The responsible manufacturing of fertilizer to boost agricultural production and crop yields provides worldwide benefits for increased food production.
Fertilizer is a natural or manufactured material which contain at least 5 percent of one of the three primary nutrients – Nitrogen (N), Phosphorous (P), or Potassium (K). N, P, and K, in different chemical forms, are key micronutrients for almost all the living species, especially for enhancing agricultural crops.
Inorganic fertilizer, and organic manure, are the two most important sources of plant nutrients. Mineral fertilizers supply the outstanding nutrient balance needed after manure and crop residues are used, and in most parts of the world, the balance supplied by mineral fertilizers is substantial.
While nitrogen is the largest fertilizer industry, phosphorus is the second largest, and potassium is the smallest. As evidence of its size, the main natural phosphorous deposit has more than 270 million tons of mined apatite (calcium phosphate with other elements). Phosphorous is mined from naturally occurring mineral deposits that were once sediments on the bottom of ancient seas.
Rock phosphate is the raw material used for the production of most commercial phosphate fertilizers. Ground rock phosphate was once applied directly to acid soils. However, due to low availability of phosphorous, high transport costs, and low crop responses, very little rock phosphate is currently used in agriculture.
Phosphate rock processing consists of the separation of phosphate from the mix of sand, clay and phosphate that makes up the matrix layer. Phosphate grade is improved through beneficiation. The beneficiated rocks can be used as a P source as they are, but the majority goes to phosphoric acid plants. Phosphoric acid is used as a raw material for the production of mixed fertilizers. The acid can also be further purified and used for food acidulants. A simplified phosphate flow sheet is illustrated below.
Equipment Choices for the Reliable Production of Phosphoric Acid
In phosphoric acid production, separating the valuable liquid from the waste solids is a critical process. The effort to optimize the efficiency and operational costs in this part of the process adds significant value to the acid at the end.
Thickening, clarification, and filtration are integral to the production of phosphoric acid. The equipment selected and the sizing depends on the customer’s process goals, feed slurry composition (e.g. - particle size distribution, slurry density, etc.), and the total cost of ownership over the life cycle of the equipment.
In phosphoric acid production, thickeners are used to concentrate the phosphate rock slurry before reacting it with acid. Due to the nature and properties of their mineralogical characteristics, phosphate can be thickened to paste consistencies, which reduces water content and saves on acid costs.
Achieving a denser underflow reduces the loss of water, provides more valuable liquid off the top of the thickener, and reduces the amount of sulfuric acid needed.
WesTech has worked with phosphoric acid plants looking for high performing thickeners and discovered that both the HiDensity™ and Deep Bed™ paste thickeners were successful in creating higher density underflow solids to reduce water loss or water use for phosphate processing.
In Morocco, a WesTech paste and thickened tailings system was put in place to dewater the phosphate rock product. When transporting the rock by slurry pipeline, the WesTech Deep Bed paste thickener was successfully used to produce a higher density, yet still pumpable underflow. The reduction in water content provided significant savings on sulfuric acid costs in the phosphoric acid production and reduced water sent to the tailings by as much as 50 percent.
Clarifiers remove fine gypsum from the phosphoric acid product to ensure a pure liquid for later production steps. The primary goal of the clarifier is to evenly distribute the feed in the tank so the liquid-solids separation takes place across the entire tank.
There are several important factors in determining the correct clarification at phosphoric acid plants:
- The ideal settling rate of solids particles helps to determine optimum production rates of the acid. A flocculant may need to be added to reduce the time required to remove phosphoric acid impurities.
- Slurry characteristics and plant preferences will determine the construction of the clarifier, from rubber lined tanks with abrasion resistant carbon bricks to exotic stainless-steel compounds.
- More effective clarification removes impurities faster, resulting in a higher quality phosphoric acid and a better downstream fertilizer product.
For both thickeners and clarifiers, a properly-designed feedwell can dissipate energy and evenly distribute the incoming feed.
It’s critical that both thickeners and clarifiers are efficient and cost effective. WesTech’s innovative EvenFlo® Feedwell and MudMax™ bed level instrument help thickeners and clarifiers perform at their best. The patented EvenFlo feedwell ensures heavy flow into the clarifier or thickener tank across a wide range of flow rates, controlling the velocity of the water entering the feedwell and producing even flow and uniformity. The MudMax is a solution for reliable bed level measurement, helping the thickener or clarifier produce consistent underflow characteristics.
With filtration in phosphoric acid plants, there is a trend towards more and larger belt filters taking a greater share of filtration duties. Belt filters separate the solids from liquid, primarily used in the dewatering of sludges in the chemical industry, mining, and water treatment.
Compared to a tilting pan or a table filter, horizontal belt filters provide the highest filter performance for the lowest initial cost. These are part of the preferred filter systems in phosphoric acid production because they take into account the unique conditions found in the plants, which are:
- High temperature (up to 105 degrees C for HH filtration)
- High acidity (40 percent P2O5 )
- High scaling environment (depending on rocks carbonate content)
- High hydraulic capacity (up to 3 m3/hm2 filtering surface - avg.)
- High washing efficiency
- High maintenance required
- Maximize P2O5 recovery
The robust design of WesTech’s horizontal belt filters provides operational flexibility, large throughput, and the ability to handle corrosive slurries, like phosphoric acid. They are also best for the large filtration area and efficient washing necessary in phosphoric acid production.
Reducing Phosphoric Acid Wastewater Disposal Costs
Wastewater that is recovered from the gypsum stack contains environmentally-challenging contaminants including residual phosphoric acid, hydrofluoric acid, fluosilicic acid, and heavy metals. WesTech’s two-stage neutralization system removes these contaminants, yielding usable plant water including a small solids effluent that offers much easier and less expensive disposal.
Treatment of any wastewater discharge, which may include provisions for gypsum stack closure, must include flouride removal and acid neutralization prior to discharge. The high-density sludge (HDS) process is a proven technology developed for the neutralization of acidic wastes and precipitation of metals from industrial discharges.
WesTech has experience with the application of the lime treatment systems using the HDS process for metals removal from both acid rock drainage and other industrial acidic wastewaters. Using both laboratory bench scale testing and the experience of a full-scale installation, WesTech engineers were successful at targeting the fluosilicic acid by applying that same HDS process to the treatment of gypsum stack wastewater (World Fertilizer Magazine March 2019 (PDF)). The fluosilicic acid neutralization is an important part of achieving maximum reaction rates, minimum effluent contaminant concentrations, and maximum lime utilization in the phosphoric acid production process.
Choose a Supplier that Knows the Industry
Phosphate is a key nutrient for raising healthy crops. To make the best use of that nutrient, transforming it into phosphoric acid for fertilizer production is necessary to efficiently feed a growing world population. However, phosphoric acid production has several unique challenges, including handling a highly caustic acid. Special care in the thickening, clarification and filtration phases of the process is needed.
To meet the difficult challenges of producing phosphoric acid, such as creating a high quality, low-cost product with the fewest impurities, WesTech’s reliable liquid-solids separation equipment ensures minimal operating and maintenance costs. Denser underflow of the thickener, even distribution in the clarifier, and larger filtration area of the belt filter are key considerations for choosing process equipment in a phosphoric acid plant.