FIELD OF THE INVENTION This invention relates to a novel apparatus for mixing fluids, especially amine and phosgene, and to a process for mixing amine and phosgene in order to obtain carbamoyl chloride and isocyanate. BACKGROUND OF THE INVENTION Many documents disclose nozzles for mixing fluids, especially reacting fluids. One particular example is found in the phosgenation reaction in which rapid mixing is a key parameter. Hence, many designs have been proposed for such nozzles, mostly with coaxial jets, which can be impinging or not. WO-A-2004004878 discloses such a jet mixer nozzle, useful for carrying out phosgenation reactions, comprising: (a) a first nozzle comprising a first flow duct having a convergent, tapered first nozzle tip having a first discharge opening; (b) a second nozzle comprising a second flow duct having a convergent, tapered second nozzle tip having a second discharge opening, wherein the second nozzle is disposed coaxially around the first nozzle and the second discharge opening is substantially coaxially aligned with the first discharge opening, a second flow chamber being defined between the second nozzle and the first nozzle; and (c) a center body disposed coaxially inside the first nozzle and having a protruding portion protruding axially beyond and through the first discharge opening, a first flow chamber being defined between the first nozzle and the center body; wherein during operation of said apparatus, the first fluid flowing in the first flow chamber and exiting through the first discharge opening forms a first annular fluid jet that is dispersed around said protruding portion; and the second fluid flowing in the second flow chamber forms at the second discharge opening a second annular fluid jet that impinges upon the first annular fluid jet, thereby mixing the first and second fluids. This mixing apparatus (and associated process) is disclosed as having many possible shapes; one specifically mentioned being a rectangular shape. What appears to be the invention in this document is the use of a center body that protrudes axially beyond and through the first discharge opening. Said central body will make it possible to have thin opening and thus thin chambers, rendering mixing quite good at the exit of the device. However, such a mixing apparatus will suffer from drawbacks, especially the possible solids deposit formation (i.e. fouling) on the tip of the protruding section, where cleaning would not be possible without having to shut down the unit. Also, this mixing device requires a central body, i.e. an extra piece to manufacture, with high precision, rendering manufacturing costs less attractive. Hence, there is a need for a mixing device that allows cleaning while in operation and which would still possess thin chambers for thin impinging flows. SUMMARY OF THE INVENTION An object of this invention is therefore to provide an apparatus for mixing at least first and second fluid, comprising: (a) a first nozzle comprising a first flow duct having a convergent, tapered first nozzle tip having a first discharge opening; (b) a second nozzle comprising a second flow duct having a convergent, tapered second nozzle tip having a second discharge opening, wherein the second nozzle is disposed coaxially around the first nozzle and the second discharge opening is substantially coaxially aligned with the first discharge opening, a second flow chamber being defined between the second nozzle and the first nozzle; and optionally a center body disposed coaxially inside the first nozzle and having no protruding portion protruding axially beyond and through the first discharge opening; wherein said tapered first nozzle tip and said tapered second nozzle tip have a cross-sectional shape that is substantially rectangular; wherein during operation of said apparatus, the first fluid flowing in the first flow chamber and exiting through the first discharge opening forms a first fluid jet, and the second fluid flowing in the second flow chamber forms at the second discharge opening a second fluid jet that impinges upon the first fluid jet, thereby mixing the first and second fluids. Another object of this invention is also to provide a process for mixing at least first and second fluid, comprising the steps of: (a) forming a first rectangular fluid jet, consisting of the first fluid, at a first axial discharge position; (b) forming a second rectangular fluid jet coaxial with and around the first rectangular fluid jet and consisting of the second fluid at a second axial discharge position so that the second rectangular fluid jet impinges upon the first rectangular fluid jet, thereby mixing the first and second fluids. The process of the invention is especially useful for the production of isocyanates. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of one embodiment of the invention; - FIG. 2 is an enlarged view of the embodiment depicted in FIG. 1; FIG. 3 is an enlarged view of another embodiment of the invention; FIG. 4 is a cross-sectional view of another embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Other objects, features and advantages will become more apparent after referring to the following specification. The invention is based on the use of a rectangular shaped nozzle, referred to hereinafter as a slot nozzle. The specific geometry allows thin flows while at the same time avoiding a central protruding element. Referring now to FIG. 1 there is shown an impinging coaxial jet mixer nozzle assembly 100 for mixing two fluids. Impinging coaxial jet mixer nozzle assembly 100 comprises inner flow duct 102 and an inner flow duct nozzle tip 104 disposed coaxially inside outer flow duct 101 and outer flow duct nozzle tip 105. Flow chamber 120 is defined as the rectangular space inside inner flow duct 102 and inner flow duct nozzle tip 104. Flow chamber 120 has two ends, supply end 130 and discharge end 110. Discharge end 110 of flow chamber 120 is formed by the discharge end of inner flow duct nozzle tip 104 and has a discharge opening, with width and length which will be defined further below. Flow chamber 121 begins as the rectangular space between outer flow duct 101 and inner flow duct 102. Flow chamber 121 continues as the rectangular space between outer flow duct nozzle tip 105 and inner flow duct 102. Flow chamber 121 continues further as the rectangular space between outer flow duct nozzle tip 105 and inner flow duct nozzle tip 104. Flow chamber 121 has two ends, supply end 131 and discharge end 132. Discharge end 132 of flow chamber 121 is formed by the discharge end of outer flow duct nozzle tip 105. Discharge end 132 of flow chamber 121 has width and length which will be defined further below. The first fluid flows through flow chamber 120 and is discharged at discharge end 110 as rectangular jet 103. Jet 103 has dimensions substantially identical to those dimensions of the discharge opening of nozzle tip 104. The second fluid flows through flow chamber 121 and is discharged at discharge end 132 as rectangular jet 106 (which has dimensions substantially identical to those dimensions of the discharge opening of nozzle tip 105) . The two coaxial jets 103 and 106 collide and mix as they exit nozzle tips 104 and 105 to form composite jet 107. The primary driving force for mixing is the kinetic .energy of jets 103 and 106. The velocities of the fluids are selected by the relative designs of the nozzles 104 and 105. The angle at which nozzle tips 104 and 105 are tapered may vary, e.g. from 30 to 60°, preferably from 40 to 50°, especially about 45°. The nozzle assembly of the present invention thus provides an apparatus for mixing at least first and second fluids, the apparatus comprising first nozzle assembly means for forming a first rectangular fluid jet 103, consisting of the first fluid, and second nozzle assembly means for forming a second rectangular fluid jet 106 coaxial with and around first rectangular fluid jet 103, the second rectangular fluid jet consisting of the second fluid, so that second rectangular fluid jet 106 impinges upon first rectangular fluid jet 103, thereby mixing the first and second fluids. It would be possible to provide further ducts for further fluids, if this is necessary. Referring now to FIG. 2 there is shown an enlarged longitudinal cross section view of the nozzle tips of FIG. 1. Here the various gaps (i.e. width and length) are chosen so as to impart the required velocities. Typically, the (superficial) velocity of the jet 103 will be 5-90 ft/sec, preferably 10-40 ft/sec. Typically, the (superficial) velocity of the jet 106 will be 5-70 ft/sec, preferably 10-40 ft/sec. The gap between nozzle tip 104 and nozzle tip 105, identified as z in FIG. 2 is typically 0.03-0.2", preferably 0.05-0.15". The gap (i.e. its width) of the discharge end 110, identified as y in FIG. 2 is typically 0.05-0.2", preferably 0.05-0.1". The gap (i.e. width D) of the discharge end 132, identified as x in FIG. 2 is typically 0.2-0.4", preferably 0.2-0.3". The discharge end 132 has typically a length L such that the ratio L/D varies from 20 to 200, preferably 60 to 150. The discharge end 110 will show a length substantially identical to the length of discharge end 132 (minus substantially twice the value of gap z) . Gaps x and y are such that y can be smaller than x or y is larger than or equal to x. The first embodiment is preferred. In one embodiment, a portion of the convergent, tapered first discharge end protrudes beyond the second discharge end. In another embodiment, the first discharge opening is axially proximate to the second discharge opening such that first nozzle tip does not protrude substantially beyond the second discharge opening and the second nozzle tip does not protrude substantially beyond the first discharge opening. Referring now to FIG. 3 there is shown an enlarged view of a further embodiment of the invention. In this embodiment, the invention further comprises a central body 201 that does not protrude in normal operation. In normal operation, the central body substantially does not interfere with the flows of fluids. Central body 201 is generally in a fixed position with respect to nozzle 105 while nozzle 104 is movable along the axial direction. Central body 201 has a tip 202 having an outer diameter that is substantially equal to gap y, so that when nozzle 104 is retracted to the upper position (identified by the dotted lines in FIG. 3) tip 202 will protrude through discharge opening 110, whereby scraping any deposits that may have been formed at the discharge opening 110. Conversely, nozzle 104 may be lowered so as to tightly fit with nozzle 105, thereby crushing any deposits that may have been formed in the vicinity of the discharge opening 132 in discharge chamber 121. It would also be possible to foresee a movable center body 201; see below with respect to FIG. 4. It would also be possible for center body 201 to have a staged tip, with a first part having a first outer width smaller than gap y, for adjustment purposes (see below) and a second part proximate the first one, said second part having a second outer diameter substantially equal to y for cleaning purposes as above. Referring now to FIG. 4 there is shown a nozzle assembly in longitudinal cross section, one embodiment of the present invention. Parts already identified in previous FIGS are identified again with the same numerals. Impinging jet mixer nozzle assembly 100 comprises a non-protruding center body -201, mechanical means for adjustment of the axial position of center body 201, mechanical means for adjustment of the axial position of inner flow duct nozzle tip 104, flow ducts 370 and 371 to supply fluids to inner and outer nozzle tips 104 and 105, and mechanical seals 340 and 341 for preventing leakage of the fluids. Impinging jet mixer assembly 100 includes spider assemblies 304 and 305. Each spider assembly comprises a hub with radiating arms. The distal ends of the radiating arms of spider assemblies 304 and 305 are affixed to the inside of inner flow duct 102. Outer shaft 330 fits through the hub of spider assembly 304. Outer shaft 330 and the hub of spider assembly 304 have matching threaded surfaces. The threaded portion of outer shaft 330 extends beyond the hub of spider assembly 304 to allow adequate travel of spider assembly 304 along outer shaft 330,. Inner shaft 331 fits through the hub of spider assembly 305. Inner shaft 331 and the hub of spider assembly 305 have matching threaded surfaces. The threaded portion of inner shaft 331 extends beyond the hub of spider assembly 305 to allow adequate travel of spider assembly 305 along outer shaft 331. There are two flow chambers in' impinging jet mixer assembly 100. The upstream side of flow chamber 370 begins with the internal space , defined by typically round inlet nozzle 301. Round inlet nozzles allow easier and simpler connections to be made to standard fluid supply piping systems. Flow chamber 370 continues into the space between inner flow duct 102 and outer shaft 330. Flow chamber 370 continues into the space between expansion joint 303 and outer shaft 330. Flow chamber 370 continues into the space between inner flow duct 102 and outer shaft 330, including openings in spider assembly 304. Flow chamber 370 continues into the space between inner flow duct 102 and inner shaft 331, including openings in spider assembly 305. Flow chamber 370 continues into the space between inner flow duct 102 and center body 201. Flow chamber 370 then continues until flow duct nozzle tip 104 to form the discharge end 110. The upstream side of flow chamber 371 begins with the internal space defined by typically round inlet nozzle 310. Flow chamber 371 continues into the space between outer flow duct 311 and inner flow duct 302. Flow chamber 371 continues into the space between outer flow duct 101 and inner flow duct 102. Flow chamber 371 continues into the space between outer flow duct 101 and expansion joint 303. Flow chamber 371 continues into the space between outer flow duct 101 and inner flow duct 102. Flow chamber 371 continues into the space between outer flow duct nozzle tip 105 and inner flow duct 102. Flow chamber 371 continues into the space between outer flow duct nozzle tip 105 and inner flow duct nozzle tip 104 to form discharge end 132. In another, preferred embodiment, central body 201 is not movable. In this embodiment, elements cited above and below for moving central body 201 will not be present. Two fluids are mixed in jet mixer assembly 100. The first fluid flows through flow chamber 370 and is discharged as inner rectangular jet 103. The second fluid flows through flow chamber 371 and is discharged from the nozzle as outer rectangular jet 106. The two jets collide and mix as they exit nozzle tips 104 and 105. The mixed streams form fluid jet 107. Impinging jet mixer assembly 100 also provides for adjustment of the axial position of both inner flow duct nozzle tip 104 and optionally center body 201. Axial 'movement of nozzle tip 104 with relation to nozzle tip 105 is achieved by rotating hand wheel 321 which rotates outer shaft 330. The axial position of outer shaft 330 is fixed by thrust bearing assembly 322. As outer shaft 330 is rotated, it produces an axial force on spider assembly 304 and moves inner flow duct 102 and inner flow duct nozzle tip 104 axially. The ability of inner flow duct 102 and inner flow duct nozzle tip 104 to move axially is provided by expansion joint 303, which either compresses or expands as inner flow duct 102 and inner flow duct nozzle tip 104 move along their axial paths. The net result is that turning hand wheel 321 enables on-line adjustment of the relative axial position of inner flow duct nozzle tip 104 to outer flow duct nozzle tip 105. As the relative axial position of inner flow duct nozzle tip 104 to outer flow duct nozzle tip 105 is adjusted, the cross-sectional area (i.e. gap z) for flow between elements 104 and 105 is adjusted, changing the thickness of impinging outer rectangular jet 106. Also, when inner flow duct 102 and inner flow duct nozzle tip 104 move along their axial paths towards the retracted position, the cleaning operation disclosed above is achieved. In the embodiment where center body is movable, axial movement of center body 201 relative to the inner flow duct nozzle tip 104 is achieved by a mechanical linkage connected to hand wheel 320. Hand wheel 320 is affixed to inner shaft 331 which is free to move axially. When hand wheel 320 is rotated, inner shaft 331 rotates. As inner shaft 331 rotates, spider assembly 305 produces an axial force on inner shaft 331. Therefore, the turning of hand wheel 320 causes inner shaft 331 to move axially. Center body 201 is affixed to inner shaft 331. The net result is that the axial position of center body 201 can be adjusted by rotating hand wheel 320. As the relative axial position of center body 201 to inner flow duct nozzle tip 104 is adjusted, it is possible for the cleaning operation disclosed above to be achieved. Impinging jet mixer assembly 100 includes several sets of flanges to facilitate assembly, maintenance, and modification. Numerals 350, 351, 352, and 353 all refer to such flanges. Flange 360 is included to enable mounting of the mixer onto a reactor vessel. Numerals 340 and 341 refer to mechanical seal assemblies which provide seals around rotating shafts 330 and 331 preventing the leakage of either fluid. Impinging jet mixer assembly 100 may also include sealing surfaces between inner flow duct nozzle tip 104 and outer flow duct nozzle tip 105 and also between center body 201 and inner flow duct nozzle tip 104. The surfaces can also be treated and/or finished with conventional surface treatments including coatings, polishing, adding ridges or grooves, if need be. The invention provides several advantages over prior art nozzle assemblies. One advantage is a reduction in thickness of the inner jet stream to be mixed, but without resorting to a protruding central body and associated costs and drawbacks. The specific geometry of the nozzle does not require impingement on other surfaces, and this avoids erosion and expensive alignment. The nozzle assembly is simple to manufacture and install. Off-line pretests are available, thereby minimizing on-line tuning of equipment. Another advantage is that the nozzle assembly is easy to dismount, allowing a rapid turnaround. One further advantage is that there are no .-continuously moving or rotating parts, avoiding thus any mechanical wear of the system. An advantage of the embodiment with movable parts is the on-line adjustability of the longitudinal cross- sectional area for flow of the outer jet stream (and possible of the inner flow as well) . On-line adjustability- denotes the ability to make adjustments without undue interference with an ongoing process. In commercial scale processes, on-line adjustability allows for frequent adjustment of the nozzles for maximum pressure drop at the discharge point of the nozzle, thereby maximizing mixing energy and mixing performance, while allowing the required flows to pass. Another advantage is improved turn-down capability of commercial processes. Fixed mixer nozzle assemblies tend to have limited ranges for optimal operation. The adjustability of one embodiment of this invention will allow a wider range of operating rates for some processes. Another advantage is the ability to stroke inner nozzle relative to center body 201 through its full travel path with the nozzle assembly installed. Commercial scale mixer assemblies can become plugged with debris or solid deposits. Stroking inner nozzle closed on center body 201 can scrape debris and deposits lodged in the inner orifice of inner flow duct nozzle tip 104. The ability to stroke inner nozzle relative to center body for cleaning purposes at the discharged end 110 is generally preferred to the alternative of shutting down a process to remove the nozzle assembly from service and replace it with a clean, unplugged nozzle assembly. A further advantage is the ability to stroke inner flow duct nozzle tip 104 through its full travel path towards nozzle tip 105 with the nozzle assembly installed. As already indicated, commercial scale mixer assemblies can become plugged with debris or solid deposits. Stroking inner flow duct nozzle tip 104 closed on nozzle tip 105 can crush some types of debris lodged between inner flow duct nozzle tip 104 and outer flow duct nozzle tip 105. Retracting back inner flow duct nozzle tip 104 open can allow debris lodged between inner flow duct nozzle tip 104 and outer flow duct nozzle tip 105 to pass. The ability to stroke inner flow duct nozzle tip 104 to clear pluggage at the discharge end of outer fluid discharge end 132 is generally preferred to the alternative of shutting down a process to remove the nozzle assembly from service and replace it with a clean, unplugged nozzle assembly. The invention is especially useful for very fast chemical reactions where fast mixing is crucial. Hence, the invention is useful as a pre-phosgenation reactor for the preparation of isocyanates. In this embodiment, the fluid flowing through the inner path is a primary amine, optionally dissolved in a solvent. In this embodiment, the fluid flowing through the outer path is phosgene, optionally dissolved in a solvent. Hence, the invention is useful for the manufacture of various isocyanates, and may e.g. be selected from aromatic, aliphatic, cycloaliphatic and araliphatic polyisocyanates. The nozzle assembly allows for minimizing side reactions, which is beneficial in terms of throughput, blend strength, yield and color. It is possible, as in the known techniques, to recycle a solution of solvent, phosgene, and isocyanate singly or in combination back into the phosgene flow. In one embodiment, it is preferred not to recycle this solution. In particular are produced the aromatic polyisocyanates such as diphenylmethane diisocyanate (MDI) (e.g. in the form of its 2,4'-, 2,2'- and 4, 4 '-isomers and mixtures thereof), and mixtures of diphenylmethane diisocyanates (MDI) and oligomers thereof known in the art as "crude" or polymeric MDI (polymethylene polyphenylene polyisocyanates) having an isocyanate functionality of greater than 2, toluene diisocyanate (TDI) (e.g. in the form of its 2,4- and 2,6-isomers and mixtures thereof), 1, 5-naphthalene diisocyanate and 1, 4-diisocyanatobenzene (PPDI) . Other organic polyisocyanates which may be obtained include the aliphatic diisocyanates such as isophorone diisocyanate (IPDI), 1, 6-diisocyanatohexane and 4, 4 ' -diisocyanatodicyclo- hexylmethane (HMDI) . Still other isocyanates that can be produced are xylene diisocyantes, phenyl isocyanates. If need be, the geometry of the nozzle assembly of the invention can be adapted to the specific isocyanate to be manufactured. Routine tests will enable one skilled in the art to define the optimum values for the gaps and lengths, as well as operative conditions. The nozzle assembly of the invention can be used in a classical continuously stirred tank reactor (with or without baffles) . The nozzle assembly can be in the vapor space or submerged. The nozzle assembly of the invention can be used in all existing equipment with minimal adaptation, thus saving costs. Also, the nozzle assembly of the invention can be used in any type of reactor; for example the nozzle assembly can be mounted at the bottom of a rotary reactor equipped with impellers and baffles or the nozzle assembly can be used as an injection device in a rotor/stator type reactor. The process conditions are those typically used. The phosgene:amine molar ratio is generally in excess and ranges from 1.1:1 to 10:1, preferably from 1.5:1 to 5:1. A solvent is generally used for the amine and the phosgene. Exemplary solvents are chlorinated aryl and alkylaryl such as monchlorobenzene (MCB) , o- and p-dichlorobenzene, trichlorobenzene and the corresponding toluene, xylene, methylbenzene, naphthalene, and many others known in the art such as toluene, xylenes, nitrobenzene, ketones, and esters. The amine blend strength can be from 5 to 40 wt% while the phosgene concentration can be from 40 to 100 wt%. The temperature of the amine flow is generally comprised from 40 to 8O0C while the temperature of the phosgene flow is generally comprised from -20 to O0C. The process is conducted at a pressure (at the mixing zone) generally from atmospheric to 100 psig. It is also possible to use one or more further reactors (esp. CSTRs) to complete the reaction. In the process for manufacturing isocyanates, it is also possible to use typical units for recycling solvent and/or excess phosgene, for removing HCl, etc.

The depicted and described preferred embodiments of the invention are exemplary only and are not exhaustive of the scope of the invention.