Kaolin belongs to inorganic materials naturally occurring on Earth. Many of these minerals have unstable elements inside their crystal-like structures. Water content is one of those volatile constituents. And determining the kaolin moisture content has been one of the most crucial steps of the production process.

In this article, you can read and figure out more about kaolin, its types and quality, its water content, its production process (slurry), and its commercial-grade forms.  

Kaolin: Types and Quality

kaolin mineral illustration

a. The Types of Kaolin

In nature, kaolin falls into the following categories: 

  • Primary kaolin or residual kaolin, due to acid rocks (granite and rhyolite) or feldspar-rich rock weathering process. This mineral formation is abundantly present in situ environments.
  • Hydrothermal kaolin, as products of alkali feldspar minerals alteration by hot groundwater.
  • Secondary kaolin or sedimentary kaolin is an accumulated (redeposited) clay mineral deposit eroded residual kaolin. Stream, lake or basin are their usual formation environments. 

b. The Quality of Kaolin

By and large, some determinants for kaolin quality are its chemical and structural composition, brightness, and the shape and size of the particles. Industry applications add more quality characteristics like particle-size distribution, disorder and degree of crystallinity, and specific surface area (SSA).

This china clay also has other desirable features for various applications. Kaolin flour can be one the most common examples. Three categories to assess the quality of kaolin flour are water content (moisture content), residue, and brightness. 

  • Kaolin water content is better to be less than 0.02 (2%). The lower the level, the higher the quality of kaolin.
  • The best residual value is below 0.09 (9%). Residue here is the finest sand found within the kaolin flour. The higher the residue, the lesser the quality of the kaolin flour is. 
  • The brightness value needs to be above 80%. It relates to the white color level in the kaolin flour. The higher the brightness level, the higher the kaolin quality.  

Moisture Content

a. Moisture Content in General

Moisture content (or water content) is the amount of water found within a substance like rocks, soil (thus soil moisture), wood, ceramics, or crops. 

A broad range of technical and scientific areas express moisture content as a ratio. It ranges from zero (0), which means highly dry, to the porosity value of the substance is at saturation. Also, it can be the percentage by water mass (gravimetric) or volumetric basis.

b. How to Measure Moisture Content

Knowing the quantity of water or the vapor content of a material can be very helpful in determining if the material is appropriate for a specific application. There are three methods to measure water content:

  • Direct measurement.  The methods are removing the water content and measuring the weight loss. They go through a drying oven by heating, using microwaves, or even infrared radiation. 
  • Laboratory measurement. This method includes chemical reactions (titrations as the Karl Fischer moisture meter method), measuring the on-heating mass loss (likely in the existence of inert gas), and freeze-drying (a low-temperature dehydration procedure). 
  • Soil moisture measurement. The first is by using geophysical methods (sensors), which can specify the water content of in situ soil. The second is the satellite microwave remote sensing method, based on a big contrast between the dielectric characteristics of dry and wet soil. The microwave signal can penetrate the ground to some extent and estimate the moisture of the surface soil.

c. Kaolin Moisture Content

When initially extracted, the kaolin moisture content is approximately 15% – 22%. After the mining process, it undergoes storing and other further drying processes, usually in large loads or piles, placed in open-sided warehouses. The water content of about 1% to 2% is the goal for finished and shipped products. 

Kaolin goes through some significant processing steps To reduce the water content thus achieve a suitable target. One process that is closely related to moisture content is slurry.  

d. Slurry 

Kaolin industries have several stages for processing the mineral, namely mining or drilling, crushing and screening, washing, filtering, press, drying, and grinding (milling). It experiences mineral size reduction, from controlled to either wet or dry. The slurry is one of the methods to separate the minerals by wet processing.

The pressing process occurs when a centrifugal slurry pump transports slurry from the slurry pool to the press machine. The objective is to reduce the water content to a targeted 35% or even less. The slurry is a mixture of solid and liquid phases like sand and water, kaolin, slurry in the process of zinc hydrometallurgy, and others. Slurry has an approximately 80% moisture content.

Related: Get Insight: Step-by-Step Procedure of Kaolin Mining Industry

Three Forms of Commercial-Grade Kaolin

Type of kaolin mineral illustration

The kaolin processing significantly depends on the nature of the crude ore and the product end-use. This white clay mineral undertakes two types of processing methods, namely wet and dry. As a result, we will have hard kaolin (produced by the dry process) and soft kaolin (by the wet process).  

Moreover, commercial-grade kaolin products are commonly available in dry powder, semi-dry noodles, and high solid-liquid slurry forms. The purpose is for the ease of processing and transporting, 

1. Kaolin Lump/Cake

Kaolin lump is a type of kaolin commercial-grade product that mostly has a moisture content (MC) of 33% to 35%.  It has undergone several phases in the processing, such as passing the 325 mesh screening, moving to the pressing filter through the high-pressure pumping. This white clay mineral usually has a larger, irregular chunk size compared to other kaolin types.

2. Kaolin Noodle

Kaolin noodle is the continuation of the kaolin cake in mineral processing. It changes into a white kaolin noodle having a moisture content of 15% – 20% by a natural drying procedure. It has a smaller size in a tiny wine-cork-like shape. 

3. Kaolin Powder

The finished product is kaolin powder, with a moisture content of about 2% to 3%.  It is a kaolin noodle that has passed the disc milling machine, crushed into the finest particles of all three.  

It becomes critical to measure or determine the precise moisture level of kaolin deposits, regardless of the finished product forms. This clay mineral can be either too dry or too wet due to its porous characteristic. Therefore, various production stages require an in-process control of kaolin water content. This control can provide a substantial positive effect on the process cost-effectiveness. 

Therefore, determining kaolin moisture content is one of the highly critical processing components. It may prevent your manufacture from wasting more money and time, thus securing profitability.