Treatment of fluoride-containing wastewater

Abstract

The invention relates to a method for treating fluoride-containing, in particular HF containing wastewater to remove fluoride and to a corresponding apparatus. In the new method calcium carbonate is reacted in a reaction step at an acidic pH≤4 with the fluoride in the wastewater to form calcium fluoride particles. Then, in a subsequent filtration step said calcium fluoride particles are separated by a porous membrane from the treated wastewater. The inventive apparatus comprises at least one reaction container/tank for reacting calcium carbonate at an acidic pH≤4 with fluoride in the wastewater to form calcium fluoride particles, as well as at least one porous membrane, in particular at least one porous ceramic membrane for separating calcium fluoride particles from the treated wastewater in a filtration step.

Claims

1. Method for treating HF-containing wastewater to remove fluoride in a crystalline process, wherein at an acidic pH≤4 calcium carbonate (CaCO.sub.3) is reacted in a reaction step with the fluoride in the wastewater to form crystalline calcium fluoride (CaF.sub.2) particles, and in a subsequent filtration step said CaF.sub.2 crystalline particles are separated by a porous ceramic membrane from the wastewater, the CaCO.sub.3 in the process performing three distinct functions: (1) neutralization of acidity by carbonate; (2) providing calcium for CaF.sub.2 crystallization; and (3) providing seed material for crystallization.

2. Method according to claim 1 characterized by a settling step before said filtration step, wherein at least a part of said CaF.sub.2 particles formed in said reaction step are allowed to settle.

3. Method according to claim 1, characterized in that said membrane is a microfiltration membrane with a cutoff from about 0.1 μm to about 1 μm.

4. Method according to claim 1, characterized in that said ceramic membrane is made from aluminium oxide (Al.sub.2O.sub.3), titanium dioxide (TiO.sub.2), zirconium dioxide (ZrO.sub.2), or silicon carbide (SiC).

5. Method according to claim 1, characterized in that CaCO.sub.3 is reacted with the fluoride at an acidic pH≤3.5.

6. Method according to claim 1, characterized in that the CaCO.sub.3 is introduced to the wastewater as a powder with an average particle size≤0.1 mm.

7. Method according to claim 1, characterized in that CaCO.sub.3 is present as a slurry.

8. Method according to claim 1, characterized in that CaCO.sub.3 is used in excess to the fluoride content of the wastewater at a CaCO.sub.3/fluoride mass ratio of 2.6 to 5.

9. Method according to claim 8, characterized in that the wastewater further includes phosphoric acid, and the method further including, after the reaction step, adding alkalinity to increase the pH to over 7.5, to fully precipitate phosphates.

10. Method according to claim 1, characterized in that CaCO.sub.3 is reacted with the fluoride for a reaction time of from 20 minutes to 40 minutes.

11. Method according to claim 10, characterized in that the wastewater further includes phosphoric acid, and the method further including adding alkalinity to increase the pH to over 7.5, to fully precipitate phosphates.

12. Method according to claim 1, characterized in that the wastewater further includes phosphoric acid, and the method further including, after the reaction step, adding alkalinity to increase the pH to over 7.5, to fully precipitate phosphates.

13. Method according to claim 12, characterized in that the addition of alkalinity raises the pH to over 8.0.

Description

(1) Further advantages and features of the overall invention will become clear from the following description of the drawing and the example in conjunction with the dependent claims. The individual features can be realized either singly or jointly in combination in one embodiment of the invention. The drawing and the example only serve for illustration and better understanding of the invention and are not to be understood as in anyway limiting the invention.

(2) The drawing schematically shows:

(3) FIG. 1: An inventive apparatus which can be used for the inventive method.

(4) FIG. 1 shows an inventive apparatus 1 in a schematic representation. Apparatus 1 comprises a reaction tank 2 and a combination tank 3, in which the settling of CaF.sub.2 particles and the separation of CaF.sub.2 particles from the treated wastewater can take place.

(5) Further, FIG. 1 shows a supply line 4 for supplying (untreated) HF containing wastewater to the reaction tank 2. If necessary or if appropriate acid can be supplied from a tank/device 6 to the (feed) wastewater via supply line 5 and pump 7. The possibility to measure and control the pH value of the untreated wastewater in line 4 is also illustrated.

(6) CaCO.sub.3 can be added to the wastewater in the reaction tank 2 (as a powder or a slurry) via supply line 8 and pump 9.

(7) Further possible details of the reaction tank 2 are not shown. Only a stirrer 10 which can be used during the reaction step is shown as an option.

(8) Further, FIG. 1 shows a supply line 11 for transferring the wastewater treated in reaction tank 2 via pump 12 into combination tank 3. It is also illustrated that the pH value of the treated wastewater in line 11 can be measured and controlled.

(9) In combination tank 3, in its bottom part there is a zone 13 in which CaF.sub.2 particles can settle from the treated waste water transferred from reaction tank 2 into combination tank 3.

(10) In the upper part of combination tank 3 a filtration system 14 comprising at least one porous membrane 15 is shown. Treated wastewater containing CaF.sub.2 particles which have not settled into the bottom part of combination tank 3, in particular into zone 13, is filtrated through these membranes 15 via pump 17 (creating (moderate) suction pressure), resulting in a clean treated wastewater stream discharged from combination tank 3 via line 16. CaF.sub.2 particles retained from membranes 15 and CaF.sub.2 particles already settled in combination tank 3 can be discharged from combination tank 3 via line 18.

EXAMPLE

(11) In this example industrial HF containing wastewater from a semiconductor factory was used as feed water for performing the inventive method in an inventive apparatus.

(12) For this purpose 100 liter (L) wastewater were collected and mixed. By measurement with an ISE (Ion Sensitive Electrode) the following data for this (feed) wastewater are provided: pH=2.4 Fluoride content: 950 ppm Temperature: 28° C.

(13) This 100 L volume was transferred as a batch into the reaction tank. Then, to this batch 380 gram (gr) dry solid CaCO.sub.3 powder (97%; grain size D90<0.1 mm) was added. This addition occurred in less than one minute under strong mixing with a stirrer.

(14) The reaction time in the reaction step (under constant stirring) was set to be 30 minutes.

(15) Then, the resulting batch of treated wastewater was transferred into the combination tank. This combination tank was equipped with SiC microfiltration membranes (cutoff 0.1 μm) with a (total) active surface of 5 dm.sup.2. In this design the filtration was run as a submerged atmospheric filtration system wherein the permeate was drawn through the pores by using gentle suction. Bigger CaF.sub.2 particles formed in the reaction step settled down to the bottom of the combination tank. Smaller CaF.sub.2 particles went up and were subjected to filtration.

(16) In this example, the filtration capacity was set at 30 L/hour (h) with a regular backwash, namely every 10 minutes during 30 seconds under fully automatic procedure. The filtration performance was constant with a varying transmembrane pressure (TMP) between 250 mbar after backwash and 450 mbar before backwash. No pre-acidification was needed as the optimal pH-value of <2.5 was already achieved from the wastewater composition. During the reaction time of 30 minutes the pH raised to 5.8.

(17) The analytical results after a total filtration time of 3 hours were as follows.

(18) Treated wastewater: Fluoride content: 7 to 10 ppm pH: 6.5 to 7.5 Turbidity: <0.5 NTU (Nephelometric Turbidity Unit) TSS (Total Suspended Solids): <1 ppm

(19) Produced crystals (analyzed by XRD (X-ray Diffraction)): 60% CaF.sub.2 35% CaCO.sub.3 (mainly from core of particles) 5% impurities >80% crystallinity and <20% amorphous structure.