GB2080282A - Agglomerating perlite ore fines prior to expansion - Google Patents

Abstract

Perlite ore fines (at least 50% of the particles being not greater than 74 mu ) are treated with an agglomerating agent which has a viscous liquid phase at a temperature above ambient temperature but below the critical dehydration temperature of the perlite ore fines, e.g. orthoboric acid or metaboric acid. The perlite ore fines and agglomerating agent are mixed, preferably by dry mixing, and then heated to the temperature of the viscous liquid phase and held at that temperature for a short period while the liquid coats at least a portion of the perlite ore fines and effects agglomeration. Thereafter, the agglomerated perlite ore fines can be expanded in a conventional perlite ore expander. The resulting expanded materials have properties essentially identical to standard expanded perlite ore and are useful for such purposes as insulating fillers and light weight aggregates.

Description

SPECIFICATION Preparation and expansion of perlite ore fines The invention here relates to the processing of perlite ore. More particularly, it relates to a method for converting perlite ore fines into expanded perlite.

Perlite is a mineral of volcanic origin which generally falls into the rhyolitic class. A unique feature of perlite is that it contains several percent of bound water. If perlite ore is rapidly heated to a temperature on the order of 1600"F (870or), the water is converted to steam and the perlite "pops", i.e., it expands rapidly to a much lower density.The amount of expansion is usually on the order of 4 to 20 times the original volume and the final density of the expanded perlite granules will normally be in the range of about 3.5 to 5 Ibs/ft3 (0.06 to 0.08 g/cm3) for use as insulating fillers or about 7 to 15 Ibs/ft3 (0.11 to 0.24 g/cm3) for plaster aggregate use. (The exact mechanism of the water-induced expansion is quite complex; see Lehmann et al, "Thermoanalytic Research on Perlite and Perlite-Type Rocks," Tonind.-Ztg.,vol. 100, no. 7, pp. 271-274 (1976). For the purposes of the invention herein, however, the above simplified description of the expansion is entirely adequate.) Perlite ore is normally expanded in large vertical chambers known as "expanders". At the bottom of each expander is a flame generating burner.The perlite ore to be expanded is dropped into the expander at a point above the flame and drops through the hot zone created by the flame. In this hot zone, the perlite particles expand and, because of their expanded volume, are then turned around and carried out of the top of the expansion chamber entrained in the exhaust gas stream. Operation of a typical perlite expander is shown in U.S. Patent No.2,572,484 with another variation shown in U.S. Patent No.

2,639,132.

It has been found over the years that the perlite ore, in order to be expanded satisfactorily, must be in the form of particles or granules having a size of from about +200 mesh (greater than 74 um) upto approximately 3 to 4 mesh (approximately i inch or 6 mm). Larger particles do not expand well because their weight pulls them through the flame zone too quickly for them to be heated to the expansion temperature and in addition their greater size and stronger structure prevents heat from penetrating to the center of the particle to convert the water to steam and also prevents the steam from fully "popping" the large rigid particle.The presence of these larger particles in the perlite ore prior to expanding causes no significant problems to the perlite ore processor, for such larger materials can be readily screened out of the raw material ore stream from the expanders and recycled to crushing or other size reduction equipmentto reduce the large granules to the optimum size for expansion.

At the other end of the size scale, however, the material known as "ore fines" which have particle sizes not greater than 200 mesh (not greater than 74 um) do pose a significant problem for the ore processor. These materials do not expand well because their light weight and small size causes them to be entrained in the exiting air and gas stream in an expander before they have dropped far enough to encounter the appropriate temperature forexpansion. In addition, such fine materials do not move well through the ore handling systems normally in use, which commonly rely in part on gravity feed.

Consequently, in the past, ore producers have considered the ore fines to be essentially waste material, although to some extent such unexpanded fine materials may fine uses (see, for instance, U.S. Patent No.4,175,158). Generally, however, since the largest market for perlite products is for expanded perlites, the ore processor is desirous of expanding as much of the ore raw material as possible.

It would be possible to handle such fine material if the individual fine particles were agglomerated into larger units which fall in the particle size range for conventional perlite expansion. However, heretofore the known processes for agglomerating "perlite fines" have been processes intended to be used with previously expanded material. In the process of expansion, perlite (which is a somewhat brittle material) often shatters upon expansion leaving a substantial amountf fragments of expanded perlite.

These fragments are themselves frequently referred to as "fines" but it must be understood that these are "expanded fines" as contrasted to the "ore fines" which are unexpanded and which are the subject of the present invention. U.S. Patents Nos. 3,235,635 and 4,175,158 both describe processes for agglomerating the expanded perlite fines by addition of water, a fluxing agent or, in the case of U.S.

Patent No.4,175,158, boric acid. An integral part of both of these described processes, however, is heating and maintaining the expanded perlite fines at temperatures on the order of about 1200 to 1700 F (650 to 925"C). At these temperatures, the fluxing materials such as soda ash or B203 (from the boric oxide) act to fuse the expanded perlite fines into larger granules.Such processes are not applicable to ore fines of perlite, however, because exposing the perlite ore to such temperatures will either cause the perlite to pop (if the exposure to the temperature is rapid) or will cause the water of hydration to slowly vaporize and be removed from the ore particle (if the perlite is slowly raised to that temperature) thus leaving no water of hydration to cause the perlite subsequently to pop.

It would therefore be highly desirable to have a process whereby the fine material in perlite ore raw materials can be agglomerated at relatively low temperatures to produce a particle of the appropriate size for conventional expansion.

The invention herein is a process for preparing perlite ore fines for expansion which comprises mixing the perlite ore fines with an agglomerating agent, the agglomerating agent having a viscous liquid phase at a temperature above room temperature but below the critical dehydration temperature of the perlite ore fines with the viscous liquid phase being capable of at least partially coating the perlite ore fines. Thereafter, the perlite ore fines are agglomerated by heating the mixture of the fines and the agglomerating agent to a temperature at which the agglomerating agent exists in the viscous liquid phase and maintaining the temperature for a period of time sufficient to obtain agglomeration of the fines by the agglomerating agent.

In another aspect, the invention comprises preparing the perlite ore fines for expansion as described above followed by rapid heating of the prepared perlite ore fines to a temperature at which the perlite ore fines expand.

The preferred agglomerating agents are materials of the boron acid system.

The present invention involves the treating of perlite ore fines to make them suitable for expansion in a conventional perlite expander. The "fine" portion of the perlite ore, as determined afterthe ore has been conventionally cleaned, crushed, screened and otherwise made ready for expansion, is normally defined as that portion of the ore which exists as particles having particle sizes of not greater than 200 mesh (i.e., not greaterthan 74 um). Twill be understood, however, that there is no absolute and distinct dividing line between materials considered "fines" and larger perlite particles considered to be conventional expandible perlite.Thus, while perlite having particle sizes of not greater than 74 um (-200 mesh) is generally considered to constitute perlite ore fines, in some circumstances particles which are larger than 74 um can also be considered to be "fine". For instance, in most conventional particle size separator devices, there is not a sharp cutoff of particles art a specified size. Ratherthere is a distribution of particle sizes with the principle segment being of the designated size but with the entire mass of separated particles also containing some material of larger andlor smaller size. Such is the case with the commercial perlite ore separation, where the "ore fines" fraction normally contains a portion having particle sizes above 74 um.Therefore, for the purposes of this invention, the "ore fines" which are beneficially agglomerated by the process of this invention will be defined as a mixture of perlite particles at least 50%, and preferably at least 70%, of which have particle sizes of not greater than 74 um.

Afterthe perlite ore fines (as defined above) have been separated from the remainder of the perlite ore following conventional mining, drying, crushing, sizing and other processes utilized in the perlite ore production industry to prepare raw materials for expansion, the fines are treated in accordance with the present invention. They are first mixed with a suitable agglomerating agent having the properties to be described below. Conveniently such agglomerating agent is a solid material at ambient temperatures and most conveniently, the agglomerating agent has been comminuted into solid particles of approximately the same particle size as the perlite ore fines. The perlite ore fines and agglomerating agent are blended and dry mixed by conventional mixing means such as ribbon blenders to form preferably a dry mixture of fines and agglomerating agent.Where the agglomerating agent is a liquid or has a liquid component at ambient conditions, it is desirable to mix the fines and agglomerating agent and then dry the mixture to facilitate subsequent handling of the mixture.

The agglomerating agent will be a material which is compatible with perlite and which has a viscous liquid phase at a temperature above ambient temperature but below the dehydration temperature of the perlite. The viscous liquid phase must be one that will effect at least partial coating of the perlite ore fines with the liquid. Materials which have such liquid phases at the indicated temperatures but " which do not also effect such coating are not suitable for use in the present invention. Examples of such materials which have been tried but found not to work are hydrated sodium silicate and hydrated sodium carbonate ("soda ash"). The preferred materials which do meet the required conditions are materials of the boron acid system, notably orthoboric acid (H3BO3) and metaboric acid (HBO2).

Orthoboric acid is a colorless or white powder at ambient temperatures with a specific gravity of 1.43.

At a temperature of approximately 340 F (1700C), orthoboric acid is transformed to metaboric acid.

Metaboric acid is a white crystalline solid at ambient temperatures with a specific gravity of approximately 2.49. At approximately 4604F (236'C), the metaboric acid melts, forming a viscous liquid which may contain some crystals and which recrylstallizes upon cooling. The phase change of metaboric acid from solid to liquid at the melting point also results in approximately a 10-fold increase in the surface area. This liquid readily coats onto the perlite ore fine particles.

Descriptions of the boron acid system and the viscous metaboric acid liquid can be found in Kracek et al, "The System, Water-Boron Oxide",Amer. J. of Sci., vol. 35-A, pp. 143-171(1938); in De Bore (ed.), Reactivity OfSolids (1961) in the section by Dollimore et al, "The Kinetics of the Thermal Decomposition of Some Substances Which Decompose to Oxides, and the Production of Active Solids by Such Processes" pp. 627-637; in Kemp, The Chemistry of Borates: Partl (1956), pp 9-21; and in Kirk-Othmer (eds.), Encyclopedia Of Chemical Technology (3rd edn., 1978), vol.4, pp.67-77.

The agglomerating agent may be present in any amount from 0.1% to about 10% by weight based on the weight of the perlite ore fines, with the agent preferably being present as from 04 to 3 weight percent.

Following mixing of the perlite ore fines and the agglomerating agent, the blended materials are subjected to heating to a temperature at which the agglomerating agent is converted to the viscous liquid phase, but which temperature does not exceed the "critical dehydration temperature" of the perlite. For the purposes of this invention, the term "critical dehydration temperature" in fact represents time-temperature combinations. It is known that the ability of perlite to expand is based generally on essentially a!l of the bound water being simultane- ously converted to steam, so that the expansion of the water on vaporization causes the perlite to expand as it seeks to escape from the particle. If, however, the bound water is vaporized slowly, each increment of vaporized water will have time to diffuse or migrate slowly out of the unexpanded particle, ultimately leaving the perlite particle so depleted of water that rapid heating of the particle under normal expansion conditions will not result in expansion. This slow "dehydration" of the particle (which is "slow" in comparison to the rate of the normal expansion-causing vaporization) occurs at varying rates which are dependent on both the temperature to which the perlite is exposed and also to the length of time of that exposure.As the temperature rises, the period of exposure time necessary to effect sufficient dehydration to prevent expansion is decreased. (At a sufficiently high temperature, of course, vaporization occurs so rapidly and so uniformly across the particle that expansion is induced, since there is not sufficient time available to let the vaporized water escape from the particle by migration and/or diffusion.) In general, at temperatures below about 1000"F (540"C), and especially below about 700"F (370"C), significant dehydration dows not occur in the time periods required for the liquification and application ofthe agglomerating agent.Therefore the effect of time can be simplified by merely requiring that the liquification and application of the agglomerating agent be conducted at a temperature below the "critical dehydration temperature," i.e., that temperature at which the time period of the liquification and application would be the minimum time in which significant dehydration could occur.

Normally the dry mixing and heating are done in separate but closely sequential steps. They could, however, be performed simultaneously by using a heated mixer. Alternatively, if the agglomerating agent is a stable material, mixing could take place long before heating with the mixed material simply being stored until time to be heated. This would also permitthe mixing and heating to be conducted at separate location which are perhaps even quite remote from each other.

The time of maintenance at temperature is dependent upon the particular agglomerating agent being used. For the boron acids, it has been found that a time of 4 minutes at 600"F (315"C) provides quite satisfactory agglomeration of perlite fines. The heating time should be kept as short as possible commensurate with good agglomeration because of the tendency of the perlite to dehydrate over prolonged heating periods, even at relatively low temperatures.

Heating times on the order of 1 to 20 minutes, preferably 3 to 10 minutes, are considered to be quite adequate.

In one aspect of the invention, the perlite ore fines which have completed their heating with the agglomerating agent are passed directly to a perlite expander where they are expanded. This is quite feasible when the expander has associated therewith a pre-heat chamber into which the mixed perlite and agglomerating agent can be placed for heating.

The pre-heat chamber discharges into the expander feed system so that the agglomerated perlite ore fines can be fed directly to the expander for expansion. Alternatively, however, the agglomerating agent will be one which can be cooled and then rapidly re-melted during expansion. This would permit the perlite ore processor to agglomerate the ore fines with the agglomerating agent at the mine, mill or other ore producing or preparation site, then store or ship the agglomerated perlite ore fines to a remote location for expansion with the agglomer ated fines being put directly into the expander feed system without any further heating. This is the preferred manner of handling the heat treating and expansion. This is due to several factors: 1) Many perlite expanders are of rather small capacity and do not have heating equipment available in conjunction with the expander.Consequently the processor wishing to use this process in conjunction with per lite expansion would frequently be forced to add a heating unit at considerable cost to his perlite expansion equipment. 2) Once the boric acid is coated on the perlite particles, the two components will not segregate as they would if they were merely blended into a simple dry mixture. 3) A more uniform agglomerated fines product can be obtained, since the ore producer treats the perlite ore fines and thus can run a single consistent operation, ratherthan having a number of different end users treating perlite or fines in a variety of different types of mixers, heaters and under perhaps significantly different conditions. Such cooling, re-heating and remelting are quite feasible when orthoboric acid is used as the agglomerating agent.

Once the perlite ore fines have been agglomerated with the agglomerating agent and heated for the appropriate time, they are then ready to be expanded in a conventional perlite expander. As noted above, the expansion can come immediately afterthe heat treating or it can be delayed for any desired length of time. Expansion of the treated perlite ore fines is in the same manner as conventional perlite expansion of the larger normal perlite particles. The perlite ore fines can be expanded by themselves or can be mixed into a common feed with the original standatd size perlite feed particles.

The examples below will illustrate the process of this invention. In these experiments, both the standard perlite ore materials and the perlite ore fines were from a commercial New Mexico perlite mining operation. Conventional mining, cleaning, crushing, sizing and other ore processing techniques were used. Agglomeration was with orthoboric acid in amounts of 1 or 2 percent by weight of the perlite ore fines, as noted.The fines portion of the ore was defined to be that portion in which 70% of the patti- cles were of 74 um particle size or smaller. HeatiFs of the ore fines/agglomerating agent mixture was fcw 4 minutes at 600"F (315"C). The "Series A" runs illustrate the expansion of the perlite to the lowest achievable loose weight "Series B" are experiments in which the perlite was expanded to a loose weight in the range of 3 to 4 Ib/ft3 (0.048 to 0.064 g/cm3), which is the conventional density for perlite use as an insulating filler. The processing conditions were otherwise the same as in "Series A". "Loose weight" is often referred to as "bulk density" or "apparent density" and refers to the nominal density of a loosely packed powder.

Table 1 below shows the particle size distributions of the perlite ore fines and the standard ore as well as various expanded materials.

TABLE 1: SERIES A Percent of Material Larger Than Indicated Particle Size After Expansion Ore Standard Ore Fines, Agglomerated Agglomerated Particle Fines, as Ore, as without Fines {1% Fines (2% Standard Size, um receivedRa) receivedba) agglomeration boric acid) boric acid) Ore 30 82 100 100 100 100 100 50 51 100 97 100 100 100 74 30 100 86 100 100 100 100 18 100 75 98 98 98 150 5 95 57 92 92 92 200 3 85 42 85 85 85 300 1 5 22 70 70 70 400 - 1 12 56 56 56 600 - - 3 30 30 30 750 - - 1 13 13 13 900 - - - 1 1 1 Median 41 165 172 440 440 440 Particle Size, um Note: a) "As received" means prior art ore or ore fines fully prepared as expander feed but not yet expanded Table 2 below illustrates typical properties of the expanded materials. The "energy consumption" is the amount of energy required to expand the indicated material.

TABLE 2: SERIES A Energy Weight Recovery, Energy 1:1 Loose Weight Recovery, /bum3 glum3 % BTUllb Callg Non-agglomerated 2.1 0.034 83.3 23000 12800 Ore Fines Agglomerated 1.8 0.029 90.0 9940 5520 Fines (1% boric acid) Agglomerated 1.67 0.027 94.5 13600 7560 Fines (2% boric acid) Standard 1.62 0.026 92.9 18300 10200 Ore Note: (a) The data here are for a small (laboratory scale) expander.

The relative energy consumption per unit of perlite weight would be expected to be 8-10 times less for a commercial scale expander.

It will be evident from these experimental data that the agglomerated perlite or fines expand to a size and loose weight essentially equivalent to that of standard perlite ore.

Similar data are presented in Tables 3 and 4 below for the "Series B" materials.

TABLE 3: SERIES B Percent of Material Larger Than Indicated Particle Size After Expansion Ore Standard Ore Fines, Agglomerated Agglomerated Particle Fines, as Ore, as without Fines (1% Fines (2% Standard Size, um received(a) received(a) agglomeration boric acid) boric acid) Ore 30 82 100 100 100 100 100 50 51 100 95 98 98 100 74 30 100 79 94 94 100 100 18 100 67 90 86 100 150 5 95 50 83 73 96 200 3 85 37 76 62 89 300 1 5 19 62 45 70 400 - - 9 36 24 44 600 - - 4 9 6 17 750 - - 1 2 2 6 900 - - 1 1 1 Median 41 165 150 340 260 375 Particle Size, um Note: a) "As received" means prior art ore or ore fines fully prepared as expander feed but not yet expanded TABLE 4:SERIES B Energy Loose Weight Recovery, Consumption (a) Ibm3 glum3 % BTUllb Calls Non-agglomerated 3.5 0.056 89.1 15800 8740 Ore Fines Agglomerated 3.0 0.048 92.4 9080 5040 Fines (1% boric acid) Agglomerated 3.5 0.056 94.4 7670 4260 Fines (2% boric acid) Standard 3.72 0.059 93.6 16200 9000 Ore Note: (a) The data here are for a small (laboratory scale) expander.

The relative energy consumption per unit of perlite weight would be expected to be 8-10 times less for a commercial scale expander.

It will be evident from these data that the agglom3rated fines of the process of this invention followng expansion are equivalent to standard expanded perlite ore used for insulating fillers. In addition, the data from both Series A and Series B indicate that the energy input required to expand the agglomerated fines is significantly less than that required to expand standard perlite ore. In addition, both the Series A and Series B experiments clearly indicate that the agglomerated perlite fines yield significantly greater recovery and significantly higher median particle sizes than the expanded perlite ore fines without any agglomeration. The latter property is particularly significant since the value of an expanded perlite as an insulating or cement filler is directly proportional to the particle size of the perlite.

Statement of Industrial Application The invention herein relates to the perlite expansion industry. It is particularly useful in the expansion of the fine portion of perlite ore. Expanded perlite ores find utility as thermal insulations in such materials as structural roofing boards and insulating wall boards for residential, commercial and industrial buildings. Expanded perlite also finds consider able use as a light weight aggregate for construction materials such as plaster, gypsum and concrete.

Construction panels made of the materials contain ing light weight aggregates are used where strength is not a prime requirement but where fire resis tance, insulation and light weight are major consid erations. Typical applications include installation in tall buildings and long span bridges.

Claims (18)

1. A process for preparing perlite ore fines for expansion which comprises mixing said perlite ore fines with an agglomerating agent which has a viscous liquid phase at a temperature above ambient temperature but below the critical dehydration temperature of said perlite ore fines, said viscous liquid phase being capable of at least partially coating said perlite ore fines, and agglomerating said ore fines by heating the mixture of said perlite ore fines and agglomerating agent to a temperature at which said agglomerating agent exists as said viscous liquid phase and maintaining said temperature for a period of time sufficient to obtain agglomeration of said perlite ore fines by said agglomerating agent.

2. A process as in Claim 1 wherein said perlite ore fines comprise a mixture of perlite ore particles at least 50% of which have particle sizes of not greaterthan 74 um.

3. A process as in Claim 2 wherein said perlite ore fines comprise a mixture of perlite ore particles at least 70% of which have particle sizes of not greaterthan 74 um.

4. A process as in Claim 2 wherein said perlite ore fines comprise a mixture of perlite ore particles substantially all of which have particle sizes of not greater than 74 um.

5. A process as in Claims 1,2,3 or 4 wherein said agglomerating agent is a boron acid.

6. A process as in Claim 5 wherein said boron acid is orthoboric acid, said orthoboric acid having a transition to metaboric acid at a temperature below the formation of said viscous liquid phase.

7. A process as in Claim 5 wherein said boron acid is metaboric acid.

8. A process as in Claims 1,2,3 or 4 wherein said mixing is dry mixing and both the perlite ore fines and agglomerating agent are in the form of granular powders.

9. A process as in Claim 8 wherein said agglomerating agent is a boron acid.

10. A process as in Claim 9 wherein said boron acid is orthoboric acid, said orthoboric acid having a transition to metaboric acid at a temperature below the formation of said viscous liquid phase.

11. A process as in Claim 9 wherein said boron acid is metaboric acid.

12. A process as in Claims 1,2,3 or 4 wherein said temperature at which said agglomerating agent exists as a viscous liquid phase is not greater than 1000 F.

13. A process as in Claim 12whereinsaidtemp erature is not greater than 700"F.

14. A process for expanding perlite ore fines which comprises preparing said perlite ore fines for expansion according to the preparation process of Claims 1,2,3 or4 followed by rapid heating of said prepared perlite ore fines to a temperature at which said perlite ore fines expand.

15. A process as in Claim 14 wherein said expansion is conducted sequentially immediately following said preparation.

16. A process as in Claim 15 wherein said expansion is conducted in an expansion chamber and said heating and maintaining steps of said preparation process are conducted in a pre-heating chamber adjacent to said expansion chamber.

17. A process as in Claim 14 wherein said preparation process is completed and said prepared perlite ore fines are cooled to ambient temperature prior to subjecting said prepared perlite ore fines to said expansion.

18. A process as in Claim 17 wherein said preparation process is conducted at a location remote from the location at which said subsequent expansion is conducted.