GB1570282A - Flocculation of solids from aqueous suspensions - Google Patents

Description

(54) FLOCCULATION OF SOLIDS FROM AQUEOUS SUSPENSIONS (71) We, ALLIED COLLOIDS LIMITED, a British Company, of Low Moor, Bradford, West Yorkshire, BD12 0JZ, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention concerns the flocculation of solids from aqueous suspensions and in particular it relates to the use of high molecular weight synthetic polymers in the flocculation of mineral suspensions.

It is well known to use flocculants to assist in the wet extraction of mineral substances from ores, in effluent treatments and in similar processes involving aqueous suspensions. The flocculants are used to increase the state of aggregation of suspended solid particles and thereby to facilitate separation of solid from liquid constituents by processes such as sedimentation, filtration and flotation.

Many flocculants are known, for example natural products such as gums and starches and synthetic high molecular weight polyelectrolytes.

Suspensions of solids having very low pH values, and especially suspensions having also high ionic contents, are often difficult to flocculate. Typical of such suspensions are the acidic titanium sulphate liquors derived from the acid leaching of titaniferous ores and slags.

In the early stages of the manufacture of titanium dioxide pigment by the so-called "sulphate process", the titaniferous ores or slags are treated with sulphuric acid to produce an aqueous acidic liquor containing the sulphates of titanium and metallic impurities in solution. Such liquor also contains suspended insoluble impurities in particulate or colloidal form which largely comprise undissolved ore and siliceous or earth matter.

Ultimately the titanium dioxide is obtained by hydrolysis of the purified oxide liquor and at some stage it is necessary to free the liquor from the above mentioned insoluble impurities. This is commonly achieved by a clarification stage using sedimentation tanks where acidic mud settles out which may be supplemented by a subsquent filtration stage in which the settled mud is dewatered. It is common practice to assist clarification and filtration by the introduction of a flocculating reagent.

It has been our purpose to devise improved flocculating agents applicable in systems containing strongly acidic liquors with high ionic content and, particularly, in the clarification of the acidic titanium sulphate liquors commonly referred to as "black liquor", and the flltration of the acidic muds obtained from the sedimentation tanks.

We have found that methylolated acrylamide polymers with at least 30A conversion of acrylamide units are highly effective flocculants for acidic titanium sulphate liquors. This finding is surprising in view of the fact that the polymer does not contain a large proportion of cationic acrylic or aminated acrylamide units hitherto regarded as necessary. Polymers containing such units for use in the clarification of black liquor are described in U.S. Patents Nos. 3,658,474, 3,719,748 and 3,859,212. Moreover, significant improvements are obtained by use of our polymers instead of the polymers currently being used commercially in the black liquor process.

According to the present invention therefore a process for flocculating black liquor solids obtained during the production of titanium dioxide is provided which comprises adding to the liquor a water-soluble polymer having a viscosity molecular weight greater than 1,000,000 and containing at least 30% of recurring N-methylol acrylamide units or N-methylolmethacrylamide units, the remaining recurring units being acrylamide or methacrylamide units or other monomer units provided that the total of the other monomer units does not exceed 10% of the total number of units.

The polymer preferably contains at least 45% of N-methylolacrylamide or N-methylolmethacrylamide units and also preferably has a viscosity molecular weight greater than 5,000,000. Small proportions of other monomers up to a total of 10% of the total number of units may be included provided that they do not reduce the effectiveness of the two-component polymer significantly.

Addition rates of the polymeric flocculants to the liquor or mud will generally be within the range 1-100 mg/litre, preferably 5-20 mg/litre.

The polymers are conveniently obtained for example by reaction of a lfneth)acryla- mide polymer of suitably high molecular weight with formaldehyde at room temperature under alkaline conditions, the proportions of reactants and the reaction time being chosen to ensure conversion of sufficient acrylamide units to give a polymer with at least 30% of methylolated units.

The invention is further illustrated in the following Examples. All of the viscosities referred to in this specification were measured in a No. 3 suspended level viscometer Type Mark BS/IP/SL at 25"C.

Example I An acrylamide polymer having a 0.9 % solution viscosity of 206 cs was dissolved in water to give a 1% solution. This solution was reacted with varying amounts of formaldehyde (shown in Table 1) at pH 11.3 for 24 hours at room temperature to give polymers which were tested as described below.

A sample of "black liquor" was prepared in the laboratory by mixing sulphated ilmenite ore with an equal weight of water and heating the mixture with stirring for 6 hours at 65"C. The density of the resulting suspension was then adjusted to 112" Tw by the addition of water and the mixture reduced by the addition of iron filings until no ferric ion reaction towards ammonium thiocyanate was given by a diluted sample.

The black liquor prepared in this manner was typical of that produced in plant practice and was regarded from the results obtained below as being of normal quality, that is to say moderately easy to clarify by the addition of flocculants.

To carry out the clarification test 500 ml quantities of the liquor were placed in graduated beakers and whilst the contents were stirred at 300 revs per minute, aqueous solutions of the flocculants to be tested were introduced into the liquors to give the specified concentrations.

Stirring was stopped one minute after the addition of the flocculants and the flocculated solids were allowed to settle. The settling abilities of the flocculants were compared by measuring the clarities of the supernatant liquids by filtering a 50 ml aliquot of the supernatant liquor after 30 minutes settling, through a glass fibre mat, and subsequently determining the weight of residual matter collected thereon. Dosages are given in weight of active flocculant per litre of liquor.

The results are given in Table 1.

TABLE 1 Clarity of supernatant Moles of CII2O per liquid g/litre mole of dose 24 12 polyacrylamide % methylolation mg/litre mg/litre 0.25 19.7 12.2 0.50 29.5 5.9 6.1 1.0 45.8 0.5 0.6 1.5 61.0 0.4 1.0 2.0 75.0 0.4 0.9 3.0 > 75.0 0.5 0.8 5.0 > 75.0 0.5 1.1 It can be seen that a large increase in effectiveness results from the methylolation of acrylamide polymer.

Example 2 Similar tests to those in Example 1 were carried out to compare the effectiveness of the methylolated acrylamide polymers (P) of the invention with polymers now used commercially for the clarification of black liquor (A and B).

Polymer P is a polymer of Example 1 with 45.8% methylolation.

Polymers A and B are commercial high molecular weight polymers containing a large proportion of cationic groups which are currently being used in the process of clarification.

The results are given in Table 2.

TABLE 2 Clarity Dose of supernatant Polymer mg/litre liquor/litre None - 14.4 A 12 2.6 P 12 2.0 A 9 8.2 P 9 2.1 A 6 10.3 P 6 3.5 B 15 14.4 It can be seen that P is significantly better than A or B in the flocculation of black liquor solids.

Example 3 Similar clarification tests to those of Example 1 were carried out on a black liquor obtained from an operating plant.

Settlement rate was determined by observation of the supernatant volume after 10, 20 and 30 minutes. In addition the clarity of the supernatant liquor was measured by removing a 10 ml aliquot sample after 30 mins settling and adding it to 50 ml of 0.1 N ferric alum in a clarity wedge. (A clarity wedge is a wedge-shaped vessel with a scale on its vertical back inner surface and the clarity of a suspension is determined by noting the first number on the scale, ranging from 3 (poor) to 24 (good), which is visible through the suspension). The results are given in Table 3 in which Polymers A and P are those of Example 2.

TABLE 3 Supernatant Volume % Polymer 10 mins 20 mins 30 mins Clarity A No definite mud line 3 P 73.0 78.0 78.0 22 It can be seen that the Polymer P gave faster settling and better clarity.

The settled mud obtained in the sedimentation stages is subjected to filtration, usually in a rotary vacuum filter. Although sedimented liquor solids are initially in a flocculated condition, during the high shear conditions to which the mud is subjected between the clarifier and the filter almost all flocculated structure is lost so that additional treatment with a flocculant is necessary to facilitate dewatering in the filter.

Example 4 In order to determine the effect of the methylolated polyacrylamide on the dewaterability of the settled mud 500 ml samples of diluted mud at pH 0.7 and 30"C were subjected to a filter leaf test. A filter leaf with an applied vacuum of 56 cm Hg was immersed in the sample and a filtration cycle od 70 seconds suction time and 140 seconds drying time was carried out. The filter cake obtained in each case was dried at 110of and calcined at 8500C to obtain the dry cake yield and cake solids content.

Polymers A and P of Example 3 were compared and the results are given in Table 4.

TABLE 4 Dose Dry cake Cake Polymer mg/l yield (g) solids % None - 2.94 60.1 A 15 4.34 64.0 A 30 4.67 64.1 P 15 6.22 65.7 It can be seen that a significant increase in dewaterability of the mud results from use of the methylolated polymer P containing no cationic units.

WHAT WE CLAIM IS:- 1. Process for flocculating black liquor solids obtained during the production of titanium dioxide which comprises adding to the liquor an effective amount of a watersoluble polymer having a viscosity molecular weight greater than 1,000,000 and containing at least 30% of recurring N-methylolacrylamide units or N-methylolmethacrylamide units, the remaining recurring units being acylamide or methacrylamide units or other monomer units provided that the total of the other monomer units does not exceed 10% of the total number of units.

2. Process as claimed in claim 1 in which the polymer contains at least 45% of Nmethylolacrylamide units or N-methylolmethacrylamide units.

3. Process as claimed in claim 1 or 2 in which the polymer has a viscosity molecular weight greater than 5,000,000.

4. Process as claimed in claim 1, 2 or 3 in which the amount of polymer added is 1 100 mg/litre.

5. Process as claimed in claim 4 in which the amount is 5-20 mg/litre.

6. Process for the flocculating black liquor solids as claimed in Claim 1 and substantially as herein described with reference to and as illustrated in Example 1 or 2.

7. Process for the flocculating black liquor solids as claimed in Claim 1 and substantially as herein described with refer

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. The results are given in Table 2. TABLE 2 Clarity Dose of supernatant Polymer mg/litre liquor/litre None - 14.4 A 12 2.6 P 12 2.0 A 9 8.2 P 9 2.1 A 6 10.3 P 6 3.5 B 15 14.4 It can be seen that P is significantly better than A or B in the flocculation of black liquor solids. Example 3 Similar clarification tests to those of Example 1 were carried out on a black liquor obtained from an operating plant. Settlement rate was determined by observation of the supernatant volume after 10, 20 and 30 minutes. In addition the clarity of the supernatant liquor was measured by removing a 10 ml aliquot sample after 30 mins settling and adding it to 50 ml of 0.1 N ferric alum in a clarity wedge. (A clarity wedge is a wedge-shaped vessel with a scale on its vertical back inner surface and the clarity of a suspension is determined by noting the first number on the scale, ranging from 3 (poor) to 24 (good), which is visible through the suspension). The results are given in Table 3 in which Polymers A and P are those of Example 2. TABLE 3 Supernatant Volume % Polymer 10 mins 20 mins 30 mins Clarity A No definite mud line 3 P 73.0 78.0 78.0 22 It can be seen that the Polymer P gave faster settling and better clarity. The settled mud obtained in the sedimentation stages is subjected to filtration, usually in a rotary vacuum filter. Although sedimented liquor solids are initially in a flocculated condition, during the high shear conditions to which the mud is subjected between the clarifier and the filter almost all flocculated structure is lost so that additional treatment with a flocculant is necessary to facilitate dewatering in the filter. Example 4 In order to determine the effect of the methylolated polyacrylamide on the dewaterability of the settled mud 500 ml samples of diluted mud at pH 0.7 and 30"C were subjected to a filter leaf test. A filter leaf with an applied vacuum of 56 cm Hg was immersed in the sample and a filtration cycle od 70 seconds suction time and 140 seconds drying time was carried out. The filter cake obtained in each case was dried at 110of and calcined at 8500C to obtain the dry cake yield and cake solids content. Polymers A and P of Example 3 were compared and the results are given in Table 4. TABLE 4 Dose Dry cake Cake Polymer mg/l yield (g) solids % None - 2.94 60.1 A 15 4.34 64.0 A 30 4.67 64.1 P 15 6.22 65.7 It can be seen that a significant increase in dewaterability of the mud results from use of the methylolated polymer P containing no cationic units. WHAT WE CLAIM IS:-

1. Process for flocculating black liquor solids obtained during the production of titanium dioxide which comprises adding to the liquor an effective amount of a watersoluble polymer having a viscosity molecular weight greater than 1,000,000 and containing at least 30% of recurring N-methylolacrylamide units or N-methylolmethacrylamide units, the remaining recurring units being acylamide or methacrylamide units or other monomer units provided that the total of the other monomer units does not exceed 10% of the total number of units.

2. Process as claimed in claim 1 in which the polymer contains at least 45% of Nmethylolacrylamide units or N-methylolmethacrylamide units.

3. Process as claimed in claim 1 or 2 in which the polymer has a viscosity molecular weight greater than 5,000,000.

4. Process as claimed in claim 1, 2 or 3 in which the amount of polymer added is 1 100 mg/litre.

5. Process as claimed in claim 4 in which the amount is 5-20 mg/litre.

6. Process for the flocculating black liquor solids as claimed in Claim 1 and substantially as herein described with reference to and as illustrated in Example 1 or 2.

7. Process for the flocculating black liquor solids as claimed in Claim 1 and substantially as herein described with refer

ence to and as illustrated in Example 3.

or 4.

8. Dewatered black liquor from which solids have been flocculated by a process as Claimed in any of the preceding claims.