What temperature should my slaking water be?


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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.

Related Links:
Do I need to heat my slaking water?


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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 you 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.



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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:

  1. Inline electric water heater with Silicon-Controlled Rectifier (SCR) or thermostat controls
  2. Direct injection Steam heater

Alternatively, any source of waste heat available in your process could be used.


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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.


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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:

  1. Heat
  2. Moving Parts
  3. Chemical Lime

Heat:

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.

Moving Parts:

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.

Chemical Lime:

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 too for treatment and protection recomendations.


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How do you handle grit produced during lime slaking?

‘Grit’ produced during slaking of quicklime can be handled in one of two ways:

  1. Separated at the discharge of the detention or paste slaker
  2. 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.


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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.


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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.


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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:

  1. Granular materials – such as quicklime, raw sodium bicarbonate, and salt
  2. Powders – such as Hydrated Lime, Magnesium Oxide, Soda Ash, Trona, and Alum
  3. 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.


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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:

  1. Will both sources be used simultaneously? This will affect bin vent filter sizing.
  2. Will a multiple sources provide an un-even silo fill? This will either affect silo structural design or fill line/target box location.
  3. 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.

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How is a slaker controlled?

Each type of slaker has its own method of control, for a detailed discussion on the types of slakers have a look here. 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 (see case study). 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.
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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.
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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.
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