Some slurries need either chemical conditioning or filter aids to improve filterability for acceptable dewatering results. Generally, slurries that require conditioning need to be treated regardless of the actual dewatering equipment used.
Filter presses can use a variety of chemical conditioners or filter aids and the selection of the appropriate conditioner is based on the customer’s process requirements. Chemical conditioners cause chemical changes in the slurry, which may not be acceptable for the overall process. Filter aids are inert materials that form a porous structure that the solids are trapped in and sometimes can be used on processes where chemical conditioning can not be used.
One of the major goals of the test program is to determine if any chemical conditioning or filter aid is required and, if required, how much is required.
The main criteria for selecting a conditioner are whether any “contamination” of the filtrate or the filter cake can be tolerated. If contamination of both the filtrate and the filter cake can be tolerated then chemical conditioners are used. If contamination of the filtrate can not be tolerated but contamination of the filter cake can be tolerated then filter aids must be used. If cake contamination can not be tolerated then no chemical conditioner or filter aid can be used.
A further complication for the selection is the preferred disposal method of the wastes produced. The filtrate and cake may need to meet strict disposal requirements that could limit the chemical conditioning options.
There are four types of chemical conditioners: coagulants, emulsion breakers, precipitants, and flocculants. Coagulants are used to form larger particles that can be precipitated or flocculated. Emulsion breakers are used to “break” very stable emulsions so the particles can be treated with a precipitant or flocculant. Precipitants cause relatively large, insoluble particles to form from the slurry. Flocculants will also form very large, insoluble particles to form but using a different chemical mechanism than a precipitant.
Common coagulants are ferric chloride (FeCl3) and alum (Al2(SO4)3), both are “inorganic chemicals” (organic and inorganic refer to whether the chemical is carbon based or not). The actual coagulating species are the ferric cation (positively charged species) and the aluminum cation respectively. These coagulants are also very acidic and will usually acidify a slurry when added. Acidification of the slurry will also help break emulsions so these two coagulants will also act as emulsion breakers. There are other coagulants, which are not emulsion breakers, that are sometimes. These are often “organic chemicals”.
Emulsion breakers are chemicals used to “break” emulsions. An emulsion is a dispersion of very small, colloidal particles that is apparently stable. Emulsions are “stabilized” by several different mechanisms. Any chemical that interferes with one or more of the stabilizing mechanisms destroys the stability of the emulsion or “breaks” the emulsion. Many coagulants are also very good emulsion breakers because coagulating the particles into larger particles destroys the stability of the emulsion. Sulfuric acid is also very good emulsion breaker because it changes the pH of the slurry and emulsions usually have a relatively narrow pH range where they are stable. The are other emulsion breakers available that do not affect the pH of the slurry.
Precipitants are usually inorganic chemicals added to the slurry to form large, insoluble particles that will settle (precipitating). The most common is hydrated lime (Ca(OH)2). It will raise the pH because it is a base (a chemical which neutralizes an acid). When the pH is high enough (usually between 11 – 12), the slurry solids will form large, easily filtered precipitates. Precipitants are often used with coagulants and emulsion breakers; however, they can be used alone on some slurries.
Flocculants are organic chemicals and are used to make large, filterable flocs. The precise chemical mechanism is totally different than with a precipitant. Many times a flocculent can be used alone or with coagulants, emulsion breakers, or precipitants (any or all may be used). The common term for flocculants is “polymer”, referring to the fact that these chemicals are very large organic compounds that are called polymers. Each polymer has three critical characteristics: charge type, charge density, and molecular weight. These characteristics vary independently.
Charge type refers to whether the polymer has a positive charge (cationic), negative charge (anionic), or no charge (non-ionic). A slurry will floc when the proper charge polymer is used.
Charge density refers to the number charges per unit length on the polymer for a cationic or anionic polymer. Non-ionic polymers, because they have no charge, do not have a charge density. The charge density can range from low to very high; polymer vendors are normally vague about the precise charge density measurements and usually refer to it in general terms.
Molecular weight refers to the size (primarily length) of the polymer. A polymer can have molecular weight from low to very high. Again, the polymer vendors are normally vague about the precise molecular weights.
Since all three properties can vary, it is necessary to test a range of polymers to identify the best combination of these properties to flocculate the slurry. A polymer vendor is usually able to recommend a range of their products that should work, reducing the amount trial and error work that needs to be done. If the results are not satisfactory, this gives a starting point for investigating other polymers.
When using polymers in combination with coagulants, emulsion breakers, or precipitants, the final slurry pH is critical. Most polymers work best a neutral pH (pH ~ 7). If it is necessary to be at relatively low or high pHs, the polymer vendor should be consulted to determine the appropriate polymers.
The primary limitation of inorganic chemicals is that they add more solids to the cake, which add to the solids to be disposed of. In addition, the cakes can not be incinerated. However, once the minimum amount of chemicals has been added to the slurry, excess chemicals do not normally adversely affect filterability.
The primary limitation of polymers is that they have an optimum dose and an effective dose range. Too much or too little polymer will cause poor slurry filterability. With some slurries, polymer conditioned cakes will be somewhat sticky and not release very well from the filter cloths.
Filter aids are inert, porous materials that give the liquid a path through the cake while trapping the solids. The best-known filter aid is diatomaceous earth (DE) though are numerous others. Filter aids are rarely used with filter presses because of the high relative dosages required compared to chemical conditioners and their relative expense.
Filter aids generally come in several grades that relate to the mean pore size in the grade. The key to effective use of filter aids is to match the mean pore size with the slurry particle size. This is not exact because particle shape will influence the optimum pore size for best filtration. Normally, the slurry particle size is not a major consideration when using chemical conditioners. Chemical conditioners tend to increase the particle size considerably.
Filter aids are normally used alone on the slurry.
Filter aids are made from a variety of materials including diatomaceous earth, perlite (from a volcanic rock), cellulose, and calcined rice hulls. While these materials are normally considered inert, one should confirm that this true, particularly when dealing with very reactive, chemically aggressive slurries.