Evaporation materials purity

One of the most serious shortcomings of the HV pulse atom-probe is the inability to pulse field evaporate materials of very low electrical conductivity such as a high purity silicon. This is due to the difficulty of transmitting ns HV pulses across the tip. This limitation is overcome by the use of laser pulses which can be focused right to the tip apex. Thus the material applicability of the atom-probe is greatly expanded by the use of laser pulses for the pulsed-field evaporation. 

The high purity of the caustic soda obtained by the membrane process eliminates the need for a caustic soda evaporator in cases where it is to be supplied to customers such as pulp mills which utilize a dilute caustic soda. This is in marked contrast to the diaphragm process which inevitably requires evaporation to separate sodium chloride. For the general trade, in which caustic soda at 50% concentration is required, a conventional multiple effect evaporator is generally utilized to concentrate the catholyte. Caustic soda from the membrane process contains a very slight amount of sodium chloride which does not cause corrosion of the evaporator materials or precipitation of sodium chloride, and thus allows easier and more stable evaporator operation than in the diaphragm process. 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

Dibenzofuran [132-64-9] M 168.2, m 82.4 . Dissolved in diethyl ether, then shaken with two portions of aqueous NaOH (2M), washed with water, separated and dried (MgS04). After evaporating the ether, dibenzofuran was crystd from aq 80% EtOH and dried under vacuum. [Cass et al. J Chem Soc 1406 7958.] High purity material was obtained by zone refining. 

The pyrazolinone should be colorless. If it is not, it may be washed with a minimum of ice-cold ethanol. This procedure is convenient and yields material of adequate purity for the subsequent reaction. Additional pyrazolinone may be obtained by evaporating the filtrate and recrvstallizing the residue from ethanol. 

Batch crystallization. Crystallization is extremely common in the production of fine and specialty chemicals. Many chemical products are in the form of solid crystals. Also, crystallization has the advantage that it can produce a product with a high purity and can be more effective than distillation from the separation of heat-sensitive materials. Crystallization has already been discussed in Chapter 10 and has two main steps. Firstly the solute to be crystallized is dissolved in a suitable solvent, unless it is already dissolved, for example, solute dissolved in a solvent from a previous a reaction step. Secondly, the solid is then deposited in the form of crystals from the solution by cooling, evaporation and so on. 

---