Crystal purification technology

1) Recrystallization technology for quality improvement
Quality can be improved drastically by recrystallization under strict production and quality control.

(Reference) Impurity levels of our magnesium sulfate before and after recrystallization.
Item Analysis before
Analysis after
Analysis method
Ca 100ppm 1.0ppm Inductively coupled
plasma – atomic
emission spectrometry
Fe 0.1ppm Below detection
limit(0.1ppm max.)
Mn 0.03ppm Below detection
limit(0.02ppm max.)
Na 2.7ppm 0.8ppm Atomic absorption
K 4.4ppm 0.9ppm

2) Purification technology by HPHT(high pressure /high temperature)
In the state of hydrothermal solution, which is exposed to high pressure and high temperature (HPHT)such as above 100 ℃, reaction progresses quickly in an autoclave ,leading to high degree of crystallization. In case of magnesium hydroxide, HPHT environment creates highly purified crystals with much smoother surface, compared with crystals obtained under normal pressure/temperature conditions.

(Reference) Crystallization of magnesium hydroxide by HPHT
Item Analysis of crystal obtained under HPHT conditions Analysis of crystal obtained under normal pressure /temperature conditions Analysis method
Purity 99.7% 97.3% Chelatometric titration
Ca 6.0ppm 87ppm Inductively coupled
plasma - atomic
emission spectrometry
Fe 3.8ppm 12.9ppm
Mn 1.8ppm 3.4ppm
Na 41ppm 89ppm Atomic absorption spectrometry
K 21ppm 61ppm

Electron micrograph of crystals under HPHT conditions

Crystal control technology

1) Crystal form control by habit modification
In case of crystallization of industrial chemicals, most chemical solutions contain impurities. Some impurities work on precipitates to change characteristics of obtained crystal or crystallization behaviors. This phenomenon is called habit modification and those impurities are defined as habit modifiers.
One form of habit modification is the change of crystal form ,which is caused by habit modifiers adhering to specific crystal face growing in a supersaturated solution.

■Example of crystal form change of magnesium sulfate 7-hydrate by habit modification

Electron micrograph of crystals
growing under HPHT conditions

Crystal form of magnesium sulfate
7-hydrate controlled by habit

Crystal form of magnesium sulfate
7-hydrate controlled by habit

2)Spherical flocculation technology by reactive crystallization
Reactive crystallization is an operation to generate particles by reaction. In a reaction, a crystal nucleus is generated by producing the state of supersturation and ,at the same time, grow produced crystals by spherical agglomeration. Reactive crystallization is applicable to produce spherical crystals of such chemicals as hardly soluble carbonates and hydroxides.

■Example of spherical crystals of hydroxides obtained by reactive crystallization

Electron micrograph of spherically flocculate
magnesium hydroxide by reactive

Electron micrograph of spherically flocculate
hydroxide by reactive crystallization

3) Crystal form control technology by HPHT
Crystal form is controlled in the state of hydrothermal solution under HPHT conditions.
High pressure lowers solubility of compound to crystallize. In the case of water soluble system, it is possible to obtain single crystals of such form that cannot be created under normal pressure/ temperature conditions as crystallization takes place under HPHT conditions. Crystallization is sometimes accompanied by reaction.

■Example of inorganic compound crystals produced under HPHT conditions.

Needle crystals

Rhombohedral crystals

Hexagonal plate crystals

Generation of crystal nucleus and crystal growth

Crystallization is a phenomenon realized by both of crystal nucleus generation and crystal growth.
Crystals are not generated immediately after supersaturated solution reaches saturation point by cooling. Crystal nucleus start to appear only after supersaturation becomes somewhat high. Supersolubility is a point for crystal nucleus generation to start. The area of solubility curve between solubility and supersolubility is called metastable domain. After the start of crystal nucleus generation, crystals continue to grow as long as solubility curve stays in this domain. However, the diameter of crystals becomes smaller when supersaturation is too high even if solubility curve is in a metastable domain. Bigger crystals can be obtained when supersaturation is low. Supersolubility changes, depending on the kind of solutions, volume of crystal nucleus, degree of agitation and so on. Industrial crystallization requires adequate crystallization conditions which enable reproduction.

Solubility curve and supersolubility curve

The relation between velocity of crystal nucleus generation and velocity of crystal growth

Examples of crystallization technology applications
Umai Chemical has been engaged in research and development of various compounds with properties or shapes which are different from conventional compounds by taking advantage of HPHT technology and reaction crystallization technology. The following are examples.

1) Complex hydroxide
By mixing a certain compound into magnesium hydroxide with extremely small particle size to cause hydrothermal reaction, it is possible to produce a special complex magnesium hydroxide which starts to decompose at the temperature 50℃ lower than magnesium hydroxide. It has more removed water content than magnesium hydroxide by about 6%.
This complex hydroxide was developed as a flame retardant for polyethylene. Its properties go well with the ignition point and the flash point of polyethylene(350℃) and it has better incombustibility, compared with the conventional flame retardant of magnesium hydroxide.

Electron micrograph of complex hydroxide (2000 times)

■Examples of differential thermal analysis(DTA)

DTA of magnesium hydroxide

DTA of complex magnesium hydroxide

Item Analysis of complex hydroxide Analysis of magnesium hydroxide
Water content 36.8% 30.8%
Decomposition temperature 367℃ 420℃

2) Spherical magnesium hydroxide
Umai Chemical developed spherical magnesium hydroxide by reaction crystallization technology which is characterized by causing reaction in state of supersaturation to produce crystal nucleus while growing so produced crystal nucleus by spherical flocculation.

Electron micrograph of spherical magnesium hydroxide (200times)

Item Analysis of spherical magnesium hydroxide Analysis method
Particle size ≦100μm Electron microscope
High purity magnesium hydroxide 98.7% Chelatometric titration
Ignition loss 30.1% 500℃ 3hours
Ca 0.01% Inductively coupled plasma-atomic emission spectrometry
Na 0.05ppm
K 0.01% max.
Fe 5ppm max.
Examples of powder engineering application
Umai Chemical has been engaged in the development of powder materials with controlled particle size and shape by using special dryer and pulverizer, The following are examples.

1) DOT series
Umai Chemical succeeded in producing spherical magnesium sulfate 2.5 hydrate(DOT- 25) by using special dryer. Since crystals are spherical, it has excellent powder fluidity.
Anhydrous DOT-00 can be also produced by burning DOT-25.

Electron micrograph of DOT-25

Particle size distribution of DOT-25

Item Analysis of DOT-25 Analysis of DOT-00
Purity(magnesium sulfate) 73.1% 97.2%
Ignition loss(water content) 26.9% 2.8%
D50(average particle size) 6.5μm 5.0μm
D90 15.0μm 30.0μm

2) Very fine particle magnesium sulfate SSN-00
Anhydrous magnesium sulfate is ground by a special pulverizer to obtain average particle size of about 6 μm .

Electron micrograph of SSN-00

Particle size distribution of SSN-00

Item Analysis of SSN-00
Purity(magnesium sulfate) 97.8%
Ignition loss(water content) 2.2%
D50(average particle size) 6.1μm
D90 16.3μm

3) Extremely fine particle magnesium sulfate USN-00
Anhydrous magnesium sulfate is ground by a special pulverizer as finely as possible to obtain the average particle size of about 3 μm but still finer than SSN-00 . All particles pass 10μm mesh.

Electron micrograph of USN-00

Particle size distribution of USN-00

Item Analysis of USN-00 Analysis of SSN-00
Purity(magnesium sulfate) 98.5% 97.8%
Ignition loss(water content) 1.5% 2.2%
D50(average particle size) 2.9μm 6.1μm
D90 4.9μm 16.3μm