Specialty Emulsions

Emulsifying Equipment

Basic equipment to prepare an emulsion includes a high-speed, higher shear mechanical device (usually a colloid mill) to divide the asphalt into tiny droplets. Also needed are an emulsifier solution tank, heated asphalt tank, pumps, and flow-metering gauges. The colloid mill has a high-speed rotor that revolves at 17-100 Hz (1,000-6,000 rpm) with mill-clearance settings in the range of about 0.25 to 0.50 mm (0.01 to 0.02 in.). Such settings yield emulsions with asphalt droplet sizes smaller than the diameter of a human hair [about 0.001 to 0.010 mm (0.00004 to 0.0004 in.)]. There is a slight variation in mill clearance settings and, thus, asphalt droplet sizes depend upon the equipment used. Some emulsion mills have fixed clearances with no latitude for variation.

Separate pumps are used to meter asphalt and the emulsifier solution into the colloid mill. Because the emulsifier solution can be highly corrosive, it may be necessary to use a pump made of corrosion resistant materials.

A schematic diagram of a typical asphalt emulsion manufacturing plant is shown in Figure A

In the general method of emulsifying asphalt, concurrent streams of molten asphalt cement and treated water are directed by pumps into the intake of the colloid mill. The asphalt and emulsifying water are subjected to intensive shear stresses as they pass through the colloid mill. The newly-formed emulsion may then be pumped through a heat exchanger. The excess heat is used to raise the temperature of the incoming emulsifying water just before it reaches the colloid mill. From the heat exchanger the emulsion may be pumped into bulk storage tanks. These tanks sometimes are equipped with some type of stirring device to keep the product uniformly blended.

Heated asphalt cement, the base of the asphalt emulsion, is fed into the colloid mill where it is divided into tiny droplets. At the same time, water containing the emulsifying agent is fed into the colloid mill. The asphalt, as it enters the colloid mill, is heated to ensure a low viscosity, and the water temperature is adjusted accordingly. These temperatures vary; they depend upon the emulsification traits of the asphalt cement and the compatibility between the asphalt and the emulsifying agent. Extremely high asphalt temperatures are not used because the temperature of the emulsion leaving the mill must be below the boiling point of water.

The method of adding the emulsifier to the water varies according to the maker's procedure. Some emulsifiers, suck as amines, must be mixed and reacted with an acid, such as hydrochloric, to attain water solubility. Others, such as fatty acids, must be mixed and reated with an alkali, such as sodium hyroxide, to attain water solubility. Mixing is most typically done in a batch mixer. The emulsifier is introduced into warm water containing acid or alkali and agitated until completely dissolved.

Asphalt and emulsifier solution must be proportioned accurately. It is done by monitoring the temperature of each phase and the mill discharge, or with meters. If the temperature regulation method of proportioning is used, the outlet temperature of the finished emulsion is calculated from the temperatures of the various emulsion ingredients

Asphalt particle size is a vital factor in making a stable emulsion. A microscopic photograph of a typical emulsion reveals the following average particle sizes:

• Smaller than 0.001 mm (1 µm).....28 percent
• 0.001-0.005 mm (1-5 µm).............57 percent
• 0.005-0.010 mm (5-10 µm)...........15 percent

These microscopic-sized asphalt droplets are dispersed in water in the presence of the chemical surface-active emulsifier (surfactant). The surfactant causes a change in the surface tension at the contact area between the asphalt droplets and the surrounding water, and this permits the asphalt to remain in a suspended state. The particles, all having a similar electrical charge, repel each other, which also aids in their remaining in a suspended state. Figure II-2 is a photomicrograph showing the sizes and distribution of the asphalt particles.