NRDC Technology offer - SYNTHETIC HYDROTALCITE

USE INTRODUCTION

Synthetic hydrotalcites (SHT), also known as anionic clays or layered double hydroxides are rarely found in nature. They are functionally akin to zeolites. The structure of SHT is very similar to brucite, Mg (OH)_2.Magnesium is octahedrally surrounded by six oxygens in the form of hydroxides; the octahedral units then, through edge sharing, form infinite sheets. The sheets are stacked on top of each other through hydrogen bonding. When some of the magnesium in the lattice is isomorphously replaced by higher charged cation, e.g. Al³⁺, then the resulting overall single Mg²⁺ – A1³⁺ - OH Layers gains a positive charge. Sorption of an equivalent amount of hydrated anions render the structure electrically neutral. The anion is frequently found to be carbonate. A typical hydrotalcite may be represented by formula such as MG_4.5 A1₂ (OH)₁₃ CO₃ 3H₂ O. Many modification of this structure can be made by changing the A1 to Mg ratio.

SHT finds application in pharmaceutical industry as an antacid. SHT has also replaced heavy metal based heat stabilizers for PVC. SHT like compounds are imported under the trade name like DHT-4A, L-55R-II and Alcamizer. They are used as halogen scavenger in plastic processes such as the production of polypropylene. It deactivates the residual quantities of acidic substances associated with acid catalysts used in producing polymers. CSMCRI has developed processes to prepare pharmaceutical grade and halogen scavenger grade SHT.

SCOPE

Major quantum of SHT is imported in India at very high price. There is no production of SHT from indigenous raw materials like bittern – a waste mother liquor remained after separation of salt from sea brine. There are number of novel applications of SHT like in catalysis, as adsorbents, as fire retardant, as anion exchangers etc. In view of these, there is an excellent scope to produce SHT and holds tremendous potential for the export.

PROCESS

Bittern and soda ash are reacted to make magnesium precursor. Aluminium sulphate and soda ash solutions are reacted to make aluminium precursor. These precursors are blended and subjected to hydrothermal treatment under optimized process conditions. The slurry is filtered, dried, pulverized and sieved to obtain pharmaceutical grade SHT. To prepare halogen scavenger grade SHT, the slurry is treated with surface modifier to make the material hydrophobic prior to filtration. The cake is washed, dried, pulverized and sieved to obtain halogen scavenger grade SHT.

HALOGEN SCAVENGER GRADE SYNTHETIC HYDROTALCITE

SPECIFICATION OF RAW MATERIALS AND UTILITIES

Basis – 100 kg/day

Raw Materials.

S.No.	Raw material	Specification	Required/day
1.	Bittern	30^o Be’	700 liters
2.	Aluminium Sulphate	Non ferric, technical grade	140 kg
3.	Soda Ash	Technical grade, BIS	240 kg
4.	Caustic Soda	Technical grade	75 kg
5.	Surface Modifier	Technical grade	6 kg

Utilities

S.No.	Utility	Required/day
1.	Electricity	60 KWH
2.	Steam	2.2 Ton
3.	DM water	8 kl
4.	Water	4 kl

Product Specifications

Specification of CSMCRI Product (SHT) and that of imported product DHT-4A are as under.

Properties	CSMCRI SHT		DHT-4A
Molar ratio MgO/A1₂ O₃	4.2		4.0 - 5.0
Specific surface area (BET) m²/gm	12		5 - 15
Loss on drying (105² C, 1hr, %wt)	0.4		0.5 max.
Heavy metals (as pb)	<100 ppm		100 ppm max.
Particle size distribution (by number)
Mean particle size (surface weighted)	0.45		0.4 - 0.7
Under 1 µm	90%		85% min.
Over 5 µm	None		None
pH of suspension
[2%(w/v)C₂ H₅ OH:H₂O (1:1]	9.1	9.0 ± 0.5
Bulk density gm/c.c	0.3	0.33 (0.06m³/20kg bag)

LIST OF MAJOR EQUIPMENTS

a) Solution preparation tanks

b) Jacketed reactor, S.S.

c) Plate @ frame filter press

d) Agitated nutsche filter

e) Autoclave, S.S. with stirrer

f) Boiler

g) Pulverizer

h) Sieve Separator

i) Dryer

j) Pumps

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