MEMBRANE FILTRATION

Porous PTFE membranes (pPTFE or ePTFE) consist of expanded PTFE sheets or tubes that have a controlled porosity of the order of 100 nm to 1µ. Because of the outstanding chemical resistance, and “non-stick” property of PTFE, they form interesting filtration media for a horde of applications, many of which in the chlor-alkali and related industries. In association with technology provider PROFILCO of the Netherlands, TECHLINK builds turn-key, single step brine filtration systems based on Tefflux® nanometric membrane technology and back-pulse membrane cleaning.

TEFFLUX® Nano-Filtration Membranes

TEFFLUX® nanometric pPTFE membranes are produced in the USA. They are seamless tubes of about 15mm diameter of expanded PTFE. The absence of seams is a strong guarantee against failures of membranes. The tubes are mounted on PP cylinders, and assembled in sets of nine, which more or less replicate the dimensions of a classic candle. For this reason, the Tefflux® nanometric membranes can be retrofitted in existing filter vessels using existing tube sheets. Alternatively, new vessels and tube sheets may be supplied made of titanium, FRP and other corrosion-free materials.
Tefflux PTFE membranes filtration candle
Single elements are grouped in clusters of nine, which are introduced though the tube-shet from the top, which avoids having to lift the tube-sheet to replace candles. The filtration vessels most often have an open top, because the downstream pressure may be atmospheric in most cases.
Tefflux PTFE membranes filtration candles cluster
Tefflux® candles are flexible, and can be bent without damage. They do not break like ceramic candles. Their lifetime is six to seven years.
Tefflux PTFE membranes filtration candles in vessel
A complete system is usually composed of three vessels, each providing about 40% of the total capacity. The filters are backflushed in turn, so that a minimum of two filters are in production state at any time. For installation convenience, and to reduce at-site work during erection, TECHLINK supplies each filter unit as an independent skid with all its appurtenances. Typically, three units are interconnected to make up a complete filtration system.
Tefflux PTFE membranes filtration candles in tubesheet
At regular intervals, automatically controlled by Δp, the brine is pushed backward through the membrane for a few seconds. The backflow expands the membrane pores and releases the impurities caught inside the pores. At the same time, the shock effect of the back-pulse breaks the “cake” of impurities accumulated around the candle, and lets it falls to the bottom of the vessels. This process is similar to the cleaning of classic candle filters, but since no body feed is required, filtration can resume immediately at the end of the back-pulse. Back-pulses consume very little brine, and can be triggered at short intervals. The sludge accumulates at the bottom of the vessel, and is only flushed away when its level reaches a preset high level.
PTFE membrane filtration backpulse system principle

Single-step membrane filtration for calcium & magnesium removal

pPTFE membrane filtration can, in one step, remove calcium and magnesium from raw brine, and make it ready for ion-exchange beds. The system eliminates the need for a clarifier and a primary filter. No body feed is required for final filtration.

Classic brine preparation

typical brine purification process diagram

Brine preparation with Tefflux

Tefflux PTFE membranes filtration for brine purification diagram

Brine preparation with one-step nano-filtration with TEFFLUX® back-pulsed membranes

  • Settler no longer needed
  • α-cellulose no longer needed
  • Floculent no longer needed
  • Primary filtration (anthracite) no longer needed
  • Automatically operates and does not require labour
  • Replace the membranes twice in twenty years only
By removing the need of a settler and an anthracite filter, and by eliminating the use of floculents and body-feed, the Tefflux® one-step brine filtration system considerably reduces the investment cost as well as the operating cost of the brine purification system. It is usually amortized in less than three years.

Magnesium rich sea salt

Magnesium rich sea salt sets a challenge to classic brine preparation systems, because the magnesium hydroxide does not separate in the settler if the concentration of magnesium is too high. The maximum ratio is usually deemed to be 1 part of magnesium to 2.5 parts of calcium by weight. Magnesium rich salts require the addition of a costly floculent and a longer residence time in the settler.

On the other hand, magnesium hydroxide is difficult to filter out, because it forms a sticky slime on filter candles, which does not break away during backflush cycles.

Techlink and Profilco have therefore developed a special method to purify magnesium rich salts using Tefflux® nanometric membrane filtration, without addition of any floculent and without wasteful prior salt washing.

Single-step membrane filtration for calcium chloride plants

Concentrated HCl and Calcium carbonate are mixed and react to form CaCl2.A large part of the CaCO3 does remains unreacted as this is dosed in excess. It needs to be filtered out from the CaCl2 solution.

In the legacy design, the solution enters a settler after pH neutralization and a polymer is added to enhance the settling process. The overflow is filtered by a filter press, and/or a precoat filter using alpha-cellulose as body feed.

Our TEFFLUX membrane filter can filter the CaCl2 solution without a settling step, and without addition of any floculent or body-feed. The sludge seems creates a nice filter cake on the membranes, that is easily discharged during short back-pulse cycles. The sludge from the filter can be recycled to the reactor, and does not contain any added organic material. The filtrate turbidity is very low ( < 05 NTU) and can be directly sent to the evaporator section.

Classic calcium chloride process

Calcium chloride process with Tefflux

CaCl2 process with one-step nano-filtration with TEFFLUX® back-pulsed membranes

  • Settler no longer needed
  • α-cellulose no longer needed
  • Floculent no longer needed
  • Automatically operates and does not require labour
  • The sludge produced is clean and free from added organic material (e-g body feed).

Filters for sodium chlorate and sodium hypo

Based on the TEFFLUX® nanometric pPTFE membranes.

Removal of all suspended particles is a condition for the stability of sodium chlorate solutions and concentrated sodium hypochlorite. In particular, suspended particles of iron and chrome oxides must be removed. Conventional filtration uses large amounts of -cellulose as body-feed, which ends up as a cumbersome waste. The TEFFLUX® nanometric pPTFE membrane technology is ideal for obtaining a perfectly clear filtrate without using any body-feed and therefore minimizing the volume of solid effluents.
  • No pre-coat required
  • Reduces waste disposal cost
  • Simplified automation
  • Maximum product clarity
  • Minimum operating & maintenance cost
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