How Rotary Dryers are Used in Chemical Processing

Rotary dryers are used to remove moisture from bulk materials through heating. This is done either directly or indirectly with heated air and/or gasses. They can consist of several drums (also called shells) or a single drum, with multi-drum rotary dryers enabling equipment to produce greater throughput in the same size space. Heating of rotary dryers with multiple drums is usually done directly via gas or oil burners. Some rotary dryers also have a combustion chamber positioned at the end of the feed, optimizing fuel usage to keep air temperatures the same throughout the drum. While their primary task is drying materials, some rotary dryers combine this process with shredding, separating, cooling, cleaning, or other processes. 

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Versatility of Rotary Dryers 

Though rotary dryers are used primarily for chemical processing and mineral extraction, they are extraordinarily versatile machines. They're also widely used in agriculture, ceramics, dairy, food, lumber, paper, rubber, and other industries.

Rotary dryers can be used for applications that include:

• Dehydrating corn gluten meal after wet milling.
• Drying rice directly from the paddy.
• Eliminating contaminants in soil.
• Extracting vegetable oils.
• Processing citrus peels and pulp for animal feed.
• Removing moisture during the processing of chemicals and minerals.
• Roasting cocoa beans, nuts, and sesame seeds.
• Mineral processing and lithium mining.

A rotary dryer's job is essentially to dry out a variety of materials before, during, and after processing, including biosolids, concentrates, copper, fly ash, gypsum, limestone, manure, metal shavings and chips, plastics, potash, rubber, salts, sugars, and various ores.

How a Rotary Dryer Works

Rotary dryers are often used to process bulk materials and powders, fed continuously into the dryer's drum, along with hot air. A rotary dryer's drum rotates, using lifting flights to carry particles upwards. Near the drum's top, particles come off the lifting flight and fall through a heated air stream.

The rotary dryer's drum is at a slight incline, fed from the higher end of the drum and discharging from the bottom part. Hot air or another heated gas blows counter currently or concurrently, and as the drum rotates, material is pulled in the direction in which the drum is revolving. Once it reaches a certain angle, material falls back to the bottom of the drum, with most drying action happening as it falls through the heated air or gas.

Rotary Dryers: Direct vs. Indirect Heating 

Heating in rotary dryers occurs either directly or indirectly. Direct drying happens when material comes into contact with heated air or gasses, whereas indirect drying happens due to contact with a drum that's externally heated. Normally, a combustion chamber is found in directly heated rotary dryer designs, in order to stop material from coming into direct contact with the burner's flame. Indirect drying is the less efficient means for removing moisture from material during processing, though this method is used for certain niche applications for which it performs better than direct rotary dryers.

A wide range of applications use rotary dryers that heat directly for processing bulk solids like minerals, mined ores, fertilizers and chemicals. In this direct configuration, material is heated via convection. For applications where materials must not make contact with heated gases, rotary dryers that utilize indirect heating should be used. Additionally, if a very narrow particle size distribution is necessary, directly drying the material would cause it to get trapped within the airflow. For this reason, adsorbents, catalysts and other specialty materials require rotary dryers that employ indirect drying methods.

Chemical Processing Applications for Rotary Dryers

Dry slurries and pastes use rotary dryers in mining activities to help chemically remove the ores from the rock they're found, their robust nature and large capacities make them particularly useful for handling abrasive materials. They're also used to dry and mix the chemicals used in making pharmaceuticals. Along with manure drying, rotary dryers can also be used to process fertilizers like DAP (di-ammonium phosphate) and NPK (nitrogen, phosphorus, and potassium) fertilizers of various ratios.

In chemical processing operations, rotary dryers help with:

• Augmenting handling, as dried chemical ingredients, are easier to convey.
• Forming material into specific textures, such as achieving maximal particle surface area in catalysts.
• Lowering transportation costs by removing liquids from materials.
• Meeting necessary requirements for post-processing of materials.
• Producing agglomerated chemical arrangements.
• Removing toxins in the material being processed
• Stabilizing and conserving solid materials.

Rotary drums play an essential role in chemical processing, where they serve not only as dryers but as coating drums, coolers, granulators, kilns, and other purposes. In fact, they play such an important part in many chemical processing plants that on the very rare occasions when they require maintenance, the whole plant needs to shut down. Because of their important role, rotary dryers for the industry require special designs that enable them to deal with factors common in chemical processing, such as abrasion, corrosion, and materials building up.

Abrasion

While chemical processing often involves handling abrasive materials, abrasion is further heightened by the rotary dryer's drum continuous revolutions. For this reason, rotary dryers for the chemical processing industry often use abrasive-resistant materials, along with liners. Additionally, many equipment manufacturers suggest reinforcing areas that experience more wear, like discharge and feed chutes.

Corrosion

The most common issue when working with chemical compounds involves corrosion. Many chemicals corrode materials relentlessly, while others do so under certain conditions. Minimizing the effects of corrosion is a requirement for most rotary dryers designed for the chemical processing industry. Depending on the type of drum used, special alloys or superior grades of steel are often used within a rotary dryer's construction. In cases where the material only becomes corrosive at when it has a high moisture content, stainless steel or similar corrosion-resistant alloys are used for the rotary dryer's inlet section, while the remainder is built from carbon steel. Sometimes protective coatings made from acrylics, epoxy polymers, latex or urethane are used instead, or in addition to these other strategies. Non-heating drums for coating or granulating will generally use various types of liners to resist corrosion.

Material Buildup

Regardless of the purpose for which it's used, buildup of materials in a rotary dryer's drum is often a concern, especially when working with hygroscopic or high-moisture materials. Material buildup can lead to serious issues during processing, depending on the application for which a dryer is used. This may increase the risk of fire, reduce the machine's efficiency, impede material flow or even damage the drum internally as larger chunks break away.

There are, however, ways in which rotary dryer design can discourage this buildup. For non-heating drums, linings that also protect against abrasion and corrosion are effective, while cast iron plating is also sometimes utilized. Rotary dryers can also incorporate internal knockers into the drum design, which work to dislodge buildup as it rotates. In certain cases, rotary dryer construction may include polished stainless steel to help prevent sticking.

Another way to limit buildup involves reconfiguring airflow, especially for rotary dryers used as kilns. For material that tends to clump, fitting a trommel screen to the lower end of the drum where material discharges will help break up clumps as material exits. Flight design that expends less force can also mitigate material buildup while placing advancing flights close to the inlet allows the material to quickly leave the area where the material is discharged to discourage buildup.

Advantages of Using a Rotary Dryer

Though other types of dryers have been used to dry bulk materials ' including fluid bed dryers and flash dryers ' rotary dryers are still largely preferred for industrial processing.

Advantages of rotary dryers include: 

• Customization: The wide range of designs, sizes, and materials from which rotary dryers can be made allows them to be easily customized to suit the material and processing objectives.
• Longevity: Due to their simple yet robust design, they can operate for decades without trouble when properly maintained.
• Polishing: When working with granulated materials, the rotary dryer's rolling action helps round off edges on the particles, making them more 'polished.'
• Reliability: With fewer moving parts and hardy construction, rotary dryers are known to be incredibly dependable machines.
• Throughput: The high capacity to process bulk solids makes them ideal for many industries, with output ranging from 1 to over 200 tons per hour.
• Versatility: Feedstock often varies when processing bulk solids, and the rotary dryer can tolerate variations in both feedstock and the conditions under which it's processed; this makes it more capable of dealing with changes in moisture content, particle size distribution, and throughput.

Rotary Dryers by Heyl Patterson

Heyl Patterson Thermal Processing makes rotary dryers that are among the most versatile on the market. Used as workhorses in processing bulk and powdered solids, our rotary dryers can handle a wide range of materials, from liquid sludges to fine powders. Additionally, Heyl Patterson rotary dryers can be set for starting and finishing moisture content, retention time, air velocity, and temperatures for the product and the air used in drying.

Our rotary dryers are ideal for processing: 

• Biomass
• Carbon fibers
• Fertilizers
• Hemp 
• Inorganic Chemicals
• Minerals
• Pulp for paper

Our rotary dryers can also be customized for various cooling designs, including air and water cooling, countercurrent air sweeping, or internal and external water cooling. To learn more about our rotary dryers, contact the experts at Heyl Patterson.

For evaporating moisture from concentrates or other products from plant operations, Rotary Dryers are designed and constructed for high efficiency and economy in fuel consumption. Whenever possible to apply heat direct to the material to be dried, Rotary Dryers of the Direct Heating Design are used. If it is not possible to apply heat direct to the material to be dried, Rotary Dryers of the Indirect Heating Design can be furnished so that the heated gases will not come in direct contact with the material.

Sizing a Rotary Dryer using a Capacity Table

Rotary Dryer is a simple, inexpensive unit for reducing the moisture content of flotation concentrates, as well as chemical and industrial products. Frequently the saving of shipping weight so effected will pay for the dryer in a few months. Difficulties from freezing while in transit are also eliminated. Many industrial projects are now using Dryers for control and production purposes on many materials.

Three main types of Rotary Dryers can be supplied. The direct heat unit is used when it is permissible for the drying gases to come in direct contact with the material being dried. Partition plates increase the heating surface. Drying may be by hot air or exhaust gases from other operations. If this drying gas has a deleterious effect on the product, then an indirect type of dryer can be supplied. A further derivation is the Tedrow Steam Dryer.

Rotary Dryer Capacity Table

ROTARY DRYERS, Direct-Heat Design

Size of Dryer

Cap. Tons per 24 Hours

Dimensions

Bentonite Dryer vs. Conventional Dryers: Which Is Best?

R.P.M.

Further reading:
Maximize Efficiency: Tips for Choosing a Bentonite Dryer

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H.P. of Motor

Shipping

Weight Lbs.

Motor Drive

LWH24'dx15''6'3'8'7'9'7½312,'dx20''6'3'8'8'2'7½313,'dx20''0'4'10'8'8','dx30''0'4'10'9'6','dx30''0'5'8'10'0'3½7½19,'dx40''0'5'8'10'10'3½7½22,800

Capacity based on 10% moisture, wet weight, in material fed to dryer, and 3.26 pounds of moisture evaporated per hour for each cubic foot of the volume of the main cylinder.

ROTARY DRYERS, Indirect-Heat Design

Size of Dryer

Cap. Tons per 24 HoursDimensions Outside BrickworkR.P.M.H.P. of Motor

Ship. Wt. Lbs.

Motor Drive Iron Work Only

LWH24'dx15''0'11'9'7'7'7½312,'dx20''0'11'9'8'0'7½313,'dx20''0'12'9'8'6','dx30''0'12'9'9'4','dx30''0'13'9'9'10'3½7½19,'dx40''0'13'9'10'8'3½7½22,800

Capacity based on 10% moisture, wet weight, in material fed to dryer, and 3.26 pounds of moisture evaporated per hour for each cubic foot of the volume of the main cylinder.

NOTE: The customer furnishes and erects the brickwork setting for the dryer in accordance with drawings furnished by Equipment Company.

ROTARY DRYERS, Tedrow Steam Design

Size

Cap. Tons per 24 Hours

Dimensions

R.P.M.

H.P.

Approximate Ship. Wt. Lbs.

Cyl. Dia.LWHBeltMotorNo. '11'0'4'8'3'10'No. '15'0'5'0'4'4'No. '19'6'6'0'5'6'2½7½

Capacity based on 10% moisture, wet weight, in material fed to dryer, and 3.26 pounds of moisture evaporated per hour for each cubic foot of the volume of the main cylinder.

Rotary Dryer Direct Heat and Indirect Heat Type -Capacity and Sizing Table

Rotary Dryer ' Kiln

Of the different types of dryers that there are the most common is the ROTARY DRUM DRYER/Kiln, This type of drier is common not only in the mining industry but you will find them in fertilizer plants, Cement plants, and peat hogs to name but a few.

The theories behind these machines are very simple, heat an air space up, and then tumble the material to be dried through this space until it is dried. All though it sounds simple there are problems that have to be solved before the required results are met. But first, so you know what we are talking about lets go through the design of a drier.

First is the KILN, this provides the heat, The BURNER is inside this portion. The fuel for the burner is usually diesel although heavy crude oil could be used in some cases. To be able to generate enough heat to dry the concentrate air must be added by way of a BLOWER.
In front of the kiln is the point that the wet concentrate enters the drier. It is put into the revolving SHELL. The shell is on a slight incline. As the Concentrate is tumbled through the hot air mass of the drier it travels down this incline to the exit of the drier.

At this exit point the concentrate is either deposited straight into a storage area or taken to the storage area by a conveyor. It is also at this point that there is an EXHAUST HOOD. This provides a controlled escape passage for the fumes and water vapor that is generated by the concentrate drying. This is a very important function and the operator will have to be sure that it is open at all times. If it should become blocked the water vapor will not be able to escape. The concentrate will become wet and sticky which will result in the discharge plugging. The wet sticky concentrate will also lower efficiency level of the drier for an extended period of time.
This happens because inside the drier shell are what are termed FLIGHTS these are flat pieces of metal that are bolted onto the shell.

 

They are there to lift the concentrate up to the top of the shells rotation and drop the concentrate through the hot air. If the water vapor isn't taken away, the concentrate becomes sticky from reabsorbing the water.
This sticky concentrate will fill the spaces between the flights.

The concentrate will not be lifted and dropped through the hot air. This results in a long term condition of poor performance even after the initial problem has been cured. These flights will remain buried in concentrate.
This removal of the water vapor is one of the functions of the blower. It assists the natural process of air movement as the hot air mass expands.
To prevent the buildup of concentrate on the flights there are often CHAINS attached to them. As the drier revolves the chains slap the flights preventing concentrate from building up on dryer's walls.

Outside of the drier shell as well, we use hammers to knock the cake clean.

These have the same purpose as the chains.  As the drier revolves, the hammers (formed by grinding balls) will fall against the side of the drier as it passes the top of the drier's revolution.

The drier shell is rotated separately from the stationary kiln section. To achieve the rotation a BULL GEAR is attached around the shell section. There are also two flat rings attached to the shell. These provide surfaces for support rollers to roll on. There is another problem that the inclined shell has, the incline causes the shell to want to slide in the direction of the incline. To prevent this additional rollers are attached to the last set of rollers.