This section on crystallisation and evaporation is based on IFS Proceedings 780, (2016), by K. Schooley, V. Bourgier and R. Lawson, Veolia Water Technologies, Plainfield, Illinois, USA

It comprises these pages:

Crystallisation – Introduction
Evaporation and crystalliser processes
Types of crystallisation equipment and equipment configurations

1. Introduction
2. Evaporation and crystallisation applications in the fertiliser industry

Introduction

Evaporation and crystallisation technologies are often essential components in fertiliser production processes. Effective performance of these technologies is critical to productivity in various aspects of plant operations, including;

• Product quality.
• Energy efficiency.
• Reliability and robustness.
• Safety and environment.
• By-product management.
• Plant integration.

When considering new or expanded evaporation and/or crystallisation capacity in a commercial facility, investment in detailed process development with attention to all of these areas will pay long-term dividends. Establishing the most effective design can be a complex task, and there are often numerous process design options to be considered.

Evaporation and crystallisation applications in the fertiliser industry

There are many applications in the fertiliser market that involve evaporation and crystallisation techniques, either to make fertiliser products from a raw material feedstock, or to recover valuable by-product fertiliser from waste streams. A typical NPK diagram showing processes that involve evaporation or crystallisation as part of the production scheme is shown in Figure 1.

There are many examples of fertiliser production processes involving evaporation and crystallisation, such as the following:

• Ammonium sulphate: Ammonium sulphate crystals can be made using pure ammonia and pure sulphuric acid using reactive crystallisation, or produced by evaporative crystallisation of a dilute ammonium sulphate stream (i.e. using a by-product from caprolactam production, coking, sulphuric acid gas scrubbing, nickel / cobalt production or recovery of waste nickel). 
• MAP/DAP: due to increasing water scarcity and the drive for more efficient fertilisation practices, there is a rise in demand for water soluble fertiliser products. As a result, the demand for water soluble fertilisers such as crystalline mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP) is on the rise. These products can be made via reactive crystallisation using technical grade quality phosphoric acid and pure ammonia. Opportunities also exist to make these products using less expensive green phosphoric acid or raffinate.
• Potash: cooling crystallisation is used extensively to produce potassium chloride (KCl or potash) from solution mining or it is used to make a higher grade KCl product from conventional mining. Many of the potassium deposits contain sylvinite (a double salt of NaCl and KCl). Therefore, sodium chloride removal from the process via the use of crystallisation can be a major portion of the process plant required to recover the valuable potash material.
• SOP: Potassium sulphate (SOP) demand is increasing due to its greater use (and higher selling price than potassium chloride) as a fertiliser on high value crops such as grapes or almonds. This can be made via multiple approaches, often involving crystallisation steps.
• Nitrate-based fertilisers: evaporation is used to concentrate ammonium nitrate and to produce products such as potassium nitrate and calcium nitrate.
• Phosphoric acid: evaporation is used to concentrate dilute phosphoric acid into a sellable product, or as raw material for phosphate fertiliser production.
• Calcium chloride: calcium chloride waste streams generated from the phosphoric acid process can be concentrated using evaporation technology to generate a saleable product.

Many of these processes traditionally start with pure raw products. However, economic pressure is encouraging companies to use lower quality (less expensive) raw materials or to use a different processing path altogether. There are also instances where a fertiliser is produced via crystallisation from a by-product or waste material taken from another manufacturing process.

Table 1 and Table 2 list some of the products and by-products of fertiliser production that are typically made using evaporation and crystallisation processes.

Links to related IFS Proceedings

129, (1972), Unit Operation of Bulk Crystallisation, A Waller.

294, (1990), Granular Ammonium Sulphate – Past, Present and Future, A M Brown, L A Hollingworth, M Fischbein.

380, (1996), Ammonium Sulphate: An Innovative Process for Production, G N Brown.

433, (1999), Fertiliser Projects: Planning, Financing and Engineering from Conception to Birth, J A Hallsworth, C Fawcett.

743 (2014), Environmental Constraints on New Plant Construction in the USA, K Ruthardt.

780 (2016), Evaporation / Crystallisation Challenges in the Fertiliser Market, K. Schooley, V. Bourgier, R. Lawson.