Filtration in Chemical Processes

Coffee filtration

Chemical processes often require the separation of compounds from a given system, whether in the manufacture of pharmaceuticals, in the synthesis of an inorganic compound, or in metal refining. Such separation depends on the purpose of the process and can range from distillation methods to column chromatography, for example. However, a simple and commonly used method is filtration.

What is filtration and what is it for?

Filtration is a process for separating solids and liquids using a porous filter that, depending on its specifications, can retain particles of different sizes. This method of separating mixtures is ancient, with its first uses dating back to 2000 BC, in a system made of porous limestone rock designed to purify water in the Egyptian region. Since then, with the advent of new technologies and the advancement of scientific knowledge, more efficient and specific filters for each process have been developed.

Moreover, filtration is not used exclusively in the chemical field — more general uses are easy to recall, such as coffee preparation and one of the stages of water treatment, for example.

Figure 1. Examples of everyday processes that use filtration. On the left, coffee preparation; on the right, a filtration tank used in water treatment plants. Sources: NaturalTec, Blog do Enem.

Types of Filtration

In general, there are two most common types of filtration: simple and vacuum. In metal refining, both are extensively used, and the choice of the ideal method depends on the type of substance being handled, the time available for the step, and other factors.

• Simple Filtration — a simple filtration system can be assembled with just filter paper, a glass funnel, and suitable glassware to collect the filtrate (usually an Erlenmeyer flask).

  
  

Figure 2. Simple filtration system.

The filter paper used also depends on the purpose of the method and can be either quantitative or qualitative. Quantitative filter paper is usually used when the mass of the separated residue is of interest for subsequent calculations, common in gravimetric analyses and quantitative chemistry. The second type is intended for general filtrations, in which the aim is simply to separate — and not quantify — the “paper + residue” system resulting from the separation. Additionally, both types of papers have different grammages, so it is important to consider parameters such as the sample’s hardness level and the expected filtration efficiency to make the correct choice. (Note: Coffee filters are not an alternative to laboratory filter papers, as they have holes along their “seam” that allow solids to pass through, making the filtration inefficient.)

It is also worth noting that this type of filtration is slower, as the liquid flows only by the action of gravity. However, changing the folding of the paper from the simple form to the fluted form increases the contact area between the solid and the separation interface, speeding up the process somewhat.

Figure 3. How to fold filter paper in simple and fluted forms, respectively.

• Vacuum filtration — unlike simple filtration, the vacuum filtration process relies not only on gravity but also on the vacuum created in the container below the filter. This creates a pressure difference between the external and internal environments, making the process much faster and leaving the filter paper drier at the end.

  
  

  In terms of setup, the apparatus is slightly more complex than that used for simple filtration. In this case, a Büchner flask (also called a filter flask), a Büchner funnel (placed over the flask), filter paper, a rubber adapter, a hose (used to connect the flask to the vacuum source), and a vacuum pump are needed. In some cases, it is advisable to use a gas washing bottle (or trap) to prevent liquids from entering the vacuum pump and affecting its operation. Thus, the trap is connected between the flask outlet and the vacuum source, as shown in Figure 4.

Figure 4. Vacuum filtration system without gas washing bottle (1) and with gas washing bottle (2).

General Precautions in the Filtration Process

As with any other process in the chemical field, it is essential to follow and verify certain parameters to ensure the efficiency of filtration and the ideal separation between filtrate and residue.

In vacuum filtration, it is necessary to ensure that the entire filter paper is in contact with the surface of the Büchner funnel, without bubbles or folds, as these interfere with sealing and prevent the ideal vacuum pressure. Also, the tip of the funnel should face away from the vacuum outlet to minimize the possibility of filtrate loss due to suction.

A common question in refining is whether filtration can be carried out with hot liquids, and the answer is no. It is recommended to filter only once the liquid is at room temperature; otherwise, acidic vapors can be generated and drawn into the vacuum pump (shortening its lifespan), and the acid can attack and react with the filter paper, causing clogging and slowing down the process.

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