Flocculants for Sand and Gravel: Aggregate Industry Water and Wastewater Treatment with Flocculants and Coagulants
Water conservation, efficient handling of solid wastes, and lack of space for settling ponds are some of the problems currently faced by the aggregate industry. Population growth increases these problems by encroaching closer to aggregate producing operations. The trend of “not-in-the-city” operations evolving into “inside city” may continue due to population growth in the USA.
The expanding city phenomenon has had a considerable impact on many aggregate operations. Not only do operators now strive to be low-cost producers, but they also must be good neighbors to local populations which are quickly consuming available water and land resources. This combination of a lack of space and water has forced many operators to change production strategies. Better water and waste management play a critical part in this strategy change.
Past practice of large ponds to settle waste solids and recover water has, due to the problems as mentioned earlier, given way to smaller lagoons and lakes that utilize flocculants and coagulants to effect faster settling rates. These specially formulated chemicals maintain and improve water clarity at the same level or better than that of large untreated ponds. Operations that require even stricter control of water and waste solids resort to the use of clarifiers, thickeners, and belt presses. However, all of these unit operations need the application of flocculants and coagulants for cost-efficient operations.
The “art” of flocculation and coagulation has a lot more “science” than meets the eye. A variety of flocculants and coagulants is available, with different chemistries, molecular weights, charge ratios, and various other properties. It is critical that the appropriate flocculant or coagulant be chosen to ensure cost-effective performance in a solids/liquid separation process. The choice is dependent on process conditions, including variables such as feed particle size, water chemistry, system pH, percent feed solids, etc. This report discusses some of the essential topics about flocculants and coagulants and their application in aggregate operations.
WHY USE COAGULANTS AND FLOCCULANTS?
Aggregate operations produce fine clay-laden slurries while creating a variety of sand and gravel products. This effluent stream then travels to a large holding pond where the solids settle out, and the supernatant water is recycled back to head works. Successful water and waste management depend on the efficiency of the water clarification phase. This step is even more critical when trying to reduce the size of the pond or increase the settling rate.
Most naturally occurring fine particles, such as those produced at an aggregate operation, have a negative surface charge. This charge sets up repulsion forces that reduce the tendency for the particles to agglomerate and settle. However, other factors such as particle size, particle density, and liquid density also exert considerable influence on the tendency of fine particles to settle. Stokes law, shown below, can be used to estimate the time for suspended solids’ precipitation from a solution.
V= 2g r2 (d1 – d2 ) / 9η
V = final velocity of particle
r = radius of particle
d1= density of particle
d2 = density of liquid
η = coefficient of viscosity
g = gravitational constant
Using Stokes Law, settling times for particles of different sizes can be estimated and are shown above. These calculations are based on a spherical particle with a specific gravity of 2.0 at 25°C.
As expected, decreasing a particle’s size reduces its settling rate. The main area of interest in the aggregate industry is particles in the size range of “silt.” Settling times are on the order of 55 minutes per foot or 0.2 inches per minute. Since Stokes law assumes that the particle is settling freely, it is reasonable to expect that in conditions such as those typical to aggregate production, some particle-to-particle interference, or hindered settling, will occur, further slowing down the settling rate. To handle the same volume of effluent in a smaller pond, operators should increase settling rates. The only parameter that can be influenced, according to Stokes Law, to effect increased settling rates is an increase in the particle size. Coagulants and flocculants will increase particle sizes, resulting in faster settling rates.
Fine sand, silicaceous, and other mineral particles have a negative surface charge. To bring these particles together, these surface charges need to be neutralized. The process of charge neutralization and bonding of particles to form microfloc particles is called coagulation. Charge neutralization is achieved by the addition of a coagulant, which neutralizes the negative surface charge by its positive charge. Coagulated particles are then aggregated to larger particle sizes and settled by the addition of a flocculant.
Use of naturally occurring coagulants has occurred for many years. Natural coagulants such as starch, glue, guar gum, and sodium alginate were used in many industrial applications early in the twentieth century. However, these natural coagulants were replaced later by more effective, synthetic formulations.
Quarternized coagulants based on ammonia and ethylene dichloride displaced many inorganic coagulants. These new coagulants based on epichlorohydrin and dimethylamine led to a decline in the use of ammonia- and ethylene dichloride-based coagulants. The method of these new coagulants expanded rapidly applied.
Some of the above coagulants are toxic to fish and should not be used in applications involving rivers and streams. However, there are several coagulants, such as Tramfloc, Inc. potable water grade coagulants, that have been developed specifically for use in such applications. These products can be safely used in applications where there are open waterways. Many aggregate operations use these coagulants under closed loop water use conditions.
After the surface charges of particles are neutralized, and microflocs are formed, flocculants are often added to bring these fine particles together. Such flocculants are usually tailor-made for particular applications to provide a cost-efficient method of improving plant efficiencies.
A flocculant brings together coagulated particles into larger aggregates and settles the particle masses.
A typical flocculant is a long chain of carbon atoms. These polymers consist of many repeating units and have molecular weights varying from 2 to 30 million Daltons. Molecular weights of flocculants are significantly higher than those of coagulants which can range from 2,000 to 200,000 Daltons.
The molecular weights of both coagulants and flocculants can be controlled and modified during production. Many different functional groups are attached to the carbon chain to give flocculants differing properties.
A flocculant charge is either neutral, anionic, or cationic. Most aggregate operations use anionic flocculants which carry a negative surface charge. r. Most aggregate flocculants are copolymers of acrylamide and sodium acrylate. Acrylamide-acrylate copolymers are anionic due to the presence of negatively charged carboxylate groups in the polymer. The ratio of sodium acrylate to acrylamide in the polymer determines anionicity.
FACTORS AFFECTING COAGULATION
The factors that govern coagulation are particle size, surface charge, and water chemistry. The smaller the particle size, the higher total surface area per unit weight of solids, so smaller particles can require higher coagulant feed rates.
The density of the surface charge of clay particles has a direct bearing on coagulation. As the surface charge increases in density, more coagulant is required for surface charge neutralization. Coagulants must be judiciously applied since excessive feeding can result in particle dispersion instead of coagulation.
Dissolved cations such as Al+3, Ca+2, Mg+2, Fe+2, and Fe+3 neutralize surface charge and reduce the coagulant requirements. These ions increase the conductivity of the process water. Generally the higher the hardness of the water, the lower the coagulant demand.
FACTORS AFFECTING FLOCCULATION
Factors affecting flocculation include, primarily polymer type, ionic strength, water pH, slurry solids, flocculant dilution, shear, molecular weight, and process conditions.
The driving force for adsorption is dictated by the mineralogical properties of the suspended solids in addition to the aqueous environment. The significant molecular groups in these types of polymer are amide and carboxylate. It is a fact that both groups do adsorb onto the mineral surface. It is true that the carboxylate group on anionic flocculants is incapable of adsorption, but that polymer extension is its primary role in most situations.
Evidence suggests that the enhanced activity of anionic polymers is related to uncoiling or extension of the polymers.
The ideal flocculant for an application balances the need for adsorbing groups (amide group) with the requirement to extend the molecule (carboxylate group). The proper ratio of these two groups is critical to their performance and will determine the degree of anionicity.
The pH of the slurry is vital in flocculant selection.
At a pH of 4 and below, nonionic polymers show the most effective performance. At this pH range, anionic polymers are coiled up just like the nonionic polymer, but they show minimal performance. This performance occurs because anionic flocculants, the amide groups (found in the nonionic polymer) are replaced by the more inert (at low pH) carboxylate groups which reduce the number of hydrogen bonding sites available in the flocculant. Thus nonionic flocculants perform better at a lower pH range.
As the pH increases, the carboxylate groups of the anionic flocculant are ionized, and uncoiling continues until the polymer is fully uncoiled and active. In the same environment, a nonionic flocculant maintains its configuration. The activity of anionic flocculants increases with slurry pH, and at a pH range of 6 to 8, a moderately anionic flocculant will show better action than a nonionic flocculant. The performance of a nonionic flocculant is the same at this pH as in the lower pH range.
At pH levels over 9.5, highly anionic flocculants perform the best, since a highly anionic flocculant is fully uncoiled and at the peak of activity at high pH levels. When competing reactions (chemical [hydrolysis] vs. physical [adsorption]) are coinciding, a significant reduction in flocculant activity can occur.
When an unfolded, and utterly wetted flocculant is fed to a slurry, adsorption takes place rapidly. It is critical that the flocculant is thoroughly mixed in the feed solids. The concentration of solids and the feed particle size has a strong influence on the distribution of flocculant in the slurry. The higher the level of solids, the more difficult it is to distribute the flocculant uniformly through the feed. Therefore, when the particle size decreases and the solids concentration increases, flocculant demand increases.
Better results can be obtained by reducing flocculant solution strength. By diluting the neat flocculant, the flocculant dispersion in the sludge or feed is more uniform as polymer contacts more fine particles, and this improves system results.
Excessive shear or mechanical action can tear apart a floc. While the distribution of the flocculant in the slurry is essential, excessive agitation will destroy the floc.
Inter-particle bridging is a mechanism by which the flocculant functions, and it is not surprising that the molecular weight can affect the performance. In most clarifiers, high molecular weight results in better results, but this is not a hard and fast rule. Each application has some unique factors to be considered. Higher molecular weight flocculants are generally more viscous and are more difficult to disperse evenly in the slurry uniformly. Since adsorption is very rapid and usually irreversible, a loss in activity results. With increasing molecular weight, the number of polymer chains per unit of weight decreases as well. There may not be enough polymer molecules to flocculate all the solids in high solids slurries.
Some sand and gravel operations use ponds as the method of effluent treatment, and water recycling, scarcity of land space and limited water supply have accelerated the trend toward the use of mechanical solids/ liquid separation methods such as thickeners, clarifiers, and belt presses. These equipment systems require coagulants and flocculants for efficient results.
It is very beneficial to carefully choose chemical feed points because manual feeding of flocculants is often wasteful and inefficient. Such is noticeable when feed to holding ponds and clarification equipment frequently changes. It is challenging to match changing feed characteristics to chemical dosage. Constant adjustment of coagulants and flocculant fed rates often results in imprecise dosages and poor results.
To improve performance, there are automatic polymer feeding systems which are PLC controlled for the highest level of efficiency and dosage accuracy. Based on specific site conditions, custom-tailored systems can be designed to improve results and minimize chemical expenses. The use of these systems has usually resulted in a decrease of chemical usage by as much as 40%. Automatic polymer feeding systems decrease operator labor requirements and often improve the overall performance of the washing operation.
Water, space, and environmental challenges will continue to cause problems for aggregate producers. Flocculants and coagulants, when appropriately chosen and applied, can avoid these difficulties. Please contact firstname.lastname@example.org for any questions or technical support.
L. J. Connelly and P. F. Richardson, “Coagulation and Flocculation in the Mining Industry,” Symposium on Solids/Liquids Separation and Mixing in Industrial Practice, AIChE (Pittsburgh Section), 1984.