US3681975A

US3681975A - Method of evaluating oil-soluble dispersants

Info

Publication number: US3681975A
Application number: US36483A
Authority: US
Inventors: Kanji Majima; Kaichi Izumi; Takao Watanabe
Original assignee: Kao Corp
Current assignee: Kao Corp
Priority date: 1969-05-13
Filing date: 1970-05-11
Publication date: 1972-08-08
Anticipated expiration: 1989-08-08
Also published as: JPS4921919B1

Abstract

A method of evaluating oil-soluble dispersants, in which a dispersion medium and a sample of dispersant to be evaluated are mixed an certain proportions with a controlled sludge or an acidic artificial sludge specifically prepared having a constant proportion of insoluble matter content to dispersion medium, the mixture is then allowed to stand for a certain period of time and thereafter the state of the artificial sludge is observed to evaluate the dispersant.

Description

United States Patent n51 3,681,975 Majima et al. 451 Aug, 8, 1972 METHOD OF EVALUATING OIL- 2,450,603 10/1948 Lomasney ..73/64 SOLUBLE DISPERSANTS 2,543,871 3/ I95] Salathiel ..73l6|.4 3 003 350 10/196] Stewart ..73/64 [72] Inventors: Karril Mallma, Wakayama; Kalchl hum, Tokyo; T I w I be, 3,025,698 3/1962 Schultze et al ..73/64 X Tokyo of Japan Primary Examiner-Louis R. Prince [73] Assignee: Kao Soap Co., Ltd., Tokyo, Japan Assistant Examiner-Joseph W. Roskos [22] Filed: y n 1970 AttorneyWoodhams, Blanchard and Flynn [2]] App]. No.: 36,483 [57] ABSTRACT A method of evaluating oil-soluble dispersants, in [30] Foreign Application Prio it I) which a dispersion medium and a sample of dispersant to be evaluated are mixed an certain proportions with May 13, Japan a convened sludge or an acidic sludge specifically prepared having a constant proportion of [52] US. Cl ..73/64, 73/53 insoluble matter content to dispersion medium, the [51] Int. 33/26 mixture is then allowed to stand for a certain period of Field of Search 61 432 R time and thereafter the state of the artificial sludge is observed to evaluate the dispersant. 56 Ref nces Cited l m 14 Claims, No Drawings UNITED STATES PATENTS 2,089,017 8/1937 Burk ..73/64 METHOD OF EVALUATING OIL-SOLUBLE DISPERSANTS BACKGROUND OF THE INVENTION l Field of the Invention The present invention relates to a method of evaluating oil-soluble dispersants such as fuel additives and lubricating oil additives.

2. Description of the Prior Art Various additives have been produced for the purpose of finely dispersing the sludge formed in fuel oil and lubricating oil and also for the purpose of preventing the formation of sludge.

in conventional methods of evaluating the dispersing power of those additives, solid powders of metals, metal oxides, metal hydroxides and carbon have been dispersed in liquids. However, those methods have been not so useful for evaluating dispersants which are to be used under circumstances such that the dispersion medium and the dispersants per se are transformed into insoluble substances, due to the difference in specific gravity which may exist between the solid powders mentioned above and the insoluble substances formed in the system. Further, commercially available carbon black and graphite have various qualities and the experimental results vary according to the characteristics of the surface of the carbon even under the same test conditions. Further, in an evaluating method wherein solid powder is used as an artificial sludge, the evaluation results are influenced by the particle size distribution of the artificial sludge and therefore, such a sensitivity as to be capable of detecting differences among respective batches of dispersants produced in the same procedure in the same plant.

in a known method of testing fuel oil additives, asphaltic substances in heavy fuel oil have been used as an artificial sludge instead of the above mentioned solid powders and there have been measured the powers of additives (dispersants) for dispersing asphaltene precipitated when a heavy fuel oil sample is dissolved in n-heptane or in naphtha. However, in this method, the results of the evaluation are not consistent due to variety of impurity contents of the heavy fuel oil samples. In addition, the method is mainly for evaluating the dispersancy in practical use and it is not generally applicable to strict and precise tests for dispersants in oil.

SUMMARY OF THE INVENTION The present invention provides a method of evaluating dispersants which method is applicable generally to dispersants in oil and which has accuracy and good reproducibility.

The method of the present invention is characterized in that a dispersion medium selected from the group consisting of parafiinic hydrocarbons and cycloparaffinic hydrocarbons and a sample of the dispersant to be evaluated are mixed in predetermined proportions with a substance to be dispersed, which has been prepared by dissolving coal pitch, petroleum pitch, coal tar, petroleum tar, wood tar, asphalt, heavy oil or the like in an aromatic hydrocarbon, filtering the insoluble matter, if any, and adjusting the concentration of the solution so that a predetermined quantity of insoluble matter may be present in the dispersion medium (the thus prepared artificial sludge will be hereinafler referred to as controlled sludge") or by incorporating spent acid sludge in the above controlled sludge (hereinafter referred to as "acidic artificial sludge"), the mixture is then allowed to stand and thereafter the state of the artificial sludge is observed in order to evaluate the dispersant.

The process for the preparation of the controlled sludge and the acidic artificial sludge to be used as the artificial sludge in the method of the present invention will be described below.

The raw materials for making the controlled sludge to be used in the method of the present invention shall have the following properties.

a. They shall be soluble in a colorless transparent aromatic hydrocarbon solvent (insoluble matters being less than 25 percent) and the color of the resulting 0.2 percent solution according to the specification of ASTM D. l 500 shall be above 3.

b. They shall contain 1 l00 percent of matters insoluble in the dispersion medium.

As raw materials for making such a sludge, there may be mentioned liquid materials or materials convertible into liquid under certain conditions, such as coal pitch, petroleum pitch, coal tar, petroleum tar, wood tar, asphalt, heavy oil and the like. The amounts of insoluble precipitate or floating matter contained in these materials are not constant and, therefore, if these materials are diluted by a dispersion medium as it is, such precipitate or floating matter will become a nucleus for flocculation and thereby cause error in the evaluation. Therefore, said raw materials are treated in the following manner.

The raw material, such as heavy fuel oil or the like, is dissolved in l times as much by weight of an aromatic hydrocarbon and the insoluble matters, if any, are filtered out. The resulting solution is concentrated by means of distillation or diluted with a concentrationcontrollng agent to obtain a certain constant predetermined proportion of matters which are insoluble in the dispersion medium. The resulting product is the controlled sludge to be used in the present invention.

By using the artificial sludge, the above controlled sludge having a constant proportion of matter insoluble in the dispersion medium which sludge is controlled by using an aromatic hydrocarbon solvent according to the present invention, the reproducibility of the evaluating test for the dispersants can be improved remarkably. However, in case a dispersant having a very high dispersing power is to be evaluated or in case the circumstances under which the dispersants are used tend to yield strong acidic substances, the differences between dispersants to be evaluated sometimes cannot be detected or the results of the evaluation, if detected, sometimes do not agree with the practical operation. in particular, in the evaluation of dispersants to be added in lubricating oils for an engine, requiring heavy fuel oil as the fuel, the sulfur contained therein is oxidized into sulfuric acid, which then oxidizes the lubricating oil to form insoluble matter called "sludge", which precipitates in the engine to plug the oil circulating system. Therefore a dispersant is used to prevent such troubles. However, if a controlled sludge is used without any additional treatment, the results of the evaluation of the dispersants do not necessarily coincide with the actual dispersing powers, because conditions at the time of the evaluation are difierent from the actual operating conditions.

We have found that an acidic artificial sludge prepared by mixing a spent acid sludge by-produced in the sulfonation of distillation residues of natural lubricating oil or of commercially available alkylbenzene or a mixture thereof with the above controlled sludge in the appropriate proportions is quite effective for improving the coagulating power of the controlled sludge.

The spent acid sludge is obtained as a by-product by sulfonating 1 mole of material to be sulfonated with l.5 moles of S0,, isolating the main product, i.e. alkylbenzene sulfonic acid and removing free sulfuric acid from the residue.

The resulting spent acid sludge is mixed with the above controlled sludge in a ratio between 0.01 18.0 percent by weight based on the weight of the sludge and the mixture is stirred at a constant temperature in the range of 35 80 C. for 0.5 2.5 hours to effect aging. The resulting product is the acidic artificial sludge.

In case said controlled sludge is used in the method of the present invention, the concentration control after the distillation is necessary if the quantity of the distillate has been controlled in the distillation-concentration stage. However, in preparing an acidic artificial sludge, the presence of a large quantity of aromatic hydrocarbons is unfavorable to stability during storage. Accordingly, it is preferable to adjust the concentration with a liquid parafiinic hydrocarbon after the whole amount of the aromatic hydrocarbons has been previously distilled out. Another preferable method comprises previously confirming the solubility of a raw material for preparing the acidic artificial sludge in an aromatic hydrocarbon by using only a small quantity of the raw material, selecting the raw material which necessitates no filtration, determining the insoluble matters of the selected raw material in a dispersion medium, incorporating a concentration-controlling agent such as liquid paraffin in a quantity calculated therefrom directly in the raw material for the acidic artiftcial sludge.

Aromatic hydrocarbons to be used in the present invention are those containing six to 26 carbon atoms and one, two or three alkyl groups in the benzene nucleus, such as benzene, toluene, xylene and mesitylene.

The concentration controlling agents to be used in the present invention are aromatic hydrocarbons and paraflinic hydrocarbons as above mentioned. Among those compounds commercially available as liquid paraffin, kerosene, spindle oil, lubricating oil, etc., those usable in the present invention have 12 34 average carbon numbers and a viscosity at 40 C. of below 150 centistokes.

The dispersion mediums to be used in the evaluation shall satisfy the following requirements:

a. The dispersion mediums do not disturb the flocculation of substances contained in the artificial sludge and which are insoluble in the dispersion medium.

b. The dispersion mediums have an appropriate power of dissolving a sample of dispersant.

c. The dispersion mediums have a color sufficiently faint so as not to disturb the judgement in the evaluanon.

Sample of dispersant: Artificial sludge:

in the presence of concentrationcontrolling agent in the absence of concentration controlling agent Dispersion medium:

0.000l-20I: by weight 0001-50; by weight (mill-25% by weight remainder In practicing the method of the present invention, the above sample of dispersant, artificial sludge and dispersion medium are mixed together in an appropriate vessel and allowed to stand. After a predetermined period of time, the precipitation state of the artificial sludge is observed to evaluate the effect of the oil-soluble dispersant used. The order of the addition to the sample of the dispersant, artificial sludge and dispersion medium should be fixed at least in a series of comparative tests, since the order exerts some influence upon precipitation velocity. From the practical and theoretical view point, the mixing order of (l) dispersion medium, (2) sample of dispersant and (3) artificial sludge is preferable. The temperature at which the mixture is left standing should be kept constant within 1 1 C. allowance at least in a series of comparative tests. In general, the higher the temperature, the higher is the flocculation velocity. From the practical view point such as the boiling point of the dispersion medium, any suitable temperature in the range of from about 10 to C. is preferable. lt is particularly preferred for obtaining a constant temperature condition irrespective of atmospheric temperature to keep the temperature in the range of about 30 -S0 C. within an allowance of less than C.

The evaluation of the dispersing power of a dispersant is usually determined by means of the following three ratings of the dispersing power. The results are evaluated as ratings i, ll and ill of dispersancy.

Rating I: The whole dispersion is homogeneous and somewhat turbid and insoluble matters are invisible to the naked eye. The dispersibility is good.

Rating ll: Borderline dispersibility. The dispersion as a whole is almost homogeneously turbid, though a small quantity of precipitate of a size visible to the naked eye is observed on the wall and at the bottom of the vessel.

Rating lll: The dispersibility is insufficient, and coagulation and precipitation occurs. The liquid phase becomes so transparent that contours of the flocculated floats and precipitates are visible distinctly.

The results of the evaluation may be shown by digits of the ratings I, ll and II] on (I) the condition of the mixture after a predetermined period of time or 2) by the amount of time required before reaching the rating ll or ill.

The method of the present invention will be illustrated by way of examples.

EXAMPLE 1 0.3 gram of a commercial heavy fuel oil (Bunker C fuel) was dissolved in 90 g of xylo] of reagent grade, and it was confirmed with the naked eye or a light microscope that no insoluble matter was present. After conforming that the color of the solution was above 3, measured according to the specification of ASTM D1500, a part of the xylol was distilled out from the solution. The xylol content of the remaining solution was 47.6 percent. Then a calculated quantity of xylol was added to the solution to make the xylol content 50.0 percent. 0.2 gram of the solution was introduced in 30 ml of heptane of reagent grade and the occurrence of a large quantity of precipitate was confirmed. A determination showed that the precipitate corresponded to 2.5 percent by weight based on the remaining solution. In the same manner, four samples from the same commercial heavy fuel oil were confirmed as regards their solubilities in xylol, insoluble matters such as precipitates, if any, were removed by means of filtration, the colors of the solutions were confirmed, and the remaining xylol contents were controlled to make the insoluble matters in heptane 2.5 percent. By using the thus obtained controlled sludges Nos. l 5, dispersibility was examined in the presence of sodium m-, pdinonylbenzene sulfonate as a dispersant. The results were as shown in Tables 1 and 2.

TABLE I Controlled sludge No. l 2 3 4 5 Insoluble matters in heptane (k by weight) 2.5 2.5 2.5 2.5 2.5 Dispersancy rating l l l I TABLE 2 Commercial heavy fuel oil 1 2 3 4 5 Insoluble matters in heptane (I1 by weight) 5.02 6.78 4.9l 5.54 8.65 Dispersancy rating l ll l I III Table 2 shows the results obtained when the treatment according to the present invention is not applied to the commercial heavy fuel oil. In view of Table 2, it is evident that the results of the evaluation varies, because the insoluble matter contents of the respective samples are different from each other. On the other hand, if controlled sludges prepared by treating the heavy fuel oil according to the present invention to make the insoluble matter contents uniform are used, the rating in five experiments are all I as shown in Table 1, indicating good reproducibility.

EXAMPLE 2 Two batches (A and B) of an alkylbenzene sulfonate were synthesized from a raw material of the same batch by the same synthesizing method. Chemical analysis and dispersancy rating according to the method of the present invention were effected on each of the samples A and B to obtain the results as shown in Table 3. The conditions in the dispersancy rating were as follows. A mixture (1 l) of commercial heavy oil and toluene at reagent grade was used as the artificial sludge. 0.2 milliliter of the mixture which had been confirmed to have neither precipitates nor floating coagulants, 29.8 ml of heptane of reagent grade as the dispersion medium and 0.01 ml of the dispersant were mixed together and then allowed to stand at 30 C. for 1 hour.

TABLE 3 Alkyibenzene sulfonate Batch A Batch B by weight) by weight) Sulfur 3.4! 3.42 Calcium 3.07 3.07 Alkali value 24.78 23.52 Specific gravity 0.977 0.976 Viscosity SSU, 2 if! F 320.5 325.3 Infrared spectrum No difference Disperancy rating I III Examination of the cause for obtaining such a large difference in the dispersancy rating between A and B proved that a small quantity of impurities was contained as an auxiliary raw material in batch B which should have been the same as in batch A.

Thus, it was proved that, according to the method of the present invention, the influence of even a small quantity of impurities which causes no change on chemical analysis or no physical change can be detected.

EXAMPLE 3 Sodium alkylbenzene sulfonates (samples C and D shown below) having the same molecular weight were prepared and their dispersing power for carbon black were compared according to a conventional method. No difi'erence between them could be detected.

Sample 0 Sample D However, there should be some difference in surface chemistry between them. Their dispersing powers were compared according to the method of the present invention under the same conditions as in Example 2 to reveal rating III for sample C and rating [for sample D.

EXAMPLE 4 Dispersants added in internal combustion engine oils are adsorbed by incomplete combustion residue or oxidated products of lubricating oil to form micelles thereby preventing blockage of the oil-circulating system. However, if the quantity of the insoluble matters is too large, the whole dispersant is adsorbed by the insoluble matters and the oil becomes deficient in the dispersant. The remaining insoluble matters precipitate in the engine to cause troubles. lt is very difficult practically to disperse again the insoluble matters once precipitated and, therefore, the lubricating oils have to be replaced by a fresh one before the dispersant is spent out. The time for the replacement cannot be de cided by a chemical method, since the dispersant is spent mainly by physical adsorption. According to the method of the present invention, the time can be decided easily. An embodiment will be shown below.

A road test by a passenger car was efiected by using a commercial lubricating oil. A small quantity of the used lubricating oil was taken out at intervals of 1,000 km operating mileage to obtain samples. The results were as shown in Table 4.

Test conditions: 0.1 ml oflubricating oil, 29.7 ml

of n-haptane and 0.2 ml of artificial sludge of Example 2 were used: 35 C. for one hour.

EXAMPLE 5 Each 0.03 ml of the following three samples were used as dispersants. 0.1 ml of the mixture of 98 g of commercial heavy oil controlled to contain 5.0 percent of n-heptane-insoluble matters and 2 g of spent acid sludge, which had been aged under stirring at 50 C. for l hour was used as the artificial sludge. Thirty ml of n heptane of reagent grade were used as dispersion medium.

The dispersant, artificial sludge and dispersion medium were mixed together, vigorously agitated and then allowed to stand at 35 C. for 1 hour. The results were as shown in Table 5.

In tests carried out on an artificial sludge containing wood tar, asphalt and heavy oil in an aromatic hydrocarbon to obtain a solution of said raw material in said aromatic hydrocarbon, removing any insoluble matter in the solution by filtration, and then adjusting the concentration of the solution to obtain an artificial sludge containing a predetermined quantity of matter insoluble in the hereinafter-mentioned dispersion medium;

mixing (l) a dispersion medium selected from the group consisting of paralfinic hydrocarbons, cycloparatfinic hydrocarbons and mixtures thereof and (2) a sample of dispersant to be evaluated, in predetermined proportions, with (3) said artificial sludge;

then allowing the mixture to stand for a predetermined period of time and thereafter observing the state of the artificial sludge to evaluate the dispersant.

2. A method according to claim 1, in which said raw material is at least percent by weight soluble in said aromatic hydrocarbon and a 0.2 percent solution thereof in said aromatic hydrocarbon has a color value above 3 when measured by ASTM D. i500 and said raw material contains from 1 to percent by weight of substances insoluble in the dispersion medium; and said raw material is dissolved in from I to 100 times as much by weight of said aromatic hydrocarbon and in which said aromatic hydrocarbons contain from six to 26 carbon atoms and from one to three alkyl groups on the benzene nucleus.

3. A method according to claim 1, in which the concentration of the solution is adjusted by distilling the solution, the amount of said dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the artificial sludge in the mixture being in the range of 0001-25 percent by weight and the balance of the mixture being dispersion medium.

4. A method according to claim 1, in which the concentration of the solution is adjusted by adding a concentration-controlling agent selected from the group consisting of aromatic hydrocarbons and paraffinic hydrocarbons having an average carbon number in the range of 12 to 34 and a viscosity at 40 C. of below centistokes, the amount of the dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the sludge in the mixture being 0.001-50 percent by weight and the balance of the mixture being dispersion medium.

5. A method according to claim 1, in which the dispersion medium has an average carbon number of not more than 20.

6. A method according to claim 1, in which the mixture is allowed to stand at a substantially constant temperature in the range of 10 to 80 C.

7. A method of evaluating oil-soluble dispersants, which comprises:

preparing an acidic artificial sludge by dissolving a raw material selected from the group consisting of coal pitch, petroleum pitch, coal tar, petroleum tar, wood tar, asphalt and heavy oil in an aromatic hydrocarbon to obtain a solution of said raw material in said aromatic hydrocarbon, removing any insoluble matter in the solution by filtration, and then adjusting the concentration of the solution to obtain an intermediate artificial sludge containing a predetennined quantity of matter insoluble in the hereinafter-mentioned dispersion medium and then adding spent acid sludge to said intermediate artificial sludge to obtain said acidic artificial sludge;

mixing (1) a dispersion medium selected from the group consisting of parafiinic hydrocarbons, cycloparaffinic hydrocarbons and mixtures thereof and (2) a sample of dispersant to be evaluated, in predetermined proportions, with (3) said acidic artificial sludge;

then allowing the mixture to stand for a predetermined period of time and thereafter observing the state of the acidic artificial sludge to evaluate the dispersant.

8. A method according to claim 7, in which the spent acid sludge is added to the artificial sludge in an amount in the range of 0.01 to 18 percent by weight, based on the weight of the artificial sludge.

9. A method according to claim 7, in which the spent 20 acid sludge is the spent acid sludge by-produced in the sulfonation of a material selected from the group consisting of distillation residue of natural lubricating oil, alkyl benzene and mixtures thereof, by sulfonating one mole of said material with l.5-10 moles of S0 and then removing the sulfonated product and free sulfuric acid to obtain the spent acid sludge.

10. A method according to claim 7, in which said raw material is at least 75 percent by weight soluble in said aromatic hydrocarbon and a 0.2 percent solution thereof in said aromatic hydrocarbon has a color value above 3 when measured by ASTM D. l 500 and said raw material contains from 1 to 100 percent by weight of substances insoluble in the dispersion medium; and said raw material is dissolved in from 1 to 100 times as much by weight of said aromatic hydrocarbon and in which said aromatic hydrocarbons contain from six to 26 carbon atoms and from one to three alkyl groups on the benzene nucleus.

11. A method according to claim 7, in which the concentration of the solution is adjusted by distilling the solution, the amount of said dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the artificial sludge in the mixture being in the range of 0.00l-25 percent by weight and the balance of the mixture being dispersion medium.

12. A method according to claim 7, in which the concentration of the solution is adjusted by adding a concentration-controlling agent selected from the group consisting of aromatic hydrocarbons and parafiinic hydrocarbons having an average carbon number in the range of 12 to 34 and a viscosity at 40 C. of below I50 centistokes, the amount of the dispersant in the mixture being in the range of 0.000 [-20 percent by weight, the amount of the sludge in the mixture being 0.001-50 percent by weight and the balance of the mixture being dispersion medium.

13. A method according to claim 7, in which the dispersion medium has an average carbon number of not more than 20.

14. A method according to claim 7, in which the mixture is allowed to stand at a substantially constant temperature in the range of 10 to C.

I l ll 1* III

Claims

2. A method according to claim 1, in which said raw material is at least 75 percent by weight soluble in said aromatic hydrocarbon and a 0.2 percent solution thereof in said aromatic hydrocarbon has a color value above 3 when measured by ASTM D.1500 and said raw material contains from 1 to 100 percent by weight of substances insoluble in the dispersion medium; and said raw material is dissolved in from 1 to 100 times as much by weight of said aromatic hydrocarbon and in which said aromatic hydrocarbons contain from six to 26 carbon atoms and from one to three alkyl groups on the benzene nucleus.
3. A method according to claim 1, in which the concentration of the solution is adjusted by distilling the solution, the amount of said dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the artificial sludge in the mixture being in the range of 0.001-25 percent by weight and the balance of the mixture being dispersion medium.
4. A method according to claim 1, in which the concentration of the solution is adjusted by adding a concentration-controlling agent selected from the group consisting of aromatic hydrocarbons and paraffinic hydrocarbons having an average carbon number in the range of 12 to 34 and a viscosity at 40* C. of below 150 centistokes, the amount of the dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the sludge in the mixture being 0.001-50 percent by weight and the balance of the mixture being dispersion medium.
5. A method according to claim 1, in which the dispersion medium has an average carbon number of not more than 20.
6. A method according to claim 1, in which the mixture is allowed to stand at a substantially constant temperature in the range of 10* to 80* C.
7. A method of evaluating oil-soluble dispersants, which comprises: preparing an acidic artificial sludge by dissolving a raw material selected from the group consisting of coal pitch, petroleum pitch, coal tar, petroleum tar, wood tar, asphalt and heavy oil in an aromatic hydrocarbon to obtain a solution of said raw material in said aromatic hydrocarbon, removing any insoluble matter in the solution by filtration, and then adjusting the concentration of the solution to obtain an intermediate artificial sludge containing a predetermined quantity of matter insoluble in the hereinafter-mentioned dispersion medium and then adding spent acid sludge to said intermediate artificial sludge to obtain said acidic artificial sludge; mixing (1) a dispersion medium selected from the group consisting of paraffinic hydrocarbons, cycloparaffinic hydrocarbons and mixtures thereof and (2) a sample of dispersant to be evaluated, in predetermined proportions, with (3) said acidic artificial sludge; then allowing the mixture to stand for a predetermined period of time and thereafter observing the state of the acidic artificial sludge to evaluate the dispersant.
8. A method according to claim 7, in which the spent acid sludge is added to the artificial sludge in an amount in the range of 0.01 to 18 percent by weight, based on the weight of the artificial sludge.
9. A method according to claim 7, in which the spent acid sludge is the spent acid sludge by-produced in the sulfonation of a material selected from the group consisting of distillation residue of natural lubricating oil, alkyl benzene and mixtures thereof, by sulfonating one mole of said material with 1.5-10 moles of SO3, and then removing the sulfonated product and free sulfuric acid to obtain the spent acid sludge.
10. A method according to claim 7, in which said raw material is at least 75 percent by weight soluble in said aromatic hydrocarbon and a 0.2 percent solution thereof in said aromatic hydrocarbon has a color value above 3 when measured by ASTM D.1500 and said raw material contains from 1 to 100 percent by weight of substances insoluble in the dispersion medium; and said raw material is dissolved in from 1 to 100 times as much by weight of said aromatic hydrocarbon and in which said aromatic hydrocarbons contain from six to 26 carbon atoms and from one to three alkyl groups on the benzene nucleus.
11. A method according to claim 7, in which the concentration of the solution is adjusted by distilling the solution, the amount of said dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the artificial sludge in the mixture being in the range of 0.001-25 percent by weight and the balance of the mixture being dispersion medium.
12. A method according to claim 7, in which the concentration of the solution is adjusted by adding a concentration-controlling agent selected from the group consisting of aromatic hydrocarbons and paraffinic hydrocarbons having an average carbon number in the range of 12 to 34 and a viscosity at 40* C. of below 150 centistokes, the amount of the dispersant in the mixture being in the range of 0.0001-20 percent by weight, the amount of the sludge in the mixture being 0.001-50 percent by weight and the balance of the mixture being dispersion medium.
13. A method according to claim 7, in which the dispersion medium has an average carbon number of not more than 20.
14. A method according to claim 7, in which the mixture is allowed to stand at a substantially constant temperature in the range of 10* to 80* C.