Introduction
Before we discuss the factors that determine Filter Press cake percent solids and percent moisture, let's look at how filter cakes are formed from slurry.
A slurry is a mixture of solid particles suspended in a liquid. Mineral and aggregate slurries are most often particles suspended in water.
When the solid particles in a slurry are separated from the water, a solid cake can be formed. The concentration of water and solids in the slurry or in the dewatered cakes can be described as percent solids or percent moisture as shown below.
The relationship between percent Solids and percent Moisture is described in the following simple equation:
If one percentage is known, it is easy and simple to calculate the other.
How to measure the concentration of a slurry or cake
An easy way to measure the concentration of a slurry or cake material is simply to dry a sample in an oven with sufficient heat to evaporate all the moisture. The procedure is quite simple and includes the following steps:
- Weigh an empty drying container on a balance.
- Weigh the wet sample on a balance.
- Dry the sample in an oven.
- Weigh the dried sample on a balance, then return the sample to the oven.
- Continue drying the sample and checking the dried weight until no further weight loss is observed.
- Calculate the sample's wet weight and dry weight by subtracting the container weight from each.
- Calculate the percent solids with the following formula: Sample Dry Weight ÷ Sample Wet Weight x 100 = Percent Solids.
- Calculate the percent moisture with the following formula: 100% - Percent Solids = Percent Moisture.
For this discussion, it is assumed water is the liquid in the slurry mixtures.
Filter Presses dewater slurry material into drip-free cakes (shown above).
Primary factors influencing the Filter Press cake percent solids or percent moisture
Many factors will determine how much liquid can be separated from a slurry to create a dewatered filter cake using a recessed/membrane plate Filter Press. Some of the primary factors include:
- Material Particle Size Distribution (PSD)
- Material composition
- Filter Press feed pressure
- Filter Press filtration time
- Additional dewatering with cake dry and membrane squeeze
1. Material particle size distribution and shape
Mineral and aggregate particles in all slurries are composed of varying sizes and shapes. Particle size measurements are made to determine the number of particles at various sizes. Particle shapes can be visually evaluated to determine roundness, flatness or irregularity.
In general, coarse particle materials with uniform round or irregular shapes will dewater more easily. Coarse particles will tend to have more void space between individual particles, allowing water to more freely flow between them.
Likewise, round particles or irregular shaped particles tend to create more void space between them and therefore are more easily dewatered.
Consider marbles packed together in a glass jar then filled with water. The water flows quickly between the marbles until the jar is full.
Likewise, if the water is drained from the marble jar, it will flow freely out of the jar with very little water remaining.
Finer particle sized materials with flat or uniform shapes will be more difficult to dewater. Fine particle materials, especially those with flat or uniform shapes, can pack tightly together as water is removed, creating limited void spaces between them for water to flow.
When water is added to a jar filled with fine sand particles, an extended period of time is required to fill the jar because the water has to penetrate all the small spaces between the particles.
If the water is removed from the jar of sand, only a limited amount would flow freely out of the jar due to the limited space between the particles. A fine layer of water tends to adhere to the surface of all particles to some degree. For a given volume of particles, the fine particles will have significantly more surface area and more retained moisture compared to larger particles.
2. Material composition
Different material compositions can have a significant effect on how much water can be removed from a slurry using plate Filter Press dewatering. All particle surfaces exhibit some level of hydrophobic (water repelling) or hydrophilic (water adhering) characteristics based on their composition.
For example, coal particles tend to repel water and therefore are more easily dewatered to lower percent moistures.
Dewatered filter cake from coal tailings.
On the other hand, clay particles tend to be more hydrophilic (water loving), making them much more difficult to dewater.
Organic materials tend to be much more difficult to dewater because they typically have water molecules chemically bonded to their structure, making it nearly impossible to remove mechanically. Many naturally occurring mineral and aggregate slurries contain some organic material, which can influence how much water can be removed using a plate Filter Press.
3. Filter Press feed pressure
Recessed and membrane plate Filter Presses are designed to operate up to a maximum amount slurry feed pressure. In general, higher slurry feed pressure will result in faster dewatering rates, i.e., lower cake percent moisture.
Today, most Filter Presses are designed for low pressure, up to 100 PSI slurry feed pressure, or high pressure, up to 225 PSI slurry feed pressure. When working with material that is very difficult to dewater, very high pressure Filter Presses have been designed up to 500 PSI or more.
4. Filter Press feed time
Plate Filter Presses operate in a batch mode rather than on a continuous basis. The final cake moisture is very dependent on the length of filtration time, but the dewatering rate in the Filter Press changes dramatically as the filtration cycle is completed.
At the beginning of the Filter Press filtration stage, the slurry is pumped quickly into the void spaces between the clamped filter plates. The slurry solids are captured on the filter cloths attached to the plate surfaces.
As the cake material becomes dewatered, the filter plates become packed with solid material from the cloth surface back to the center of the cakes, making it more and more difficult for dewatering to occur.
An example dewatering curve is shown in the chart below with cake moisture plotted versus time. Initial dewatering rates are very high, but these quickly slow as the filter cake material begins to build up on the filter cloths.
Reasonable dewatering rates continue for some time, then the rate is very low with little gained in percent moisture reduction over long periods of time. When the slower dewatering rate period is reached, the filtration is stopped at an acceptable level to discharge the cake and allow another cycle to begin.
5. Additional dewatering with cake dry and membrane squeeze
Once the filtration period has been completed, there are two other steps that can be used to further reduce the percent moisture in the filter cake:
- Cake dry
- Membrane squeeze
Both of these steps occur after the slurry feed pump has been stopped; therefore, the throughput rate of the Filter Press becomes zero for the balance of the batch cycle.
Cake dry
Cake dry involves pushing compressed air through the cakes while they are still being held between the filter plates. The filtrate top corner discharge pipping is used to force compressed air into the top of the filter plates. Compressed air is directed down through the cake material, forcing additional moisture out of the cake through the bottom corner filtrate discharge pipes.
The filter plates are configured with special cake dry porting that allows the compressed air only to exit the filter plates after passing through the formed cakes.
The majority of moisture removed with cake dry happens very quickly when the cake dry is initiated and is shown in the dewatering example below. Further moisture reduction occurs very slowly with the continuation of compressed air passing through the cake.
Cake dry is typically used when the cake material is very porous and when very low cake moisture levels are desired.
Membrane squeeze
Similar to the cake dry step, the membrane squeeze step is used to remove additional water from the filter cakes after they have been formed between the filter plates.
Membrane plates have flexible surfaces that are expanded with water or compressed air. The expanded plate surfaces compress the cakes, forcing additional water to be removed.
The amount of reduced moisture achieved with membrane squeeze is very dependent on the cake material compressibility. Typically, the more the material can be compressed, the more moisture can be removed.
Once the membrane squeeze step occurs, no additional moisture is removed from the cake material with additional squeeze time. An example of a membrane squeeze is shown the in dewatering curve below.
How much liquid can be removed from the slurry largely depends on the characteristics of the solid material within. Feed pressure and feed time also have an effect on dewatering.
In addition to process and design factors like feed pressure and filtration time, the addition of a cake dry or membrane squeeze step can further reduce the percent moisture of the filter cakes.
